diff --git a/package-lock.json b/package-lock.json
index dc2b6ab..0b01796 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -9,26 +9,26 @@
       "version": "0.3.0",
       "license": "GPL-3.0-or-later",
       "devDependencies": {
-        "@types/node": "^22.1.0",
-        "prettier": "^3.3.3",
-        "prettier-plugin-organize-imports": "^4.0.0",
-        "typescript": "^5.5.4"
+        "@types/node": "^22.10.1",
+        "prettier": "^3.4.2",
+        "prettier-plugin-organize-imports": "^4.1.0",
+        "typescript": "^5.7.2"
       }
     },
     "node_modules/@types/node": {
-      "version": "22.1.0",
-      "resolved": "https://registry.npmjs.org/@types/node/-/node-22.1.0.tgz",
-      "integrity": "sha512-AOmuRF0R2/5j1knA3c6G3HOk523Ga+l+ZXltX8SF1+5oqcXijjfTd8fY3XRZqSihEu9XhtQnKYLmkFaoxgsJHw==",
+      "version": "22.10.1",
+      "resolved": "https://registry.npmjs.org/@types/node/-/node-22.10.1.tgz",
+      "integrity": "sha512-qKgsUwfHZV2WCWLAnVP1JqnpE6Im6h3Y0+fYgMTasNQ7V++CBX5OT1as0g0f+OyubbFqhf6XVNIsmN4IIhEgGQ==",
       "dev": true,
       "license": "MIT",
       "dependencies": {
-        "undici-types": "~6.13.0"
+        "undici-types": "~6.20.0"
       }
     },
     "node_modules/prettier": {
-      "version": "3.3.3",
-      "resolved": "https://registry.npmjs.org/prettier/-/prettier-3.3.3.tgz",
-      "integrity": "sha512-i2tDNA0O5IrMO757lfrdQZCc2jPNDVntV0m/+4whiDfWaTKfMNgR7Qz0NAeGz/nRqF4m5/6CLzbP4/liHt12Ew==",
+      "version": "3.4.2",
+      "resolved": "https://registry.npmjs.org/prettier/-/prettier-3.4.2.tgz",
+      "integrity": "sha512-e9MewbtFo+Fevyuxn/4rrcDAaq0IYxPGLvObpQjiZBMAzB9IGmzlnG9RZy3FFas+eBMu2vA0CszMeduow5dIuQ==",
       "dev": true,
       "license": "MIT",
       "bin": {
@@ -42,30 +42,26 @@
       }
     },
     "node_modules/prettier-plugin-organize-imports": {
-      "version": "4.0.0",
-      "resolved": "https://registry.npmjs.org/prettier-plugin-organize-imports/-/prettier-plugin-organize-imports-4.0.0.tgz",
-      "integrity": "sha512-vnKSdgv9aOlqKeEFGhf9SCBsTyzDSyScy1k7E0R1Uo4L0cTcOV7c1XQaT7jfXIOc/p08WLBfN2QUQA9zDSZMxA==",
+      "version": "4.1.0",
+      "resolved": "https://registry.npmjs.org/prettier-plugin-organize-imports/-/prettier-plugin-organize-imports-4.1.0.tgz",
+      "integrity": "sha512-5aWRdCgv645xaa58X8lOxzZoiHAldAPChljr/MT0crXVOWTZ+Svl4hIWlz+niYSlO6ikE5UXkN1JrRvIP2ut0A==",
       "dev": true,
       "license": "MIT",
       "peerDependencies": {
-        "@vue/language-plugin-pug": "^2.0.24",
         "prettier": ">=2.0",
         "typescript": ">=2.9",
-        "vue-tsc": "^2.0.24"
+        "vue-tsc": "^2.1.0"
       },
       "peerDependenciesMeta": {
-        "@vue/language-plugin-pug": {
-          "optional": true
-        },
         "vue-tsc": {
           "optional": true
         }
       }
     },
     "node_modules/typescript": {
-      "version": "5.5.4",
-      "resolved": "https://registry.npmjs.org/typescript/-/typescript-5.5.4.tgz",
-      "integrity": "sha512-Mtq29sKDAEYP7aljRgtPOpTvOfbwRWlS6dPRzwjdE+C0R4brX/GUyhHSecbHMFLNBLcJIPt9nl9yG5TZ1weH+Q==",
+      "version": "5.7.2",
+      "resolved": "https://registry.npmjs.org/typescript/-/typescript-5.7.2.tgz",
+      "integrity": "sha512-i5t66RHxDvVN40HfDd1PsEThGNnlMCMT3jMUuoh9/0TaqWevNontacunWyN02LA9/fIbEWlcHZcgTKb9QoaLfg==",
       "dev": true,
       "license": "Apache-2.0",
       "bin": {
@@ -77,9 +73,9 @@
       }
     },
     "node_modules/undici-types": {
-      "version": "6.13.0",
-      "resolved": "https://registry.npmjs.org/undici-types/-/undici-types-6.13.0.tgz",
-      "integrity": "sha512-xtFJHudx8S2DSoujjMd1WeWvn7KKWFRESZTMeL1RptAYERu29D6jphMjjY+vn96jvN3kVPDNxU/E13VTaXj6jg==",
+      "version": "6.20.0",
+      "resolved": "https://registry.npmjs.org/undici-types/-/undici-types-6.20.0.tgz",
+      "integrity": "sha512-Ny6QZ2Nju20vw1SRHe3d9jVu6gJ+4e3+MMpqu7pqE5HT6WsTSlce++GQmK5UXS8mzV8DSYHrQH+Xrf2jVcuKNg==",
       "dev": true,
       "license": "MIT"
     }
diff --git a/package.json b/package.json
index d3c7a06..3b7b821 100644
--- a/package.json
+++ b/package.json
@@ -18,10 +18,10 @@
     "format": "prettier src --write"
   },
   "devDependencies": {
-    "@types/node": "^22.1.0",
-    "prettier": "^3.3.3",
-    "prettier-plugin-organize-imports": "^4.0.0",
-    "typescript": "^5.5.4"
+    "@types/node": "^22.10.1",
+    "prettier": "^3.4.2",
+    "prettier-plugin-organize-imports": "^4.1.0",
+    "typescript": "^5.7.2"
   },
   "license": "GPL-3.0-or-later"
 }
diff --git a/src/mos/model/test/line.test.ts b/src/mos/model/test/line.test.ts
index 808c6b5..b5b32d7 100644
--- a/src/mos/model/test/line.test.ts
+++ b/src/mos/model/test/line.test.ts
@@ -1,6 +1,6 @@
 import assert from "node:assert"
 import { test } from "node:test"
-import { line } from "../line.js"
+import { line } from "../line.ts"
 
 test("mos -- line", () => {
   assert.deepEqual(line(1)([2, 3]), 5)
diff --git a/src/mos/tensor/operator.ts b/src/mos/tensor/operator.ts
index 9c9d665..25adb12 100644
--- a/src/mos/tensor/operator.ts
+++ b/src/mos/tensor/operator.ts
@@ -1,4 +1,4 @@
-import { assertTensor1, isScalar, isTensor1, type Tensor } from "./tensor.js"
+import { assertTensor1, isScalar, isTensor1, type Tensor } from "./tensor.ts"
 
 export function sum1(t: Tensor): number {
   assertTensor1(t)
diff --git a/src/mos/tensor/test/operator.test.ts b/src/mos/tensor/test/operator.test.ts
index 7ca9654..5fabf04 100644
--- a/src/mos/tensor/test/operator.test.ts
+++ b/src/mos/tensor/test/operator.test.ts
@@ -1,6 +1,6 @@
 import assert from "node:assert"
 import test from "node:test"
-import { sum, sum1 } from "../operator.js"
+import { sum, sum1 } from "../operator.ts"
 
 test("mos -- sum1", () => {
   assert.equal(sum1([1, 2, 3, 4, 5, 6]), 21)
diff --git a/src/mos/tensor/test/tensor.test.ts b/src/mos/tensor/test/tensor.test.ts
index db7c499..ceac501 100644
--- a/src/mos/tensor/test/tensor.test.ts
+++ b/src/mos/tensor/test/tensor.test.ts
@@ -1,6 +1,6 @@
 import assert from "node:assert"
 import test from "node:test"
-import { rank, shape } from "../tensor.js"
+import { rank, shape } from "../tensor.ts"
 
 test("mos -- shape", () => {
   assert.deepEqual(shape(1), [])
diff --git a/src/system-a/block/Block.ts b/src/system-a/block/Block.ts
index 7b15d56..a16a12b 100644
--- a/src/system-a/block/Block.ts
+++ b/src/system-a/block/Block.ts
@@ -1,4 +1,4 @@
-import type { Shape, Tensor } from "../tensor/index.js"
+import type { Shape, Tensor } from "../tensor/index.ts"
 
 export type BlockFn = (t: Tensor) => (...ps: Array<Tensor>) => Tensor
 
diff --git a/src/system-a/block/blockStack.ts b/src/system-a/block/blockStack.ts
index ebe82e2..7cd937e 100644
--- a/src/system-a/block/blockStack.ts
+++ b/src/system-a/block/blockStack.ts
@@ -1,6 +1,6 @@
-import { arrayAppend } from "../../utils/arrayAppend.js"
-import { Block, type BlockFn } from "./Block.js"
-import { emptyBlock } from "./emptyBlock.js"
+import { arrayAppend } from "../../utils/arrayAppend.ts"
+import { Block, type BlockFn } from "./Block.ts"
+import { emptyBlock } from "./emptyBlock.ts"
 
 // NOTE Be careful about the order of applications,
 // `f` first, then `g`.
diff --git a/src/system-a/block/emptyBlock.ts b/src/system-a/block/emptyBlock.ts
index cb2ffa0..566a4af 100644
--- a/src/system-a/block/emptyBlock.ts
+++ b/src/system-a/block/emptyBlock.ts
@@ -1,4 +1,4 @@
-import { Block } from "./Block.js"
+import { Block } from "./Block.ts"
 
 export const emptyBlock = Block(
   (t) =>
diff --git a/src/system-a/block/index.ts b/src/system-a/block/index.ts
index 7bc5023..6767c98 100644
--- a/src/system-a/block/index.ts
+++ b/src/system-a/block/index.ts
@@ -1,3 +1,3 @@
-export * from "./Block.js"
-export * from "./blockStack.js"
-export * from "./emptyBlock.js"
+export * from "./Block.ts"
+export * from "./blockStack.ts"
+export * from "./emptyBlock.ts"
diff --git a/src/system-a/gradient-descent/Representation.ts b/src/system-a/gradient-descent/Representation.ts
index 55b9fb8..4cb5cc2 100644
--- a/src/system-a/gradient-descent/Representation.ts
+++ b/src/system-a/gradient-descent/Representation.ts
@@ -1,4 +1,4 @@
-import type { Tensor } from "../tensor/index.js"
+import type { Tensor } from "../tensor/index.ts"
 
 export interface Representation<R> {
   inflate: (p: Tensor) => R
diff --git a/src/system-a/gradient-descent/gradientDescent.ts b/src/system-a/gradient-descent/gradientDescent.ts
index f93e94b..2ac8f6f 100644
--- a/src/system-a/gradient-descent/gradientDescent.ts
+++ b/src/system-a/gradient-descent/gradientDescent.ts
@@ -1,7 +1,7 @@
-import { zip } from "../../utils/zip.js"
-import { gradient } from "../gradient/index.js"
-import { assertTensorArray, type Tensor } from "../tensor/index.js"
-import type { Representation } from "./Representation.js"
+import { zip } from "../../utils/zip.ts"
+import { gradient } from "../gradient/index.ts"
+import { assertTensorArray, type Tensor } from "../tensor/index.ts"
+import type { Representation } from "./Representation.ts"
 
 export type GradientDescentFn = (
   objective: (...ps: Array<Tensor>) => Tensor,
diff --git a/src/system-a/gradient-descent/gradientDescentAdam.ts b/src/system-a/gradient-descent/gradientDescentAdam.ts
index 0997d31..9f5bb4e 100644
--- a/src/system-a/gradient-descent/gradientDescentAdam.ts
+++ b/src/system-a/gradient-descent/gradientDescentAdam.ts
@@ -1,8 +1,8 @@
-import { tensorZeros, type Tensor } from "../tensor/index.js"
-import { add, div, mul, square, squareRoot, sub } from "../toys/index.js"
-import type { Representation } from "./Representation.js"
-import { gradientDescent } from "./gradientDescent.js"
-import { smooth } from "./smooth.js"
+import { tensorZeros, type Tensor } from "../tensor/index.ts"
+import { add, div, mul, square, squareRoot, sub } from "../toys/index.ts"
+import type { Representation } from "./Representation.ts"
+import { gradientDescent } from "./gradientDescent.ts"
+import { smooth } from "./smooth.ts"
 
 const stabilizer = 1e-8
 
diff --git a/src/system-a/gradient-descent/gradientDescentLonely.ts b/src/system-a/gradient-descent/gradientDescentLonely.ts
index c24f8a8..acbe055 100644
--- a/src/system-a/gradient-descent/gradientDescentLonely.ts
+++ b/src/system-a/gradient-descent/gradientDescentLonely.ts
@@ -1,7 +1,7 @@
-import type { Tensor } from "../tensor/index.js"
-import { mul, sub } from "../toys/index.js"
-import type { Representation } from "./Representation.js"
-import { gradientDescent } from "./gradientDescent.js"
+import type { Tensor } from "../tensor/index.ts"
+import { mul, sub } from "../toys/index.ts"
+import type { Representation } from "./Representation.ts"
+import { gradientDescent } from "./gradientDescent.ts"
 
 export function lonelyRepresentation(options: {
   learningRate: number
diff --git a/src/system-a/gradient-descent/gradientDescentNaked.ts b/src/system-a/gradient-descent/gradientDescentNaked.ts
index 4cf218d..1f5588c 100644
--- a/src/system-a/gradient-descent/gradientDescentNaked.ts
+++ b/src/system-a/gradient-descent/gradientDescentNaked.ts
@@ -1,7 +1,7 @@
-import type { Tensor } from "../tensor/index.js"
-import { mul, sub } from "../toys/index.js"
-import type { Representation } from "./Representation.js"
-import { gradientDescent } from "./gradientDescent.js"
+import type { Tensor } from "../tensor/index.ts"
+import { mul, sub } from "../toys/index.ts"
+import type { Representation } from "./Representation.ts"
+import { gradientDescent } from "./gradientDescent.ts"
 
 export function nakedRepresentation(options: {
   learningRate: number
diff --git a/src/system-a/gradient-descent/gradientDescentRms.ts b/src/system-a/gradient-descent/gradientDescentRms.ts
index c36251d..2876bff 100644
--- a/src/system-a/gradient-descent/gradientDescentRms.ts
+++ b/src/system-a/gradient-descent/gradientDescentRms.ts
@@ -1,8 +1,8 @@
-import { tensorZeros, type Tensor } from "../tensor/index.js"
-import { add, div, mul, square, squareRoot, sub } from "../toys/index.js"
-import type { Representation } from "./Representation.js"
-import { gradientDescent } from "./gradientDescent.js"
-import { smooth } from "./smooth.js"
+import { tensorZeros, type Tensor } from "../tensor/index.ts"
+import { add, div, mul, square, squareRoot, sub } from "../toys/index.ts"
+import type { Representation } from "./Representation.ts"
+import { gradientDescent } from "./gradientDescent.ts"
+import { smooth } from "./smooth.ts"
 
 const stabilizer = 1e-8
 
diff --git a/src/system-a/gradient-descent/gradientDescentVelocity.ts b/src/system-a/gradient-descent/gradientDescentVelocity.ts
index 73ef47e..106af66 100644
--- a/src/system-a/gradient-descent/gradientDescentVelocity.ts
+++ b/src/system-a/gradient-descent/gradientDescentVelocity.ts
@@ -1,7 +1,7 @@
-import { tensorZeros, type Tensor } from "../tensor/index.js"
-import { add, mul, sub } from "../toys/index.js"
-import type { Representation } from "./Representation.js"
-import { gradientDescent } from "./gradientDescent.js"
+import { tensorZeros, type Tensor } from "../tensor/index.ts"
+import { add, mul, sub } from "../toys/index.ts"
+import type { Representation } from "./Representation.ts"
+import { gradientDescent } from "./gradientDescent.ts"
 
 export function velocityRepresentation(options: {
   learningRate: number
diff --git a/src/system-a/gradient-descent/index.ts b/src/system-a/gradient-descent/index.ts
index fae4abd..c6352f3 100644
--- a/src/system-a/gradient-descent/index.ts
+++ b/src/system-a/gradient-descent/index.ts
@@ -1 +1 @@
-export * from "./gradientDescent.js"
+export * from "./gradientDescent.ts"
diff --git a/src/system-a/gradient-descent/smooth.ts b/src/system-a/gradient-descent/smooth.ts
index 892a5d5..c199988 100644
--- a/src/system-a/gradient-descent/smooth.ts
+++ b/src/system-a/gradient-descent/smooth.ts
@@ -1,5 +1,5 @@
-import type { Tensor } from "../tensor/index.js"
-import { add, mul } from "../toys/index.js"
+import type { Tensor } from "../tensor/index.ts"
+import { add, mul } from "../toys/index.ts"
 
 export function smooth(decayRate: number, average: Tensor, g: Tensor): Tensor {
   return add(mul(decayRate, average), mul(1 - decayRate, g))
diff --git a/src/system-a/gradient/GradientState.ts b/src/system-a/gradient/GradientState.ts
index e14d487..364ac56 100644
--- a/src/system-a/gradient/GradientState.ts
+++ b/src/system-a/gradient/GradientState.ts
@@ -1,4 +1,4 @@
-import { type Scalar } from "../tensor/index.js"
+import { type Scalar } from "../tensor/index.ts"
 
 export type GradientState = Map<Scalar, number>
 
diff --git a/src/system-a/gradient/gradient.test.ts b/src/system-a/gradient/gradient.test.ts
index aa53fef..4926b04 100644
--- a/src/system-a/gradient/gradient.test.ts
+++ b/src/system-a/gradient/gradient.test.ts
@@ -1,7 +1,7 @@
 import assert from "node:assert"
 import { test } from "node:test"
-import { add, mul, sum } from "../toys/index.js"
-import { gradient } from "./gradient.js"
+import { add, mul, sum } from "../toys/index.ts"
+import { gradient } from "./gradient.ts"
 
 test("gradient -- add", () => {
   assert.deepStrictEqual(gradient(add, [1, 1]), [1, 1])
diff --git a/src/system-a/gradient/gradient.ts b/src/system-a/gradient/gradient.ts
index 56310b8..fdc3d49 100644
--- a/src/system-a/gradient/gradient.ts
+++ b/src/system-a/gradient/gradient.ts
@@ -4,12 +4,12 @@ import {
   scalarTruncate,
   tensorMap,
   type Tensor,
-} from "../tensor/index.js"
+} from "../tensor/index.ts"
 import {
   emptyGradientState,
   gradientStateGetWithDefault,
   type GradientState,
-} from "./GradientState.js"
+} from "./GradientState.ts"
 
 // The effect of `gradient` on a `DifferentiableFn`
 // is `sum` of all elements of it's result tensor.
diff --git a/src/system-a/gradient/index.ts b/src/system-a/gradient/index.ts
index 56111e3..cd6273c 100644
--- a/src/system-a/gradient/index.ts
+++ b/src/system-a/gradient/index.ts
@@ -1,2 +1,2 @@
-export * from "./gradient.js"
-export * from "./GradientState.js"
+export * from "./gradient.ts"
+export * from "./GradientState.ts"
diff --git a/src/system-a/loss/index.ts b/src/system-a/loss/index.ts
index b269dde..f6fa6d2 100644
--- a/src/system-a/loss/index.ts
+++ b/src/system-a/loss/index.ts
@@ -1 +1 @@
-export * from "./l2Loss.js"
+export * from "./l2Loss.ts"
diff --git a/src/system-a/loss/l2Loss.ts b/src/system-a/loss/l2Loss.ts
index 24a2449..7f9d3bc 100644
--- a/src/system-a/loss/l2Loss.ts
+++ b/src/system-a/loss/l2Loss.ts
@@ -1,5 +1,5 @@
-import { type Tensor } from "../tensor/index.js"
-import { square, sub, sum } from "../toys/index.js"
+import { type Tensor } from "../tensor/index.ts"
+import { square, sub, sum } from "../toys/index.ts"
 
 export type Target = (xs: Tensor) => (...ps: Array<Tensor>) => Tensor
 export type Expectant = (xs: Tensor, ys: Tensor) => Objective
diff --git a/src/system-a/models/argmax.test.ts b/src/system-a/models/argmax.test.ts
index 78d30e7..683a4c2 100644
--- a/src/system-a/models/argmax.test.ts
+++ b/src/system-a/models/argmax.test.ts
@@ -1,6 +1,6 @@
 import assert from "node:assert"
 import { test } from "node:test"
-import { argmax } from "./argmax.js"
+import { argmax } from "./argmax.ts"
 
 test("argmax", () => {
   assert.deepStrictEqual(argmax([1, 2, 3]), 2)
diff --git a/src/system-a/models/argmax.ts b/src/system-a/models/argmax.ts
index 3e0578c..9c38628 100644
--- a/src/system-a/models/argmax.ts
+++ b/src/system-a/models/argmax.ts
@@ -1,4 +1,4 @@
-import { scalarReal, type Scalar } from "../tensor/Scalar.js"
+import { scalarReal, type Scalar } from "../tensor/Scalar.ts"
 
 // NOTE `argmax` is only meaningful for `Tensor1`,
 // for general nested `Tensor` a list of indexed
diff --git a/src/system-a/models/compareClassification.test.ts b/src/system-a/models/compareClassification.test.ts
index 12717a2..ead2ddd 100644
--- a/src/system-a/models/compareClassification.test.ts
+++ b/src/system-a/models/compareClassification.test.ts
@@ -1,6 +1,6 @@
 import assert from "node:assert"
 import { test } from "node:test"
-import { compareClassification } from "./compareClassification.js"
+import { compareClassification } from "./compareClassification.ts"
 
 test("compareClassification", () => {
   assert.deepStrictEqual(compareClassification([1, 2, 3], [10, 20, 30]), 1)
diff --git a/src/system-a/models/compareClassification.ts b/src/system-a/models/compareClassification.ts
index 30a3773..16cf813 100644
--- a/src/system-a/models/compareClassification.ts
+++ b/src/system-a/models/compareClassification.ts
@@ -1,6 +1,6 @@
-import type { Scalar } from "../tensor/Scalar.js"
-import { extend2 } from "../toys/extend.js"
-import { argmax } from "./argmax.js"
+import type { Scalar } from "../tensor/Scalar.ts"
+import { extend2 } from "../toys/extend.ts"
+import { argmax } from "./argmax.ts"
 
 export type Classification = Array<Scalar>
 
diff --git a/src/system-a/models/iris/irisModel.test.ts b/src/system-a/models/iris/irisModel.test.ts
index 3af73b3..88a4d48 100644
--- a/src/system-a/models/iris/irisModel.test.ts
+++ b/src/system-a/models/iris/irisModel.test.ts
@@ -1,7 +1,7 @@
 import { test } from "node:test"
-import { modelAccuracy } from "../modelAccuracy.js"
-import { irisTestXs, irisTestYs } from "./irisDataset.js"
-import { irisModel } from "./irisModel.js"
+import { modelAccuracy } from "../modelAccuracy.ts"
+import { irisTestXs, irisTestYs } from "./irisDataset.ts"
+import { irisModel } from "./irisModel.ts"
 
 test("irisModel", () => {
   // console.log(irisModel(irisTestXs))
diff --git a/src/system-a/models/iris/irisModel.ts b/src/system-a/models/iris/irisModel.ts
index 7a18881..106164c 100644
--- a/src/system-a/models/iris/irisModel.ts
+++ b/src/system-a/models/iris/irisModel.ts
@@ -1,16 +1,16 @@
-import { blockStack } from "../../block/index.js"
-import { gradientDescentAdam } from "../../gradient-descent/gradientDescentAdam.js"
-import { gradientDescentNaked } from "../../gradient-descent/gradientDescentNaked.js"
-import { gradientDescentRms } from "../../gradient-descent/gradientDescentRms.js"
-import { l2Loss } from "../../loss/index.js"
+import { blockStack } from "../../block/index.ts"
+import { gradientDescentAdam } from "../../gradient-descent/gradientDescentAdam.ts"
+import { gradientDescentNaked } from "../../gradient-descent/gradientDescentNaked.ts"
+import { gradientDescentRms } from "../../gradient-descent/gradientDescentRms.ts"
+import { l2Loss } from "../../loss/index.ts"
 import {
   denseBlock,
   denseBlockInitParameters,
-} from "../../perceptrons/relu/index.js"
-import type { Tensor } from "../../tensor/Tensor.js"
-import { samplingObjective } from "../../tensor/samplingObjective.js"
-import { model } from "../model.js"
-import { irisTrainXs, irisTrainYs } from "./irisDataset.js"
+} from "../../perceptrons/relu/index.ts"
+import type { Tensor } from "../../tensor/Tensor.ts"
+import { samplingObjective } from "../../tensor/samplingObjective.ts"
+import { model } from "../model.ts"
+import { irisTrainXs, irisTrainYs } from "./irisDataset.ts"
 
 // export const irisNetwork = blockStack([
 //   denseBlock(4, 6),
diff --git a/src/system-a/models/model.ts b/src/system-a/models/model.ts
index 8992ea5..a876f0b 100644
--- a/src/system-a/models/model.ts
+++ b/src/system-a/models/model.ts
@@ -1,5 +1,5 @@
-import type { Target } from "../loss/index.js"
-import type { Tensor } from "../tensor/Tensor.js"
+import type { Target } from "../loss/index.ts"
+import type { Tensor } from "../tensor/Tensor.ts"
 
 export type Model = (t: Tensor) => Tensor
 
diff --git a/src/system-a/models/modelAccuracy.ts b/src/system-a/models/modelAccuracy.ts
index 022a600..46e5d7d 100644
--- a/src/system-a/models/modelAccuracy.ts
+++ b/src/system-a/models/modelAccuracy.ts
@@ -1,9 +1,9 @@
-import type { Tensor } from "../tensor/Tensor.js"
-import { assertTensorArray } from "../tensor/assertions.js"
-import { sum } from "../toys/sum.js"
-import { div } from "../toys/toys.js"
-import { compareClassification } from "./compareClassification.js"
-import type { Model } from "./model.js"
+import type { Tensor } from "../tensor/Tensor.ts"
+import { assertTensorArray } from "../tensor/assertions.ts"
+import { sum } from "../toys/sum.ts"
+import { div } from "../toys/toys.ts"
+import { compareClassification } from "./compareClassification.ts"
+import type { Model } from "./model.ts"
 
 export function modelAccuracy(model: Model, xs: Tensor, ys: Tensor): Tensor {
   return accuracy(model(xs), ys)
diff --git a/src/system-a/perceptrons/relu/denseBlock.ts b/src/system-a/perceptrons/relu/denseBlock.ts
index a610551..1f7a66d 100644
--- a/src/system-a/perceptrons/relu/denseBlock.ts
+++ b/src/system-a/perceptrons/relu/denseBlock.ts
@@ -1,9 +1,9 @@
-import { Block } from "../../block/index.js"
-import type { Tensor } from "../../tensor/Tensor.js"
-import { randomTensor } from "../../tensor/randomTensor.js"
-import type { Shape } from "../../tensor/shape.js"
-import { zeroTensor } from "../../tensor/zeroTensor.js"
-import { leakyRelu } from "./leakyRelu.js"
+import { Block } from "../../block/index.ts"
+import type { Tensor } from "../../tensor/Tensor.ts"
+import { randomTensor } from "../../tensor/randomTensor.ts"
+import type { Shape } from "../../tensor/shape.ts"
+import { zeroTensor } from "../../tensor/zeroTensor.ts"
+import { leakyRelu } from "./leakyRelu.ts"
 
 export function denseBlock(inputSize: number, layerWidth: number): Block {
   return Block(leakyRelu, [[layerWidth, inputSize], [layerWidth]])
diff --git a/src/system-a/perceptrons/relu/index.ts b/src/system-a/perceptrons/relu/index.ts
index ea15e5d..783cfb5 100644
--- a/src/system-a/perceptrons/relu/index.ts
+++ b/src/system-a/perceptrons/relu/index.ts
@@ -1,3 +1,3 @@
-export * from "./denseBlock.js"
-export * from "./leakyRelu.js"
-export * from "./relu.js"
+export * from "./denseBlock.ts"
+export * from "./leakyRelu.ts"
+export * from "./relu.ts"
diff --git a/src/system-a/perceptrons/relu/leakyRelu.ts b/src/system-a/perceptrons/relu/leakyRelu.ts
index 23b863f..be3dcdf 100644
--- a/src/system-a/perceptrons/relu/leakyRelu.ts
+++ b/src/system-a/perceptrons/relu/leakyRelu.ts
@@ -1,6 +1,6 @@
-import type { Scalar, Tensor } from "../../tensor/index.js"
-import { extend1, lt, mul } from "../../toys/index.js"
-import { linear } from "./linear.js"
+import type { Scalar, Tensor } from "../../tensor/index.ts"
+import { extend1, lt, mul } from "../../toys/index.ts"
+import { linear } from "./linear.ts"
 
 export function leakyRectifyScalar(s: Scalar): Scalar {
   return lt(s, 0) ? (mul(-0.01, s) as Scalar) : s
diff --git a/src/system-a/perceptrons/relu/linear.test.ts b/src/system-a/perceptrons/relu/linear.test.ts
index 62fae23..e163a1b 100644
--- a/src/system-a/perceptrons/relu/linear.test.ts
+++ b/src/system-a/perceptrons/relu/linear.test.ts
@@ -1,6 +1,6 @@
 import assert from "node:assert"
 import { test } from "node:test"
-import { linear, linearWrong } from "./linear.js"
+import { linear, linearWrong } from "./linear.ts"
 
 test("linear -- matrixVactorMul is necessary", () => {
   assert.deepStrictEqual(
diff --git a/src/system-a/perceptrons/relu/linear.ts b/src/system-a/perceptrons/relu/linear.ts
index 120ad36..852c2c8 100644
--- a/src/system-a/perceptrons/relu/linear.ts
+++ b/src/system-a/perceptrons/relu/linear.ts
@@ -1,5 +1,5 @@
-import type { Tensor } from "../../tensor/index.js"
-import { add, dot, extend2, mul, sum } from "../../toys/index.js"
+import type { Tensor } from "../../tensor/index.ts"
+import { add, dot, extend2, mul, sum } from "../../toys/index.ts"
 
 const mul21 = extend2(mul, 2, 1)
 
diff --git a/src/system-a/perceptrons/relu/multiLayerRelu.ts b/src/system-a/perceptrons/relu/multiLayerRelu.ts
index 91beb42..840dc1f 100644
--- a/src/system-a/perceptrons/relu/multiLayerRelu.ts
+++ b/src/system-a/perceptrons/relu/multiLayerRelu.ts
@@ -1,5 +1,5 @@
-import type { Tensor } from "../../tensor/Tensor.js"
-import { relu } from "./relu.js"
+import type { Tensor } from "../../tensor/Tensor.ts"
+import { relu } from "./relu.ts"
 
 type Target = (t: Tensor) => (...ps: Array<Tensor>) => Tensor
 
diff --git a/src/system-a/perceptrons/relu/relu.test.ts b/src/system-a/perceptrons/relu/relu.test.ts
index 3151cc3..c9d5bec 100644
--- a/src/system-a/perceptrons/relu/relu.test.ts
+++ b/src/system-a/perceptrons/relu/relu.test.ts
@@ -1,7 +1,7 @@
 import assert from "node:assert"
 import { test } from "node:test"
-import { assertTensorAlmostEqual } from "../../tensor/assertions.js"
-import { rectify, relu } from "./relu.js"
+import { assertTensorAlmostEqual } from "../../tensor/assertions.ts"
+import { rectify, relu } from "./relu.ts"
 
 test("rectify", () => {
   assert.deepStrictEqual(
diff --git a/src/system-a/perceptrons/relu/relu.ts b/src/system-a/perceptrons/relu/relu.ts
index 57da84b..3bf138f 100644
--- a/src/system-a/perceptrons/relu/relu.ts
+++ b/src/system-a/perceptrons/relu/relu.ts
@@ -1,6 +1,6 @@
-import type { Scalar, Tensor } from "../../tensor/index.js"
-import { extend1, lt } from "../../toys/index.js"
-import { linear } from "./linear.js"
+import type { Scalar, Tensor } from "../../tensor/index.ts"
+import { extend1, lt } from "../../toys/index.ts"
+import { linear } from "./linear.ts"
 
 export function rectifyScalar(s: Scalar): Scalar {
   return lt(s, 0) ? 0 : s
diff --git a/src/system-a/targets/line.test.ts b/src/system-a/targets/line.test.ts
index 48ae395..d06fcfa 100644
--- a/src/system-a/targets/line.test.ts
+++ b/src/system-a/targets/line.test.ts
@@ -1,13 +1,13 @@
 import assert from "assert"
 import { test } from "node:test"
-import { gradientDescentLonely } from "../gradient-descent/gradientDescentLonely.js"
-import { gradientDescentNaked } from "../gradient-descent/gradientDescentNaked.js"
-import { gradientDescentVelocity } from "../gradient-descent/gradientDescentVelocity.js"
-import type { GradientDescentFn } from "../gradient-descent/index.js"
-import { l2Loss } from "../loss/index.js"
-import { assertTensorAlmostEqual } from "../tensor/index.js"
-import { samplingObjective } from "../tensor/samplingObjective.js"
-import { line } from "./line.js"
+import { gradientDescentLonely } from "../gradient-descent/gradientDescentLonely.ts"
+import { gradientDescentNaked } from "../gradient-descent/gradientDescentNaked.ts"
+import { gradientDescentVelocity } from "../gradient-descent/gradientDescentVelocity.ts"
+import type { GradientDescentFn } from "../gradient-descent/index.ts"
+import { l2Loss } from "../loss/index.ts"
+import { assertTensorAlmostEqual } from "../tensor/index.ts"
+import { samplingObjective } from "../tensor/samplingObjective.ts"
+import { line } from "./line.ts"
 
 test("line", () => {
   assert.deepStrictEqual(line(1)(2, 3), 5)
diff --git a/src/system-a/targets/line.ts b/src/system-a/targets/line.ts
index 3c6e97e..8218cfa 100644
--- a/src/system-a/targets/line.ts
+++ b/src/system-a/targets/line.ts
@@ -1,5 +1,5 @@
-import { type Tensor } from "../tensor/index.js"
-import { add, mul } from "../toys/index.js"
+import { type Tensor } from "../tensor/index.ts"
+import { add, mul } from "../toys/index.ts"
 
 export function line(x: Tensor): (...ps: [Tensor, Tensor]) => Tensor {
   return (ps0, ps1) => add(mul(ps0, x), ps1)
diff --git a/src/system-a/targets/plane.test.ts b/src/system-a/targets/plane.test.ts
index 34fa6c5..92d9581 100644
--- a/src/system-a/targets/plane.test.ts
+++ b/src/system-a/targets/plane.test.ts
@@ -1,15 +1,15 @@
 import assert from "node:assert"
 import { test } from "node:test"
-import { gradientDescentAdam } from "../gradient-descent/gradientDescentAdam.js"
-import { gradientDescentLonely } from "../gradient-descent/gradientDescentLonely.js"
-import { gradientDescentNaked } from "../gradient-descent/gradientDescentNaked.js"
-import { gradientDescentRms } from "../gradient-descent/gradientDescentRms.js"
-import { gradientDescentVelocity } from "../gradient-descent/gradientDescentVelocity.js"
-import type { GradientDescentFn } from "../gradient-descent/index.js"
-import { l2Loss } from "../loss/index.js"
-import { assertTensorAlmostEqual } from "../tensor/index.js"
-import { samplingObjective } from "../tensor/samplingObjective.js"
-import { plane } from "./plane.js"
+import { gradientDescentAdam } from "../gradient-descent/gradientDescentAdam.ts"
+import { gradientDescentLonely } from "../gradient-descent/gradientDescentLonely.ts"
+import { gradientDescentNaked } from "../gradient-descent/gradientDescentNaked.ts"
+import { gradientDescentRms } from "../gradient-descent/gradientDescentRms.ts"
+import { gradientDescentVelocity } from "../gradient-descent/gradientDescentVelocity.ts"
+import type { GradientDescentFn } from "../gradient-descent/index.ts"
+import { l2Loss } from "../loss/index.ts"
+import { assertTensorAlmostEqual } from "../tensor/index.ts"
+import { samplingObjective } from "../tensor/samplingObjective.ts"
+import { plane } from "./plane.ts"
 
 test("plane", () => {
   assert.deepStrictEqual(plane([1, 3])([1, 2], 3), 10)
diff --git a/src/system-a/targets/plane.ts b/src/system-a/targets/plane.ts
index 10d76a7..ec375f0 100644
--- a/src/system-a/targets/plane.ts
+++ b/src/system-a/targets/plane.ts
@@ -1,5 +1,5 @@
-import { type Tensor } from "../tensor/index.js"
-import { add, dot } from "../toys/index.js"
+import { type Tensor } from "../tensor/index.ts"
+import { add, dot } from "../toys/index.ts"
 
 export function plane(t: Tensor): (...ps: [Tensor, Tensor]) => Tensor {
   return (ps0, ps1) => add(dot(ps0, t), ps1)
diff --git a/src/system-a/targets/quad.test.ts b/src/system-a/targets/quad.test.ts
index d0845c5..dcc2340 100644
--- a/src/system-a/targets/quad.test.ts
+++ b/src/system-a/targets/quad.test.ts
@@ -1,13 +1,13 @@
 import assert from "node:assert"
 import { test } from "node:test"
-import type { GradientDescentFn } from "../gradient-descent/gradientDescent.js"
-import { gradientDescentLonely } from "../gradient-descent/gradientDescentLonely.js"
-import { gradientDescentNaked } from "../gradient-descent/gradientDescentNaked.js"
-import { gradientDescentVelocity } from "../gradient-descent/gradientDescentVelocity.js"
-import { l2Loss } from "../loss/index.js"
-import { assertTensorAlmostEqual } from "../tensor/index.js"
-import { samplingObjective } from "../tensor/samplingObjective.js"
-import { quad } from "./quad.js"
+import type { GradientDescentFn } from "../gradient-descent/gradientDescent.ts"
+import { gradientDescentLonely } from "../gradient-descent/gradientDescentLonely.ts"
+import { gradientDescentNaked } from "../gradient-descent/gradientDescentNaked.ts"
+import { gradientDescentVelocity } from "../gradient-descent/gradientDescentVelocity.ts"
+import { l2Loss } from "../loss/index.ts"
+import { assertTensorAlmostEqual } from "../tensor/index.ts"
+import { samplingObjective } from "../tensor/samplingObjective.ts"
+import { quad } from "./quad.ts"
 
 test("quad", () => {
   assert.deepStrictEqual(quad(3)(4.5, 2.1, 7.8), 54.6)
diff --git a/src/system-a/targets/quad.ts b/src/system-a/targets/quad.ts
index 88d0a7b..ead78bb 100644
--- a/src/system-a/targets/quad.ts
+++ b/src/system-a/targets/quad.ts
@@ -1,5 +1,5 @@
-import { type Tensor } from "../tensor/index.js"
-import { add, mul, square } from "../toys/index.js"
+import { type Tensor } from "../tensor/index.ts"
+import { add, mul, square } from "../toys/index.ts"
 
 export function quad(x: Tensor): (...ps: [Tensor, Tensor, Tensor]) => Tensor {
   return (ps0, ps1, ps2) => add(mul(ps0, square(x)), add(mul(ps1, x), ps2))
diff --git a/src/system-a/tensor/Link.ts b/src/system-a/tensor/Link.ts
index 7648190..f5366f9 100644
--- a/src/system-a/tensor/Link.ts
+++ b/src/system-a/tensor/Link.ts
@@ -2,8 +2,8 @@ import {
   gradientStateGetWithDefault,
   gradientStateSet,
   type GradientState,
-} from "../gradient/index.js"
-import { type Scalar } from "./Scalar.js"
+} from "../gradient/index.ts"
+import { type Scalar } from "./Scalar.ts"
 
 export type Link = (
   y: Scalar,
diff --git a/src/system-a/tensor/Scalar.ts b/src/system-a/tensor/Scalar.ts
index 0a8902f..f918de1 100644
--- a/src/system-a/tensor/Scalar.ts
+++ b/src/system-a/tensor/Scalar.ts
@@ -1,4 +1,4 @@
-import { endOfChain, type Link } from "./Link.js"
+import { endOfChain, type Link } from "./Link.ts"
 
 export type Dual = { "@type": "Dual"; real: number; link: Link }
 
diff --git a/src/system-a/tensor/Tensor.ts b/src/system-a/tensor/Tensor.ts
index 4aef6e2..acf6d81 100644
--- a/src/system-a/tensor/Tensor.ts
+++ b/src/system-a/tensor/Tensor.ts
@@ -1,4 +1,4 @@
-import { isScalar, type Scalar } from "./Scalar.js"
+import { isScalar, type Scalar } from "./Scalar.ts"
 
 export type Tensor = Scalar | Array<Tensor>
 
diff --git a/src/system-a/tensor/assertions.ts b/src/system-a/tensor/assertions.ts
index d3861af..65be6d6 100644
--- a/src/system-a/tensor/assertions.ts
+++ b/src/system-a/tensor/assertions.ts
@@ -1,6 +1,6 @@
-import { isScalar, type Scalar } from "./Scalar.js"
-import { isTensor, type Tensor } from "./Tensor.js"
-import { tensorAlmostEqual } from "./tensorAlmostEqual.js"
+import { isScalar, type Scalar } from "./Scalar.ts"
+import { isTensor, type Tensor } from "./Tensor.ts"
+import { tensorAlmostEqual } from "./tensorAlmostEqual.ts"
 
 export function assertScalar(t: any): asserts t is Scalar {
   if (!isScalar(t)) {
diff --git a/src/system-a/tensor/index.ts b/src/system-a/tensor/index.ts
index 7077179..b60557b 100644
--- a/src/system-a/tensor/index.ts
+++ b/src/system-a/tensor/index.ts
@@ -1,13 +1,13 @@
-export * from "./assertions.js"
-export * from "./Link.js"
-export * from "./randomTensor.js"
-export * from "./rank.js"
-export * from "./Scalar.js"
-export * from "./shape.js"
-export * from "./Tensor.js"
-export * from "./tensorAlmostEqual.js"
-export * from "./tensorEvery.js"
-export * from "./tensorMap.js"
-export * from "./tensorReal.js"
-export * from "./tensorZeros.js"
-export * from "./zeroTensor.js"
+export * from "./assertions.ts"
+export * from "./Link.ts"
+export * from "./randomTensor.ts"
+export * from "./rank.ts"
+export * from "./Scalar.ts"
+export * from "./shape.ts"
+export * from "./Tensor.ts"
+export * from "./tensorAlmostEqual.ts"
+export * from "./tensorEvery.ts"
+export * from "./tensorMap.ts"
+export * from "./tensorReal.ts"
+export * from "./tensorZeros.ts"
+export * from "./zeroTensor.ts"
diff --git a/src/system-a/tensor/randomTensor.ts b/src/system-a/tensor/randomTensor.ts
index 56584a0..1c8ad96 100644
--- a/src/system-a/tensor/randomTensor.ts
+++ b/src/system-a/tensor/randomTensor.ts
@@ -1,7 +1,7 @@
-import { randomGaussian } from "../../utils/randomGaussian.js"
-import { repeatApply } from "../../utils/repeatApply.js"
-import type { Tensor } from "./Tensor.js"
-import type { Shape } from "./shape.js"
+import { randomGaussian } from "../../utils/randomGaussian.ts"
+import { repeatApply } from "../../utils/repeatApply.ts"
+import type { Tensor } from "./Tensor.ts"
+import type { Shape } from "./shape.ts"
 
 export function randomTensor(
   mean: number,
diff --git a/src/system-a/tensor/rank.ts b/src/system-a/tensor/rank.ts
index ee5b923..e7ce017 100644
--- a/src/system-a/tensor/rank.ts
+++ b/src/system-a/tensor/rank.ts
@@ -1,5 +1,5 @@
-import { type Tensor } from "./Tensor.js"
-import { shape } from "./shape.js"
+import { type Tensor } from "./Tensor.ts"
+import { shape } from "./shape.ts"
 
 export function rank(t: Tensor): number {
   return shape(t).length
diff --git a/src/system-a/tensor/samplingObjective.ts b/src/system-a/tensor/samplingObjective.ts
index 77d7f23..5677bbc 100644
--- a/src/system-a/tensor/samplingObjective.ts
+++ b/src/system-a/tensor/samplingObjective.ts
@@ -1,7 +1,7 @@
-import { refs } from "../../utils/refs.js"
-import { samples } from "../../utils/samples.js"
-import type { Expectant, Objective } from "../loss/index.js"
-import { type Tensor } from "../tensor/index.js"
+import { refs } from "../../utils/refs.ts"
+import { samples } from "../../utils/samples.ts"
+import type { Expectant, Objective } from "../loss/index.ts"
+import { type Tensor } from "../tensor/index.ts"
 
 export function samplingObjective(
   expectant: Expectant,
diff --git a/src/system-a/tensor/shape.test.ts b/src/system-a/tensor/shape.test.ts
index eb830c9..c61e85a 100644
--- a/src/system-a/tensor/shape.test.ts
+++ b/src/system-a/tensor/shape.test.ts
@@ -1,6 +1,6 @@
 import assert from "node:assert"
 import { test } from "node:test"
-import { shape } from "./shape.js"
+import { shape } from "./shape.ts"
 
 test("shape", () => {
   assert.deepStrictEqual(shape(1), [])
diff --git a/src/system-a/tensor/shape.ts b/src/system-a/tensor/shape.ts
index 3641876..cba339a 100644
--- a/src/system-a/tensor/shape.ts
+++ b/src/system-a/tensor/shape.ts
@@ -1,8 +1,8 @@
-import { arrayEqual } from "../../utils/arrayEqual.js"
-import { assertArrayTheSame } from "../../utils/assertArrayTheSame.js"
-import { isScalar } from "./Scalar.js"
-import { type Tensor } from "./Tensor.js"
-import { assertTensorArray } from "./assertions.js"
+import { arrayEqual } from "../../utils/arrayEqual.ts"
+import { assertArrayTheSame } from "../../utils/assertArrayTheSame.ts"
+import { isScalar } from "./Scalar.ts"
+import { type Tensor } from "./Tensor.ts"
+import { assertTensorArray } from "./assertions.ts"
 
 export type Shape = Array<number>
 
diff --git a/src/system-a/tensor/tensorAlmostEqual.ts b/src/system-a/tensor/tensorAlmostEqual.ts
index e6f054f..01fb51e 100644
--- a/src/system-a/tensor/tensorAlmostEqual.ts
+++ b/src/system-a/tensor/tensorAlmostEqual.ts
@@ -1,7 +1,7 @@
-import { sub } from "../toys/index.js"
-import { scalarReal } from "./Scalar.js"
-import { type Tensor } from "./Tensor.js"
-import { tensorEvery } from "./tensorEvery.js"
+import { sub } from "../toys/index.ts"
+import { scalarReal } from "./Scalar.ts"
+import { type Tensor } from "./Tensor.ts"
+import { tensorEvery } from "./tensorEvery.ts"
 
 export function tensorAlmostEqual(
   x: Tensor,
diff --git a/src/system-a/tensor/tensorEvery.ts b/src/system-a/tensor/tensorEvery.ts
index cec33da..bbc55fd 100644
--- a/src/system-a/tensor/tensorEvery.ts
+++ b/src/system-a/tensor/tensorEvery.ts
@@ -1,5 +1,5 @@
-import { isScalar, type Scalar } from "./Scalar.js"
-import { type Tensor } from "./Tensor.js"
+import { isScalar, type Scalar } from "./Scalar.ts"
+import { type Tensor } from "./Tensor.ts"
 
 export function tensorEvery(
   tensor: Tensor,
diff --git a/src/system-a/tensor/tensorMap.ts b/src/system-a/tensor/tensorMap.ts
index 3825aa9..facb0e1 100644
--- a/src/system-a/tensor/tensorMap.ts
+++ b/src/system-a/tensor/tensorMap.ts
@@ -1,5 +1,5 @@
-import { isScalar, type Scalar } from "./Scalar.js"
-import { type Tensor } from "./Tensor.js"
+import { isScalar, type Scalar } from "./Scalar.ts"
+import { type Tensor } from "./Tensor.ts"
 
 export function tensorMap(tensor: Tensor, fn: (x: Scalar) => Scalar): Tensor {
   if (isScalar(tensor)) {
diff --git a/src/system-a/tensor/tensorReal.ts b/src/system-a/tensor/tensorReal.ts
index 0221177..4b06e98 100644
--- a/src/system-a/tensor/tensorReal.ts
+++ b/src/system-a/tensor/tensorReal.ts
@@ -1,6 +1,6 @@
-import { scalarReal } from "./Scalar.js"
-import { type Tensor } from "./Tensor.js"
-import { tensorMap } from "./tensorMap.js"
+import { scalarReal } from "./Scalar.ts"
+import { type Tensor } from "./Tensor.ts"
+import { tensorMap } from "./tensorMap.ts"
 
 export function tensorReal(tensor: Tensor): Tensor {
   return tensorMap(tensor, scalarReal)
diff --git a/src/system-a/tensor/tensorZeros.test.ts b/src/system-a/tensor/tensorZeros.test.ts
index f745a7a..84f4913 100644
--- a/src/system-a/tensor/tensorZeros.test.ts
+++ b/src/system-a/tensor/tensorZeros.test.ts
@@ -1,6 +1,6 @@
 import assert from "node:assert"
 import { test } from "node:test"
-import { tensorZeros } from "./tensorZeros.js"
+import { tensorZeros } from "./tensorZeros.ts"
 
 test("tensorZeros", () => {
   assert.deepStrictEqual(
diff --git a/src/system-a/tensor/tensorZeros.ts b/src/system-a/tensor/tensorZeros.ts
index e5b865b..e415881 100644
--- a/src/system-a/tensor/tensorZeros.ts
+++ b/src/system-a/tensor/tensorZeros.ts
@@ -1,5 +1,5 @@
-import { type Tensor } from "./Tensor.js"
-import { tensorMap } from "./tensorMap.js"
+import { type Tensor } from "./Tensor.ts"
+import { tensorMap } from "./tensorMap.ts"
 
 export function tensorZeros(tensor: Tensor): Tensor {
   return tensorMap(tensor, (_) => 0)
diff --git a/src/system-a/tensor/zeroTensor.test.ts b/src/system-a/tensor/zeroTensor.test.ts
index c2e0749..c8f98ab 100644
--- a/src/system-a/tensor/zeroTensor.test.ts
+++ b/src/system-a/tensor/zeroTensor.test.ts
@@ -1,6 +1,6 @@
 import assert from "node:assert"
 import { test } from "node:test"
-import { zeroTensor } from "./zeroTensor.js"
+import { zeroTensor } from "./zeroTensor.ts"
 
 test("zeroTensor", () => {
   assert.deepStrictEqual(zeroTensor([2, 3]), [
diff --git a/src/system-a/tensor/zeroTensor.ts b/src/system-a/tensor/zeroTensor.ts
index 0cb6c43..5ef7e17 100644
--- a/src/system-a/tensor/zeroTensor.ts
+++ b/src/system-a/tensor/zeroTensor.ts
@@ -1,6 +1,6 @@
-import { repeatApply } from "../../utils/repeatApply.js"
-import type { Tensor } from "./Tensor.js"
-import type { Shape } from "./shape.js"
+import { repeatApply } from "../../utils/repeatApply.ts"
+import type { Tensor } from "./Tensor.ts"
+import type { Shape } from "./shape.ts"
 
 export function zeroTensor(shape: Shape): Tensor {
   if (shape.length === 0) return 0
diff --git a/src/system-a/toys/comparator.ts b/src/system-a/toys/comparator.ts
index b0375e2..6d25778 100644
--- a/src/system-a/toys/comparator.ts
+++ b/src/system-a/toys/comparator.ts
@@ -1,4 +1,4 @@
-import { scalarReal, type Scalar } from "../tensor/index.js"
+import { scalarReal, type Scalar } from "../tensor/index.ts"
 
 export function comparator(
   p: (x: number, y: number) => boolean,
diff --git a/src/system-a/toys/dot.test.ts b/src/system-a/toys/dot.test.ts
index ca0a1f1..54d4946 100644
--- a/src/system-a/toys/dot.test.ts
+++ b/src/system-a/toys/dot.test.ts
@@ -1,6 +1,6 @@
 import assert from "node:assert"
 import { test } from "node:test"
-import { dot } from "./dot.js"
+import { dot } from "./dot.ts"
 
 test("dot", () => {
   assert.deepStrictEqual(dot([1, 2, 3], [1, 2, 3]), 14)
diff --git a/src/system-a/toys/dot.ts b/src/system-a/toys/dot.ts
index 60ffd75..039196e 100644
--- a/src/system-a/toys/dot.ts
+++ b/src/system-a/toys/dot.ts
@@ -1,5 +1,5 @@
-import { type Tensor } from "../tensor/index.js"
-import { mul, sum } from "./index.js"
+import { type Tensor } from "../tensor/index.ts"
+import { mul, sum } from "./index.ts"
 
 export function dot(w: Tensor, x: Tensor): Tensor {
   return sum(mul(w, x))
diff --git a/src/system-a/toys/extend.test.ts b/src/system-a/toys/extend.test.ts
index 8f9992d..7d151ec 100644
--- a/src/system-a/toys/extend.test.ts
+++ b/src/system-a/toys/extend.test.ts
@@ -1,8 +1,8 @@
 import assert from "node:assert"
 import { test } from "node:test"
-import { shape } from "../tensor/index.js"
-import { extend2 } from "./extend.js"
-import { mul } from "./toys.js"
+import { shape } from "../tensor/index.ts"
+import { extend2 } from "./extend.ts"
+import { mul } from "./toys.ts"
 
 test("extend2 -- mul 0 0", () => {
   assert.deepStrictEqual(
diff --git a/src/system-a/toys/extend.ts b/src/system-a/toys/extend.ts
index ef0e26c..c68a324 100644
--- a/src/system-a/toys/extend.ts
+++ b/src/system-a/toys/extend.ts
@@ -1,5 +1,5 @@
-import { zip } from "../../utils/zip.js"
-import { assertTensorArray, rank, type Tensor } from "../tensor/index.js"
+import { zip } from "../../utils/zip.ts"
+import { assertTensorArray, rank, type Tensor } from "../tensor/index.ts"
 
 export function extend1<A, B extends Tensor>(
   fn: (x: A) => B,
diff --git a/src/system-a/toys/flatten.test.ts b/src/system-a/toys/flatten.test.ts
index 1982e17..3c92a21 100644
--- a/src/system-a/toys/flatten.test.ts
+++ b/src/system-a/toys/flatten.test.ts
@@ -1,6 +1,6 @@
 import assert from "node:assert"
 import { test } from "node:test"
-import { flatten } from "./flatten.js"
+import { flatten } from "./flatten.ts"
 
 test("flatten", () => {
   assert.deepStrictEqual(
diff --git a/src/system-a/toys/flatten.ts b/src/system-a/toys/flatten.ts
index 537e1f6..22e4b98 100644
--- a/src/system-a/toys/flatten.ts
+++ b/src/system-a/toys/flatten.ts
@@ -1,5 +1,5 @@
-import { type Tensor } from "../tensor/index.js"
-import { extend1 } from "./extend.js"
+import { type Tensor } from "../tensor/index.ts"
+import { extend1 } from "./extend.ts"
 
 export function flatten2(t: Array<Array<Tensor>>): Array<Tensor> {
   return t.flatMap((x) => x)
diff --git a/src/system-a/toys/index.ts b/src/system-a/toys/index.ts
index 6775355..aa40022 100644
--- a/src/system-a/toys/index.ts
+++ b/src/system-a/toys/index.ts
@@ -1,4 +1,4 @@
-export * from "./dot.js"
-export * from "./extend.js"
-export * from "./sum.js"
-export * from "./toys.js"
+export * from "./dot.ts"
+export * from "./extend.ts"
+export * from "./sum.ts"
+export * from "./toys.ts"
diff --git a/src/system-a/toys/mul.test.ts b/src/system-a/toys/mul.test.ts
index 1a3c171..19e70bf 100644
--- a/src/system-a/toys/mul.test.ts
+++ b/src/system-a/toys/mul.test.ts
@@ -1,6 +1,6 @@
 import assert from "node:assert"
 import { test } from "node:test"
-import { mul } from "./index.js"
+import { mul } from "./index.ts"
 
 // NOTE Related issues about `ext2`:
 // https://github.com/themetaschemer/malt/issues/56
diff --git a/src/system-a/toys/primitive.ts b/src/system-a/toys/primitive.ts
index 67e4f45..73f1622 100644
--- a/src/system-a/toys/primitive.ts
+++ b/src/system-a/toys/primitive.ts
@@ -4,7 +4,7 @@ import {
   scalarLink,
   scalarReal,
   type Scalar,
-} from "../tensor/index.js"
+} from "../tensor/index.ts"
 
 export function primitive1(
   realFn: (ra: number) => number,
diff --git a/src/system-a/toys/sum.test.ts b/src/system-a/toys/sum.test.ts
index 102739a..7b51abf 100644
--- a/src/system-a/toys/sum.test.ts
+++ b/src/system-a/toys/sum.test.ts
@@ -1,6 +1,6 @@
 import assert from "node:assert"
 import { test } from "node:test"
-import { sum } from "./sum.js"
+import { sum } from "./sum.ts"
 
 test("sum", () => {
   assert.deepStrictEqual(sum([1, 2, 3]), 6)
diff --git a/src/system-a/toys/sum.ts b/src/system-a/toys/sum.ts
index 759d1da..5bd6195 100644
--- a/src/system-a/toys/sum.ts
+++ b/src/system-a/toys/sum.ts
@@ -1,6 +1,6 @@
-import type { Scalar } from "../tensor/index.js"
-import { extend1 } from "./extend.js"
-import { addScalar } from "./index.js"
+import type { Scalar } from "../tensor/index.ts"
+import { extend1 } from "./extend.ts"
+import { addScalar } from "./index.ts"
 
 export function sum1(xs: Array<Scalar>): Scalar {
   return xs.reduce((x, result) => addScalar(x, result), 0)
diff --git a/src/system-a/toys/toys-by-hand.ts b/src/system-a/toys/toys-by-hand.ts
index 1a1ff39..fdf10fa 100644
--- a/src/system-a/toys/toys-by-hand.ts
+++ b/src/system-a/toys/toys-by-hand.ts
@@ -1,4 +1,4 @@
-import { Dual, scalarLink, scalarReal, type Scalar } from "../tensor/index.js"
+import { Dual, scalarLink, scalarReal, type Scalar } from "../tensor/index.ts"
 
 function addScalar(da: Scalar, db: Scalar): Scalar {
   return Dual(scalarReal(da) + scalarReal(db), (_d, z, state) => {
diff --git a/src/system-a/toys/toys.ts b/src/system-a/toys/toys.ts
index ba5799b..c0d00e6 100644
--- a/src/system-a/toys/toys.ts
+++ b/src/system-a/toys/toys.ts
@@ -1,6 +1,6 @@
-import { comparator } from "./comparator.js"
-import { extend1, extend2 } from "./extend.js"
-import { primitive1, primitive2 } from "./primitive.js"
+import { comparator } from "./comparator.ts"
+import { extend1, extend2 } from "./extend.ts"
+import { primitive1, primitive2 } from "./primitive.ts"
 
 export const expScalar = primitive1(Math.exp, (ra, z) => Math.exp(ra) * z)
 
diff --git a/src/the-book/appendix-a.test.ts b/src/the-book/appendix-a.test.ts
index 9fbdee8..b029fa0 100644
--- a/src/the-book/appendix-a.test.ts
+++ b/src/the-book/appendix-a.test.ts
@@ -8,7 +8,7 @@ import {
   mulScalar,
   scalarTruncate,
   tensorMap,
-} from "./appendix-a.js"
+} from "./appendix-a.ts"
 
 test("appendix-a -- tensorMap", () => {
   assert.deepStrictEqual(
diff --git a/src/the-book/chapter-1.test.ts b/src/the-book/chapter-1.test.ts
index f58125a..1faed8c 100644
--- a/src/the-book/chapter-1.test.ts
+++ b/src/the-book/chapter-1.test.ts
@@ -1,6 +1,6 @@
 import assert from "node:assert"
 import { test } from "node:test"
-import { line } from "./chapter-1.js"
+import { line } from "./chapter-1.ts"
 
 test("chapter-1 -- line", () => {
   assert.deepStrictEqual(line(1)([2, 3]), 5)
diff --git a/src/the-book/chapter-2.test.ts b/src/the-book/chapter-2.test.ts
index 3620913..834c84c 100644
--- a/src/the-book/chapter-2.test.ts
+++ b/src/the-book/chapter-2.test.ts
@@ -1,6 +1,6 @@
 import assert from "node:assert"
 import { test } from "node:test"
-import { shape } from "./chapter-2.js"
+import { shape } from "./chapter-2.ts"
 
 test("chapter-2 -- shape", () => {
   assert.deepStrictEqual(shape(1), [])
diff --git a/src/the-book/chapter-3.test.ts b/src/the-book/chapter-3.test.ts
index 51f96f1..a7d5824 100644
--- a/src/the-book/chapter-3.test.ts
+++ b/src/the-book/chapter-3.test.ts
@@ -1,7 +1,7 @@
 import assert from "node:assert"
 import { test } from "node:test"
-import { l2Loss } from "./chapter-3.js"
-import { line } from "./interlude-i.js"
+import { l2Loss } from "./chapter-3.ts"
+import { line } from "./interlude-i.ts"
 
 test("chapter-3 -- l2Loss", () => {
   const lineXs = [2, 1, 4, 3]
diff --git a/src/the-book/chapter-3.ts b/src/the-book/chapter-3.ts
index 96114ee..b8a3ad0 100644
--- a/src/the-book/chapter-3.ts
+++ b/src/the-book/chapter-3.ts
@@ -1,5 +1,5 @@
-import { type Tensor } from "./chapter-2.js"
-import { square, sub, sum } from "./interlude-i.js"
+import { type Tensor } from "./chapter-2.ts"
+import { square, sub, sum } from "./interlude-i.ts"
 
 export function l2Loss<Parameters extends Tensor>(
   target: (xs: Tensor) => (ps: Parameters) => Tensor,
diff --git a/src/the-book/interlude-i.test.ts b/src/the-book/interlude-i.test.ts
index c478a4d..753d847 100644
--- a/src/the-book/interlude-i.test.ts
+++ b/src/the-book/interlude-i.test.ts
@@ -1,6 +1,6 @@
 import assert from "node:assert"
 import { test } from "node:test"
-import { add, line, mul, square, squareRoot, sub, sum } from "./interlude-i.js"
+import { add, line, mul, square, squareRoot, sub, sum } from "./interlude-i.ts"
 
 test("interlude-i -- add", () => {
   assert.deepStrictEqual(add(1, 1), 2)
diff --git a/src/the-book/interlude-i.ts b/src/the-book/interlude-i.ts
index eab712b..e6bfbb2 100644
--- a/src/the-book/interlude-i.ts
+++ b/src/the-book/interlude-i.ts
@@ -1,11 +1,11 @@
-import { zip } from "../utils/zip.js"
+import { zip } from "../utils/zip.ts"
 import {
   assertNotScalar,
   assertTensor1,
   isScalar,
   rank,
   type Tensor,
-} from "./chapter-2.js"
+} from "./chapter-2.ts"
 
 export function extend2(
   fn: (x: number, y: number) => number,
diff --git a/src/utils/arrayAppend.test.ts b/src/utils/arrayAppend.test.ts
index 35c1745..1f6795f 100644
--- a/src/utils/arrayAppend.test.ts
+++ b/src/utils/arrayAppend.test.ts
@@ -1,6 +1,6 @@
 import assert from "node:assert"
 import { test } from "node:test"
-import { arrayAppend } from "./arrayAppend.js"
+import { arrayAppend } from "./arrayAppend.ts"
 
 test("arrayAppend", () => {
   assert.deepStrictEqual(arrayAppend([10], [3]), [10, 3])
diff --git a/src/utils/refs.test.ts b/src/utils/refs.test.ts
index 30167d3..eb935b0 100644
--- a/src/utils/refs.test.ts
+++ b/src/utils/refs.test.ts
@@ -1,6 +1,6 @@
 import assert from "node:assert"
 import { test } from "node:test"
-import { refs } from "./refs.js"
+import { refs } from "./refs.ts"
 
 test("refs", () => {
   assert.deepStrictEqual(refs([0, 1, 2, 3, 4, 5], [1, 3, 5]), [1, 3, 5])
diff --git a/src/utils/samples.test.ts b/src/utils/samples.test.ts
index 685e0a7..1c29251 100644
--- a/src/utils/samples.test.ts
+++ b/src/utils/samples.test.ts
@@ -1,6 +1,6 @@
 import assert from "node:assert"
 import { test } from "node:test"
-import { samples } from "./samples.js"
+import { samples } from "./samples.ts"
 
 test("samples", () => {
   assert.deepStrictEqual(samples(10, 3).length, 3)
diff --git a/src/utils/samples.ts b/src/utils/samples.ts
index d062c40..3613695 100644
--- a/src/utils/samples.ts
+++ b/src/utils/samples.ts
@@ -1,4 +1,4 @@
-import { randomNat } from "./randomNat.js"
+import { randomNat } from "./randomNat.ts"
 
 export function samples(n: number, s: number): Array<number> {
   const results: Array<number> = []
diff --git a/tsconfig.json b/tsconfig.json
index 4b7619c..80308fe 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -6,6 +6,7 @@
     "target": "es6",
     "strict": true,
     "verbatimModuleSyntax": true,
+    "rewriteRelativeImportExtensions": true,
     "declaration": true,
     "sourceMap": true,
     "declarationMap": true,