diff --git a/package.json b/package.json
index 4f55f1e..585bb12 100644
--- a/package.json
+++ b/package.json
@@ -22,8 +22,8 @@
     "watch": "npm run build:node -- --watch",
     "test": "npm-run-all build test:unit-test",
     "build:format": "prettier --write \"src/**/*.ts\" \"test/**/*.js\"",
-    "build:node": "esbuild ./src/index.js --bundle --sourcemap --outdir=dist --platform=node",
-    "build:web": "esbuild ./src/index.js --bundle --sourcemap --platform=browser --global-name=mistreevous --outfile=dist/bundle.js",
+    "build:node": "esbuild ./src/index.ts --bundle --sourcemap --outdir=dist --platform=node",
+    "build:web": "esbuild ./src/index.ts --bundle --sourcemap --platform=browser --global-name=mistreevous --outfile=dist/bundle.js",
     "build:typecheck": "tsc --emitDeclarationOnly",
     "test:unit-test": "mocha \"test/**/*.js\""
   },
diff --git a/test/nodes/composite/Lotto.test.js b/test/nodes/composite/Lotto.test.js
index 7376e31..83af99b 100644
--- a/test/nodes/composite/Lotto.test.js
+++ b/test/nodes/composite/Lotto.test.js
@@ -418,7 +418,7 @@ describe("A Lotto node", () => {
             });
         });
 
-        describe("move to the RUNNING state if the selected child node does not move to the SUCCESS or FAILED state", () => {
+        describe("move to the RUNNING state if the selected child node is in the RUNNING state", () => {
             it("(MDSL)", () => {
                 const definition = "root { lotto { action [someAction] } }";
                 const agent = { someAction: () => {} };
diff --git a/test/nodes/composite/Selector.test.js b/test/nodes/composite/Selector.test.js
index e1cf352..99cf463 100644
--- a/test/nodes/composite/Selector.test.js
+++ b/test/nodes/composite/Selector.test.js
@@ -3,6 +3,9 @@ const chai = require("chai");
 
 var assert = chai.assert;
 
+const findNode = (tree, type, caption) =>
+    tree.getFlattenedNodeDetails().find((node) => node.type === type && node.caption === caption);
+
 describe("A Selector node", () => {
     describe("on tree initialisation", () => {
         describe("will error if the node does not have at least one child", () => {
@@ -32,5 +35,79 @@ describe("A Selector node", () => {
         });
     });
 
-    describe("when updated as part of a tree step will", () => {});
+    describe("when updated as part of a tree step will", () => {
+        describe("move to the FAILED state if all child nodes move to the FAILED state", () => {});
+
+        describe("move to the SUCCEEDED state if any child node moves to the SUCCEEDED state", () => {});
+
+        describe("move to the RUNNING state if any child node is in the RUNNING state", () => {
+            it("(MDSL)", () => {
+                const definition =
+                    "root { selector { action [actionFail] action [actionRunning] action [actionSucceed] } }";
+                const agent = {
+                    actionSucceed: () => mistreevous.State.SUCCEEDED,
+                    actionRunning: () => {},
+                    actionFail: () => mistreevous.State.FAILED
+                };
+                const tree = new mistreevous.BehaviourTree(definition, agent);
+
+                assert.strictEqual(findNode(tree, "root", "ROOT").state, mistreevous.State.READY);
+                assert.strictEqual(findNode(tree, "selector", "SELECTOR").state, mistreevous.State.READY);
+                assert.strictEqual(findNode(tree, "action", "actionFail").state, mistreevous.State.READY);
+                assert.strictEqual(findNode(tree, "action", "actionRunning").state, mistreevous.State.READY);
+                assert.strictEqual(findNode(tree, "action", "actionSucceed").state, mistreevous.State.READY);
+
+                tree.step();
+
+                assert.strictEqual(findNode(tree, "root", "ROOT").state, mistreevous.State.RUNNING);
+                assert.strictEqual(findNode(tree, "selector", "SELECTOR").state, mistreevous.State.RUNNING);
+                assert.strictEqual(findNode(tree, "action", "actionFail").state, mistreevous.State.FAILED);
+                assert.strictEqual(findNode(tree, "action", "actionRunning").state, mistreevous.State.RUNNING);
+                assert.strictEqual(findNode(tree, "action", "actionSucceed").state, mistreevous.State.READY);
+            });
+
+            it("(JSON)", () => {
+                const definition = {
+                    type: "root",
+                    child: {
+                        type: "selector",
+                        children: [
+                            {
+                                type: "action",
+                                call: "actionFail"
+                            },
+                            {
+                                type: "action",
+                                call: "actionRunning"
+                            },
+                            {
+                                type: "action",
+                                call: "actionSucceed"
+                            }
+                        ]
+                    }
+                };
+                const agent = {
+                    actionSucceed: () => mistreevous.State.SUCCEEDED,
+                    actionRunning: () => {},
+                    actionFail: () => mistreevous.State.FAILED
+                };
+                const tree = new mistreevous.BehaviourTree(definition, agent);
+
+                assert.strictEqual(findNode(tree, "root", "ROOT").state, mistreevous.State.READY);
+                assert.strictEqual(findNode(tree, "selector", "SELECTOR").state, mistreevous.State.READY);
+                assert.strictEqual(findNode(tree, "action", "actionFail").state, mistreevous.State.READY);
+                assert.strictEqual(findNode(tree, "action", "actionRunning").state, mistreevous.State.READY);
+                assert.strictEqual(findNode(tree, "action", "actionSucceed").state, mistreevous.State.READY);
+
+                tree.step();
+
+                assert.strictEqual(findNode(tree, "root", "ROOT").state, mistreevous.State.RUNNING);
+                assert.strictEqual(findNode(tree, "selector", "SELECTOR").state, mistreevous.State.RUNNING);
+                assert.strictEqual(findNode(tree, "action", "actionFail").state, mistreevous.State.FAILED);
+                assert.strictEqual(findNode(tree, "action", "actionRunning").state, mistreevous.State.RUNNING);
+                assert.strictEqual(findNode(tree, "action", "actionSucceed").state, mistreevous.State.READY);
+            });
+        });
+    });
 });
diff --git a/test/nodes/decorator/Fail.test.js b/test/nodes/decorator/Fail.test.js
index eaac606..1f0bc00 100644
--- a/test/nodes/decorator/Fail.test.js
+++ b/test/nodes/decorator/Fail.test.js
@@ -115,7 +115,7 @@ describe("A Fail node", () => {
             });
         });
 
-        describe("move to the RUNNING state if the child node does not move to the SUCCESS or FAILED state", () => {
+        describe("move to the RUNNING state if the child node moves to the RUNNING state", () => {
             it("(MDSL)", () => {
                 const definition = "root { fail { action [someAction] } }";
                 const agent = { someAction: () => {} };
diff --git a/test/nodes/decorator/Succeed.test.js b/test/nodes/decorator/Succeed.test.js
index 1296588..1205a85 100644
--- a/test/nodes/decorator/Succeed.test.js
+++ b/test/nodes/decorator/Succeed.test.js
@@ -115,7 +115,7 @@ describe("A Succeed node", () => {
             });
         });
 
-        describe("move to the RUNNING state if the child node does not move to the SUCCESS or FAILED state", () => {
+        describe("move to the RUNNING state if the child node moves to the RUNNING state", () => {
             it("(MDSL)", () => {
                 const definition = "root { succeed { action [someAction] } }";
                 const agent = { someAction: () => {} };