diff --git a/.eslintrc b/.eslintrc
index 15b8728b..a0ad0f94 100644
--- a/.eslintrc
+++ b/.eslintrc
@@ -4,9 +4,11 @@
   ],
   "env": {
     "es6": true,
-    "node": true,
-    "browser": true,
-    "jasmine": true
+    "node": false,
+    "jest": false,
+    "browser": false,
+    "shared-node-browser": true,
+    "jasmine": false
   },
   "parserOptions": {
     "ecmaVersion": 2020,
@@ -21,6 +23,60 @@
     "no-param-reassign": ["off"],
     "import/no-extraneous-dependencies": ["error", {
       "devDependencies": ["test/**/*.js"]
-    }]
-  }
+    }],
+    "import/extensions": ["error", "ignorePackages"]
+  },
+  "overrides": [
+    {
+      "files": ["lib/browser/**/*.js", "demos/**/*.js"],
+      "env": {
+        "browser": true
+      }
+    },
+    {
+      "files": ["lib/node/**/*.js"],
+      "env": {
+        "node": true
+      },
+      "rules": {
+        "no-restricted-globals": [
+          "error",
+          {
+            "name": "__filename",
+            "message": "Use import.meta.url instead"
+          },
+          {
+            "name": "__dirname",
+            "message": "Not available in ESM"
+          },
+          {
+            "name": "exports",
+            "message": "Not available in ESM"
+          },
+          {
+            "name": "module",
+            "message": "Not available in ESM"
+          },
+          {
+            "name": "require",
+            "message": "Use import instead"
+          }
+        ]
+      }
+    },
+    {
+      "files": ["test/**/*.js", "demos/**/*.js"],
+      "env": {
+        "browser": true,
+        "node": true,
+        "jasmine": true
+      },
+      "parserOptions": {
+        "sourceType": "script"
+      },
+      "rules": {
+        "import/extensions": "off"
+      }
+    }
+  ]
 }
diff --git a/.gitignore b/.gitignore
index b83bec94..5c9147fe 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,3 +1,5 @@
+.eslintcache
+.vscode/settings.json
 node_modules
 demos/reactnative/.expo
 lib.es5
diff --git a/demos/browser/demo.js b/demos/browser/demo.js
index 38f71164..97d22cb7 100644
--- a/demos/browser/demo.js
+++ b/demos/browser/demo.js
@@ -1,6 +1,8 @@
 /* global tus */
 /* eslint-disable no-console, no-alert */
 
+'use strict'
+
 let upload          = null
 let uploadIsRunning = false
 const toggleBtn       = document.querySelector('#toggle-btn')
diff --git a/demos/browser/video.js b/demos/browser/video.js
index 7ab95049..a3215ae6 100644
--- a/demos/browser/video.js
+++ b/demos/browser/video.js
@@ -1,6 +1,8 @@
 /* global tus */
 /* eslint-disable no-console, no-alert */
 
+'use strict'
+
 let stopRecording   = null
 let upload          = null
 const recordBtn       = document.querySelector('#record-btn')
diff --git a/demos/cordova/hooks/before_prepare/copy_tus_files.js b/demos/cordova/hooks/before_prepare/copy_tus_files.js
index 2954f745..fc686395 100755
--- a/demos/cordova/hooks/before_prepare/copy_tus_files.js
+++ b/demos/cordova/hooks/before_prepare/copy_tus_files.js
@@ -1,5 +1,7 @@
 #!/usr/bin/env node
 
+'use strict'
+
 const fs = require('fs')
 const path = require('path')
 
diff --git a/demos/cordova/www/js/index.js b/demos/cordova/www/js/index.js
index aa45456d..6e84a9a0 100644
--- a/demos/cordova/www/js/index.js
+++ b/demos/cordova/www/js/index.js
@@ -1,6 +1,8 @@
 /* global tus Camera */
 /* eslint-disable no-alert */
 
+'use strict'
+
 let upload = null
 let uploadIsRunning = false
 let file = null
diff --git a/demos/nodejs/index.js b/demos/nodejs/index.js
index dd338217..2631b451 100644
--- a/demos/nodejs/index.js
+++ b/demos/nodejs/index.js
@@ -1,5 +1,7 @@
 /* eslint-disable no-console */
 
+'use strict'
+
 const fs = require('fs')
 const tus = require('../..')
 
diff --git a/lib/browser/fileReader.js b/lib/browser/fileReader.js
index 91c14c44..cc10a927 100644
--- a/lib/browser/fileReader.js
+++ b/lib/browser/fileReader.js
@@ -1,8 +1,8 @@
-import isReactNative from './isReactNative'
-import uriToBlob from './uriToBlob'
+import isReactNative from './isReactNative.js'
+import uriToBlob from './uriToBlob.js'
 
-import FileSource from './sources/FileSource'
-import StreamSource from './sources/StreamSource'
+import FileSource from './sources/FileSource.js'
+import StreamSource from './sources/StreamSource.js'
 
 export default class FileReader {
   openFile (input, chunkSize) {
diff --git a/lib/browser/fileSignature.js b/lib/browser/fileSignature.js
index 6e88ce05..bd94085a 100644
--- a/lib/browser/fileSignature.js
+++ b/lib/browser/fileSignature.js
@@ -1,4 +1,4 @@
-import isReactNative from './isReactNative'
+import isReactNative from './isReactNative.js'
 
 // TODO: Differenciate between input types
 
diff --git a/lib/browser/index.js b/lib/browser/index.js
index 9022748e..49666b0b 100644
--- a/lib/browser/index.js
+++ b/lib/browser/index.js
@@ -1,12 +1,12 @@
-import BaseUpload from '../upload'
-import NoopUrlStorage from '../noopUrlStorage'
-import { enableDebugLog } from '../logger'
-import DetailedError from '../error'
+import BaseUpload from '../upload.js'
+import NoopUrlStorage from '../noopUrlStorage.js'
+import { enableDebugLog } from '../logger.js'
+import DetailedError from '../error.js'
 
-import { canStoreURLs, WebStorageUrlStorage } from './urlStorage'
-import DefaultHttpStack from './httpStack'
-import FileReader from './fileReader'
-import fingerprint from './fileSignature'
+import { canStoreURLs, WebStorageUrlStorage } from './urlStorage.js'
+import DefaultHttpStack from './httpStack.js'
+import FileReader from './fileReader.js'
+import fingerprint from './fileSignature.js'
 
 const defaultOptions = {
   ...BaseUpload.defaultOptions,
diff --git a/lib/browser/sources/FileSource.js b/lib/browser/sources/FileSource.js
index 50a8d352..eb3b0888 100644
--- a/lib/browser/sources/FileSource.js
+++ b/lib/browser/sources/FileSource.js
@@ -1,5 +1,5 @@
-import isCordova from './isCordova'
-import readAsByteArray from './readAsByteArray'
+import isCordova from './isCordova.js'
+import readAsByteArray from './readAsByteArray.js'
 
 export default class FileSource {
   // Make this.size a method
diff --git a/lib/node/fileReader.js b/lib/node/fileReader.js
index 0bf2888a..a0757d8c 100644
--- a/lib/node/fileReader.js
+++ b/lib/node/fileReader.js
@@ -1,9 +1,9 @@
 import { ReadStream } from 'fs'
 import isStream from 'is-stream'
 
-import BufferSource from './sources/BufferSource'
-import getFileSource from './sources/FileSource'
-import StreamSource from './sources/StreamSource'
+import BufferSource from './sources/BufferSource.js'
+import getFileSource from './sources/FileSource.js'
+import StreamSource from './sources/StreamSource.js'
 
 export default class FileReader {
   openFile (input, chunkSize) {
diff --git a/lib/node/index.js b/lib/node/index.js
index c90ea32f..4667461a 100644
--- a/lib/node/index.js
+++ b/lib/node/index.js
@@ -1,13 +1,13 @@
-import BaseUpload from '../upload'
-import NoopUrlStorage from '../noopUrlStorage'
-import { enableDebugLog } from '../logger'
-import DetailedError from '../error'
+import BaseUpload from '../upload.js'
+import NoopUrlStorage from '../noopUrlStorage.js'
+import { enableDebugLog } from '../logger.js'
+import DetailedError from '../error.js'
 
-import { FileUrlStorage, canStoreURLs } from './urlStorage'
-import DefaultHttpStack from './httpStack'
-import FileReader from './fileReader'
-import fingerprint from './fileSignature'
-import StreamSource from './sources/StreamSource'
+import { FileUrlStorage, canStoreURLs } from './urlStorage.js'
+import DefaultHttpStack from './httpStack.js'
+import FileReader from './fileReader.js'
+import fingerprint from './fileSignature.js'
+import StreamSource from './sources/StreamSource.js'
 
 const defaultOptions = {
   ...BaseUpload.defaultOptions,
diff --git a/lib/upload.js b/lib/upload.js
index 8943b4ae..25ff92f0 100644
--- a/lib/upload.js
+++ b/lib/upload.js
@@ -1,8 +1,8 @@
 import { Base64 } from 'js-base64'
 import URL from 'url-parse'
-import DetailedError from './error'
-import { log } from './logger'
-import uuid from './uuid'
+import DetailedError from './error.js'
+import { log } from './logger.js'
+import uuid from './uuid.js'
 
 const defaultOptions = {
   endpoint: null,
@@ -924,8 +924,8 @@ function sendRequest (req, body, options) {
 function isOnline () {
   let online = true
   if (typeof window !== 'undefined'
-        && 'navigator' in window
-        && window.navigator.onLine === false) {
+        && 'navigator' in window // eslint-disable-line no-undef
+        && window.navigator.onLine === false) { // eslint-disable-line no-undef
     online = false
   }
 
diff --git a/package.json b/package.json
index 3e5d31f0..b0c24234 100644
--- a/package.json
+++ b/package.json
@@ -99,6 +99,6 @@
     "test-browserstack": "karma start test/karma/browserstack.conf.js",
     "test-node": "jasmine test/spec/node-index.js",
     "test-types": "tsd",
-    "lint": "eslint ."
+    "lint": "eslint . --cache"
   }
 }
diff --git a/test/karma/base.conf.js b/test/karma/base.conf.js
index 34ff939d..ebfba7f5 100644
--- a/test/karma/base.conf.js
+++ b/test/karma/base.conf.js
@@ -1,5 +1,7 @@
 // Karma base configuration
 
+'use strict'
+
 module.exports = (config) => {
   config.set({
     // base path that will be used to resolve all patterns (eg. files, exclude)
diff --git a/test/karma/browserstack.conf.js b/test/karma/browserstack.conf.js
index 29fb1efd..ded7d98d 100644
--- a/test/karma/browserstack.conf.js
+++ b/test/karma/browserstack.conf.js
@@ -1,3 +1,5 @@
+'use strict'
+
 // Karma configuration for testing using BrowserStack
 const baseConfig = require('./base.conf')
 
diff --git a/test/karma/puppeteer.conf.js b/test/karma/puppeteer.conf.js
index dfffdc5c..b908ef6f 100644
--- a/test/karma/puppeteer.conf.js
+++ b/test/karma/puppeteer.conf.js
@@ -1,3 +1,5 @@
+'use strict'
+
 // Karma configuration for testing using Puppeteer
 const baseConfig = require('./base.conf')
 
diff --git a/test/spec/browser-index.js b/test/spec/browser-index.js
index c9705dad..856d61bf 100644
--- a/test/spec/browser-index.js
+++ b/test/spec/browser-index.js
@@ -1,7 +1,9 @@
+'use strict'
+
 // The regenerator runtime is needed since the test use functions
 // with the async/await keywords. See
 // https://babeljs.io/docs/en/babel-plugin-transform-regenerator
-import 'regenerator-runtime/runtime'
+require('regenerator-runtime/runtime')
 
 beforeEach(() => {
   // Clear localStorage before every test to prevent stored URLs to
diff --git a/test/spec/helpers/assertUrlStorage.js b/test/spec/helpers/assertUrlStorage.js
index b4c248da..9bd39388 100644
--- a/test/spec/helpers/assertUrlStorage.js
+++ b/test/spec/helpers/assertUrlStorage.js
@@ -1,3 +1,5 @@
+'use strict'
+
 module.exports = async function assertUrlStorage (urlStorage) {
   // In the beginning of the test, the storage should be empty.
   let result = await urlStorage.findAllUploads()
diff --git a/test/spec/helpers/utils.js b/test/spec/helpers/utils.js
index d5a8d8ed..5b07a0d0 100644
--- a/test/spec/helpers/utils.js
+++ b/test/spec/helpers/utils.js
@@ -1,4 +1,7 @@
 /* eslint-disable max-classes-per-file */
+
+'use strict'
+
 const isBrowser  = typeof window !== 'undefined'
 const isNode     = !isBrowser
 
diff --git a/test/spec/node-index.js b/test/spec/node-index.js
index fef8fc39..9db81a70 100644
--- a/test/spec/node-index.js
+++ b/test/spec/node-index.js
@@ -1,3 +1,5 @@
+'use strict'
+
 require('./test-common')
 require('./test-node-specific')
 require('./test-parallel-uploads')
diff --git a/test/spec/test-browser-specific.js b/test/spec/test-browser-specific.js
index 95840935..0c34ea56 100644
--- a/test/spec/test-browser-specific.js
+++ b/test/spec/test-browser-specific.js
@@ -1,3 +1,5 @@
+'use strict'
+
 const assertUrlStorage = require('./helpers/assertUrlStorage')
 const { TestHttpStack, waitableFunction, wait } = require('./helpers/utils')
 const tus = require('../..')
diff --git a/test/spec/test-common.js b/test/spec/test-common.js
index 1801b59f..1de4305f 100644
--- a/test/spec/test-common.js
+++ b/test/spec/test-common.js
@@ -1,3 +1,5 @@
+'use strict'
+
 const { TestHttpStack, waitableFunction, wait, getBlob } = require('./helpers/utils')
 const tus = require('../..')
 
diff --git a/test/spec/test-end-to-end.js b/test/spec/test-end-to-end.js
index 79fa5aa8..e5500089 100644
--- a/test/spec/test-end-to-end.js
+++ b/test/spec/test-end-to-end.js
@@ -1,3 +1,5 @@
+'use strict'
+
 const axios = require('axios')
 const { getBlob } = require('./helpers/utils')
 const tus = require('../..')
diff --git a/test/spec/test-node-specific.js b/test/spec/test-node-specific.js
index c68a0915..9644bb52 100644
--- a/test/spec/test-node-specific.js
+++ b/test/spec/test-node-specific.js
@@ -1,3 +1,5 @@
+'use strict'
+
 const stream = require('stream')
 const temp = require('temp')
 const fs = require('fs')
diff --git a/test/spec/test-parallel-uploads.js b/test/spec/test-parallel-uploads.js
index c8a5ffee..1002693f 100644
--- a/test/spec/test-parallel-uploads.js
+++ b/test/spec/test-parallel-uploads.js
@@ -1,3 +1,5 @@
+'use strict'
+
 const { TestHttpStack, waitableFunction, wait, getBlob } = require('./helpers/utils')
 const tus = require('../..')
 
diff --git a/test/spec/test-terminate.js b/test/spec/test-terminate.js
index dddacc34..f257ae08 100644
--- a/test/spec/test-terminate.js
+++ b/test/spec/test-terminate.js
@@ -1,3 +1,5 @@
+'use strict'
+
 const { TestHttpStack, getBlob } = require('./helpers/utils')
 const tus = require('../..')