enables strict mode in your package

NPM version No deps JavaScript Style Guide


With npm do

npm install strict-mode --save-dev

NOTA BENE you may want to install strict-mode as a development dependency, see Bonus Tip below.


Please note that this package is intended to be used server side. If used with browserify it is a no op.

Suppose that the main attribute in your package.json is index.js. If you want that all the modules in your package have strict mode enabled, just wrap your index.js this way

require('strict-mode')(function () {

// your index.js content

// every *require* call inside this function will have strict mode enabled



Strict mode is a best practice but adding a "use strict"; on top of every .js file in your package could

On the other hand the use-strict package solution is too invasive, cause it applies strictness to all future modules loaded.

Use case

At the time of this writing Node v4 stable version was released few days ago. Finally we can use class, let, const (among other new exciting features) but you will notice that if you do not turn on strict mode an exception will raise. For instance, a file Point2d.js with content

class Point2d {
  constructor (x, y) {
    this.x = x
    this.y = y

module.exports = Point2d

when imported will complain

SyntaxError: Block-scoped declarations (let, const, function, class) not yet supported outside strict mode

but if you wrap the import with strict-mode everything will just work

require('strict-mode')(function () {

  var Point2d = require('./Point2d')
  // require all other classes you need.

  // You can also export them safely
  exports.Point2d = Point2d

Bonus tip

You could use strict-mode just as a development dependency.

Following the instructions below, you are not going to deploy strict-mode on production but when you run your tests if some code is not strict, then you will get an error.

Suppose you have the following folder structure in your package.

├── package.json
├── src
│   └── index.js    # your package.json `main` entry
└── test
    ├── test1.js
    └── test2.js

Assuming you package name is, emh my-package, create a file test/my-package.js containing

require('strict-mode')(function () {
  module.exports = require('../src/index.js')

Now if you set the environment variable NODE_PATH=test, you can use require('my-package') in your tests.

For instance if you are using tape you can add this to your package.json

    "test": "NODE_PATH=test tape test/*js"

You can also achieve the same result without NODE_PATH environment variable, using this nasty trick (to cheat npm :^) Instead of test/my-package.js, create a test/node_modules/my-package/index.js containing

require('strict-mode')(function () {
  module.exports = require('../../../src/index.js')

Notice also that you will probably need to force git add, for instance

git add -f test/node_modules/my-package/index.js


Code stolen from isaacs’ use-strict. A big thank to MDN cause it is an awesome knowledge base for everything related with the Web: in particular, I could find some valid counterexamples of strict mode to include in my tests.