Strict mode in JavaScript with `use strict`


  • Runs the JS interpreter in a strict mode, meaning it fails loudly for bad practices in JS
  • Strict mode also eliminates silent errors. When something pretty major is going wrong, you won’t have to hunt for it.
  • If you’re writing good code, 'use strict' shouldn’t cause you any problems
  • Part of ES5/ES2015, official JS standard
  • Goes on top your code, must be the first line in your file. Must be a string 'use strict'
  • Keep in mind concatenation. It must be the first line and not concatenated somewhere in the middle, or it won’t work.
  • It can also be used inside your functions
  • If you get an error, it’ll stop executing the rest of the file
  • does webpack-dev-server run/compile in strict mode?


  • if you define variable without using the var (or let or const) keyword, it’ll throw an error (because you didn’t define a scope, var/let/constdefine the scope of the variable)

this is bad

badVariable = 35

it’s bad because you didn’t use the var (or let or const) keyword, which are needed to define the scope of the variable. You have cluttered the global name space by creating a global variable.

Now this bad code will still run normally, but if you use strict, it’ll fail and give you an error

'use strict';
badVariable = 35 // Uncaught ReferenceError: badVariable is not defined(…)
  • If you write to an object that is non-writable, it’ll throw an error in strict mode
NaN.foobar = true; // Uncaught TypeError: Cannot create property 'foo' on number 'NaN'(…)

NaN is just an object used as example here, you wouldn’t actually write the above code.

  • if you try to delete (Object) properties that can’t be deleted
delete Object.prototype //Uncaught TypeError: Cannot delete property 'prototype' of function Object() { [native code] }(…)
  • it used to throw an error for duplicate keys in an object but it doesn’t anymore in ES6.
  • Octal number syntax (leading zeros with numbers, like 020) will throw a syntax error
let someOctal = 500 + 090; // Uncaught SyntaxError: Octal literals are not allowed in strict mode.
  • delete can not be used to delete variables or plain objects (you can only Object properties)
let cutie = 'i am cute'
delete cutie // Uncaught SyntaxError: Delete of an unqualified identifier in strict mode.