use strict in JavaScript


Ever wondered what use strict does in JavaScript?


use strict is a feature of ECMAScript 5 allowing you to put a block in a strict execution mode. This can help at finding certain problems at an earlier time.

  • It prevents you to access global objects and other unsafe operations.
  • It ensures that you always define your variables.
  • It avoids bad coding style by throwing exceptions for them.
function strictFunction() {
   // put function in strict mode
   'use strict';

   return 'Hello from strict mode!';

function notStrictFunction() {
   return 'Hello from not strict mode'; 

A more detailed explanation can be found here:


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