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Strict Mode

Strict mode is JavaScript is not always available in all browser engines, but the aim is to reduce mistakes.

Advantages;

  • Eliminates some JavaScript silent errors by changing them to throw errors.
  • Fixes mistakes that make it difficult for JavaScript engines to perform optimizations: strict mode code can sometimes run faster than identical code that’s not strict mode.
  • Prohibits some syntax likely to be defined in future versions of ECMAScript.

Entering strict mode​

Add "use strict" before any JavaScript code to run it in strict mode.

// Whole-script strict mode syntax
"use strict";
const v = "Hi! I'm a strict mode script!";

Example of partial strict mode

function myStrictFunction() {
  // Function-level strict mode syntax
  "use strict";

  function nested() {
    return "And so am I!";
  }

  return `Hi! I'm a strict mode function! ${nested()}`;
}

function myNotStrictFunction() {
  return "I'm not strict.";
}