What is JavaScript’s strict mode?

In a nutshell

Strict mode in JavaScript makes several changes:

• Throws errors for otherwise silent JavaScript errors.
• Fixes mistakes that otherwise make it difficult for JavaScript engines to optimize code. As a result, strict mode can sometimes be more faster than non-strict mode.
• Prohibits syntax that’s likely to be used in future ECMAScript versions.

Changes introduced in strict mode

No assignment to undeclared variables

In non-strict mode, the below code wouldn’t throw any errors.

"use strict";
let mistypeVariable;

// Assuming no global variable mistypeVarible exists
// this line throws a ReferenceError due to the
// misspelling of "mistypeVariable" (lack of an "a")
mistypeVarible = 17;

Throws errors if object property assignment fails

In non-strict mode, the below code wouldn’t throw any errors.

"use strict";

// Assignment to a non-writable global
var undefined = 5; // TypeError
var Infinity = 5; // TypeError

// Assignment to a non-writable property
const obj1 = {};
Object.defineProperty(obj1, "x", { value: 42, writable: false });
obj1.x = 9; // TypeError

// Assignment to a getter-only property
const obj2 = {
  get x() {
    return 17;
  },
};
obj2.x = 5; // TypeError

// Assignment to a new property on a non-extensible object
const fixed = {};
Object.preventExtensions(fixed);
fixed.newProp = "ohai"; // TypeError

this never refers to globalThis

In sloppy mode (non-strict mode), if this is not specified, it will refer to the global context. In strict mode, it’ll be undefined.

"use strict";

function fun() {
  return this;
}
console.assert(fun() === undefined);

Throws error if deleting an object property fails

In non-strict mode, the below code wouldn’t throw any errors.

"use strict";
delete Object.prototype; // TypeError
delete [].length; // TypeError

eval doesn’t introduce new variables into the surrounding scope

var x = 17;
var evalX = eval("'use strict'; var x = 42; x;");
console.assert(x === 17);
console.assert(evalX === 42);

No duplicate parameter names

function sum(a, a, c) {
  // syntax error
  "use strict";
  return a + a + c; // wrong if this code ran
}

No setting properties on primitive values

"use strict";

false.true = ""; // TypeError
(14).sailing = "home"; // TypeError
"with".you = "far away"; // TypeError

More reserved keywords

The following words can’t be used as variable names:

• implements
• interface
• let
• package
• private
• protected
• public
• static
• yield

How to invoke strict mode

Scripts

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

Functions

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.";
}

Other notes

• Strict mode is enabled by default for JavaScript modules or classes.

Sources/related

• Strict mode – MDN
• How to check if JavaScript code is running in strict mode?