JavaScript - Strict Mode

Overview

Estimated time: 10–15 minutes

Strict mode opts into safer semantics: no implicit globals, safer this, and more errors for silent mistakes.

Learning Objectives

• Enable strict mode at file/function scope.
• Recognize key behavior changes in strict mode.

Enabling

'use strict'; // file scope
function f(){ 'use strict'; /* function scope only */ }

Key Differences

• No implicit globals (assigning to undeclared variables throws).
• this in plain function calls is undefined (not window).
• Duplicate parameter names are errors.
• Some silent failures (e.g., writing to read-only properties) throw.

Examples

'use strict';
// 1) Implicit global becomes error
// oops = 1; // ReferenceError

// 2) this is undefined in plain function calls
function who(){ return this; }
console.log(who() === undefined); // true

// 3) Duplicate params
// function bad(a, a) {} // SyntaxError