Strict equality (===) - JavaScript 编辑

The strict equality operator (===) checks whether its two operands are equal, returning a Boolean result. Unlike the equality operator, the strict equality operator always considers operands of different types to be different.

The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.

The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.

Syntax

x === y

Description

The strict equality operators (=== and !==) use the Strict Equality Comparison Algorithm to compare two operands.

• If the operands are of different types, return false.
• If both operands are objects, return true only if they refer to the same object.
• If both operands are null or both operands are undefined, return true.
• If either operand is NaN, return false.
• Otherwise, compare the two operand's values:
  • Numbers must have the same numeric values. +0 and -0 are considered to be the same value.
  • Strings must have the same characters in the same order.
  • Booleans must be both true or both false.

The most notable difference between this operator and the equality (==) operator is that if the operands are of different types, the == operator attempts to convert them to the same type before comparing.

Examples

Comparing operands of the same type

console.log("hello" === "hello");   // true
console.log("hello" === "hola");    // false

console.log(3 === 3);               // true
console.log(3 === 4);               // false

console.log(true === true);         // true
console.log(true === false);        // false

console.log(null === null);         // true

Comparing operands of different types

console.log("3" === 3);           // false

console.log(true === 1);          // false

console.log(null === undefined);  // false

Comparing objects

const object1 = {
  name: "hello"
}

const object2 = {
  name: "hello"
}

console.log(object1 === object2);  // false
console.log(object1 === object1);  // true

Specifications

Specification
ECMAScript (ECMA-262) The definition of 'Equality operators' in that specification.

Browser compatibility

BCD tables only load in the browser

See also

• Equality operator
• Inequality operator
• Strict inequality operator