Equality (==)

The equality (==) operator checks whether its two operands are equal, returning a Boolean result. Unlike the strict equality operator, it attempts to convert and compare operands that are of different types.

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Syntax

x == y

Description

The equality operators (== and !=) provide the IsLooselyEqual semantic. This can be roughly summarized as follows:

  1. If the operands have the same type, they are compared as follows:
    • Object: return true only if both operands reference the same object.
    • String: return true only if both operands have the same characters in the same order.
    • Number: return true only if both operands have the same value. +0 and -0 are treated as the same value. If either operand is NaN, return false; so, NaN is never equal to NaN.
    • Boolean: return true only if operands are both true or both false.
    • BigInt: return true only if both operands have the same value.
    • Symbol: return true only if both operands reference the same symbol.
  2. If one of the operands is null or undefined, the other must also be null or undefined to return true. Otherwise return false.
  3. If one of the operands is an object and the other is a primitive, convert the object to a primitive.
  4. At this step, both operands are converted to primitives (one of String, Number, Boolean, Symbol, and BigInt). The rest of the conversion is done case-by-case.
    • If they are of the same type, compare them using step 1.
    • If one of the operands is a Symbol but the other is not, return false.
    • If one of the operands is a Boolean but the other is not, convert the boolean to a number: true is converted to 1, and false is converted to 0. Then compare the two operands loosely again.
    • Number to String: convert the string to a number. Conversion failure results in NaN, which will guarantee the equality to be false.
    • Number to BigInt: compare by their numeric value. If the number is ±Infinity or NaN, return false.
    • String to BigInt: convert the string to a BigInt using the same algorithm as the BigInt() constructor. If conversion fails, return false.

Loose equality is symmetric: A == B always has identical semantics to B == A for any values of A and B (except for the order of applied conversions).

The most notable difference between this operator and the strict equality (===) operator is that the strict equality operator does not attempt type conversion. Instead, the strict equality operator always considers operands of different types to be different. The strict equality operator essentially carries out only step 1, and then returns false for all other cases.

There's a "willful violation" of the above algorithm: if one of the operands is document.all, it is treated as if it's undefined. This means that document.all == null is true, but document.all === undefined && document.all === null is false.

Examples

Comparison with no type conversion

1 == 1; // true
"hello" == "hello"; // true

Comparison with type conversion

"1" == 1; // true
1 == "1"; // true
0 == false; // true
0 == null; // false
0 == undefined; // false
0 == !!null; // true, look at Logical NOT operator
0 == !!undefined; // true, look at Logical NOT operator
null == undefined; // true

const number1 = new Number(3);
const number2 = new Number(3);
number1 == 3; // true
number1 == number2; // false

Comparison of objects

const object1 = {
  key: "value",
};

const object2 = {
  key: "value",
};

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

Comparing strings and String objects

Note that strings constructed using new String() are objects. If you compare one of these with a string literal, the String object will be converted to a string literal and the contents will be compared. However, if both operands are String objects, then they are compared as objects and must reference the same object for comparison to succeed: 

const string1 = "hello";
const string2 = String("hello");
const string3 = new String("hello");
const string4 = new String("hello");

console.log(string1 == string2); // true
console.log(string1 == string3); // true
console.log(string2 == string3); // true
console.log(string3 == string4); // false
console.log(string4 == string4); // true

Comparing Dates and strings

const d = new Date("December 17, 1995 03:24:00");
const s = d.toString(); // for example: "Sun Dec 17 1995 03:24:00 GMT-0800 (Pacific Standard Time)"
console.log(d == s); //true

Comparing arrays and strings

const a = [1, 2, 3];
const b = "1,2,3";
a == b; // true, `a` converts to string

const c = [true, 0.5, "hey"];
const d = c.toString(); // "true,0.5,hey"
c == d; // true

Specifications

Specification
ECMAScript Language Specification
# sec-equality-operators

Browser compatibility

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See also