JavaScript: Equality

Double Equal

The double equal == does type conversion implicitly.

console.log([] == "");
console.log("0" == 0);
console.log("" == 0);
console.log(1 == true);
// all true

Tip: never use double equal. Use triple equal instead.

Triple Equal

Triple Equal can compare most Primitive Values such as string, number, Symbol, undefined, null, except NaN.

Object/Array cannot be compared with triple equal.

console.log("x" === "x"); // true
console.log(3 === 3); // true
console.log(3 === 3.0); // true
console.log(Infinity === Infinity); // true

console.log(NaN === NaN); // false
console.log([] === []); // false
console.log({} === {}); // false

Triple equale does not convert value types implicitly.

console.log(([] === "") === false);
console.log(("" === {}) === false);
console.log(("0" === 0) === false);
console.log(("" === 0) === false);
console.log((1 === true) === false);
// all true

Test NaN

• Object.is
• Number.isNaN

Test Infinity

Number.isFinite

Test Equality of Objects

Test Equality of Objects