Strict equality

2 snippets in JavaScript

JSJavaScript

Equality: == vs === vs Object.is

syntax

a === b   // strict (no coercion)
a == b    // loose (with coercion)
Object.is(a, b)  // same-value equality

example

console.log(0 == false);         // true
console.log(0 === false);        // false
console.log(NaN === NaN);        // false
console.log(Object.is(NaN, NaN)); // true
console.log(Object.is(0, -0));    // false

Note Always use === for comparisons. Object.is() handles edge cases like NaN and -0 that even === gets wrong.

== vs === Confusion

JS · Common Mistakes

syntax

// Wrong: == with type coercion
// Right: === for strict comparison

example

// These all evaluate to true (unexpected!)
console.log("" == false);    // true
console.log(0 == "");        // true
console.log(null == undefined); // true
console.log("0" == false);   // true

// Use strict equality
console.log("" === false);   // false
console.log(0 === "");       // false
console.log(null === undefined); // false

Note Rule of thumb: always use === and !==. The only acceptable == use is value == null to check both null and undefined at once.