JavaScript Equality Operators

Those operators accept two values and return a boolean:

• == checks for equality
• != checks for inequality
• === checks for strict equality
• !== checks for strict inequality

Let’s talk what we mean for strict. == does not convert the second operand to the type of the first. It follows fixed rules, regardless of order: null and undefined only equal each other; if one side is a number and the other a string, the string becomes a number; booleans become numbers; objects get converted to primitives; BigInt compares by mathematical value with numbers and strings. === skips that and compares type and value as they are. I also have a short == vs === post.

Examples:

const a = true

a == true //true
a === true //true

1 == 1 //true
1 == '1' //true
1 === 1 //true
1 === '1' //false

You cannot check objects for equality: two objects are never equal to each other. The only case when a check might be true is if two variables reference the same object.

Some peculiarities to be aware: NaN is always different from NaN. Use Number.isNaN() to check for it.

Object.is() (ES2015) is like ===, except Object.is(NaN, NaN) is true and Object.is(0, -0) is false. I wrote about Object.is().

NaN == NaN //false

null and undefined values are equal if compared in non-strict mode:

null == undefined //true
null === undefined //false

I built a free JS equality explorer where you can try any comparison and see the coercion steps behind it.

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