What Is "Same-Value-Zero" in JavaScript?

In JavaScript, "Same-Value-Zero" (or SameValueZero) is merely a comparison algorithm (used for comparison of two values), which is, for example, used by Array.prototype.includes() and TypedArray.prototype.includes(), as well as, Map and Set methods for comparing key equality.

The Same-Value-Zero algorithm is similar to doing the strict equality (===) value comparison. However, they differ in terms of how they perform NaN equality — i.e. strict equality treats NaN as unequal to itself, whereas Same-Value-Zero does not:

`x`	`y`	`===`	`SameValueZero`
`NaN`	`NaN`	`false`	`true`

For example:

// ES7+
console.log([NaN].includes(NaN)); // true

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

Other than this one difference, both (Same-Value-Zero algorithm and strict equality) yield the same result:

`x`	`y`	`===`	`SameValueZero`
`undefined`	`undefined`	`true`	`true`
`null`	`null`	`true`	`true`
`true`	`true`	`true`	`true`
`false`	`false`	`true`	`true`
`'foo'`	`'foo'`	`true`	`true`
`0`	`0`	`true`	`true`
`+0`	`-0`	`true`	`true`
`+0`	`0`	`true`	`true`
`-0`	`0`	`true`	`true`
`0n`	`-0n`	`true`	`true`
`0`	`false`	`false`	`false`
`""`	`false`	`false`	`false`
`""`	`0`	`false`	`false`
`'0'`	`0`	`false`	`false`
`'23'`	`23`	`false`	`false`
`[]`	`[]`	`false`	`false`
`[1, 2]`	`[1, 2]`	`false`	`false`
`[1, 2]`	`'1,2'`	`false`	`false`
`new String('foo')`	`'foo'`	`false`	`false`
`null`	`undefined`	`false`	`false`
`null`	`false`	`false`	`false`
`undefined`	`false`	`false`	`false`
`{ foo: 'bar' }`	`{ foo: 'bar' }`	`false`	`false`
`new String('foo')`	`new String('foo')`	`false`	`false`
`0`	`null`	`false`	`false`
`0`	`NaN`	`false`	`false`
`'foo'`	`NaN`	`false`	`false`

This post was published 13 Sep, 2022 by Daniyal Hamid. Daniyal currently works as the Head of Engineering in Germany and has 20+ years of experience in software engineering, design and marketing. Please show your love and support by sharing this post.