The Object is() method

Object.is() compares two values and returns true if they are the same value. It was introduced in ES2015, and it works almost like ===, with two differences: it treats NaN as equal to itself, and it tells 0 apart from -0.

Usage:

Object.is(a, b)

The result is always false unless:

• a and b are the same exact object
• a and b are equal strings (strings are equal when composed by the same characters, in the same order)
• a and b are equal numbers (numbers are equal when their value is equal)
• a and b are both undefined, both null, both NaN, both true or both false

How is it different from ===?

For most values, the two behave the same:

Object.is('hello', 'hello') //true
Object.is(21, 21) //true
Object.is(null, undefined) //false

The first difference is NaN. In JavaScript, NaN === NaN is famously false. Object.is() fixes that:

NaN === NaN //false
Object.is(NaN, NaN) //true

This is the main reason to reach for Object.is(). Say you parsed some user input and want to know if the result is stuck at NaN. Comparing with === can never work, so before Object.is() you had to use Number.isNaN().

The second difference goes the other way. === considers 0 and -0 equal, while Object.is() does not:

0 === -0 //true
Object.is(0, -0) //false

0 and -0 are different values in JavaScript, so pay attention in this special case (convert all to +0 using the + unary operator before comparing, for example).

Where is it used in practice?

React uses this same-value comparison internally. When you call a state setter from useState(), React uses Object.is() to compare the new value with the old one, and skips the re-render if they match. Knowing how Object.is() behaves explains why setting state to the same string or number does nothing.

One pitfall

Object.is() is not a deep comparison. Two objects with identical content are still two different objects:

Object.is({ age: 36 }, { age: 36 }) //false

It only returns true for objects when both sides point to the same reference:

const flavio = { age: 36 }
Object.is(flavio, flavio) //true

If you need to compare object contents, compare the properties you care about one by one, or serialize both sides first. Object.is() won’t do it for you.

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