You Don't Know JS Yet: Get Started by Kyle Simpson

Author:Kyle Simpson
Language: eng
Format: mobi, epub
Publisher: GetiPub & Leanpub
Published: 2020-01-27T22:00:00+00:00

In the case of NaN, the === operator lies and says that an occurrence of NaN is not equal to another NaN. In the case of -0 (yes, this is a real, distinct value you can use intentionally in your programs!), the === operator lies and says it’s equal to the regular 0 value.

Since the lying about such comparisons can be bothersome, it’s best to avoid using === for them. For NaN comparisons, use the Number.isNaN(..) utility, which does not lie. For -0 comparison, use the Object.is(..) utility, which also does not lie. Object.is(..) can also be used for non-lying NaN checks, if you prefer. Humorously, you could think of Object.is(..) as the “quadruple-equals” ====, the really-really-strict comparison!

There are deeper historical and technical reasons for these lies, but that doesn’t change the fact that === is not actually strictly exactly equal comparison, in the strictest sense.

The story gets even more complicated when we consider comparisons of object values (non-primitives). Consider:

[ 1, 2, 3 ] === [ 1, 2, 3 ]; // false

{ a: 42 } === { a: 42 } // false

(x => x * 2) === (x => x * 2) // false

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