My favorite type coercion example

karataev profile image Eugene Karataev ・2 min read
[] == ![] // true

How is an empty array equal to a not empty array?! It doesn't make sense!
That's the reason why there are a lot of articles and videos on wtfjs theme.

Most developers prefer not to use double equals at all to avoid such issues.
IDE code checker may warn you not to use ==.
IDE warn

And this is a good decision to always use ===.

But I really like Kyle Simpson's YDKJS series and his why attitude. Why does JS behave like that? Let's dive in deeper!

== operator allows types coercion. It means that if two compared values are of different types, JS will try to coerce them to the same type before comparison.

What's the coercion algorithm? Actually it's pretty straightforward and defined in ECMAScript language specification.

It might look scary at first, so let's follow this algorithm step by step using the example from the beginning of the post.

[] == ![] - expression statement of our interest
First of all, before the actual comparision JS engine will execute all operands.

Step 1
[] == false
The right operand is the boolean type, so the result of comparision [] == ToNumber(false) is returned.

Step 2
[] == 0
[] is the object and 0 is the number, ToPrimitive([]) == 0 is returned.

Step 3
"" == 0
"" is the string and 0 is the number, ToNumber(x) == y is returned.

Step 4
0 == 0
true is returned.

It's fascinating how much work a JS engine does under the hood. But it follows the clear algorithm, no magic involved.
Read the specification, learn how JS works, be a better developer 😉

P.S. I really love this tool which decomposes any JS comparison expression according to the ECMAScript language specification.

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