Comparing null and undefined with booleans in Javascript

Question

According to ES5 section 11.9.3 it says that

If Type(x) is Boolean, return the result of the comparison ToNumber(x) == y.
If Type(y) is Boolean, return the result of the comparison x == ToNumber(y).

When I tried to compare

let a = null, b = null;

a == false; // false
b == false; // false
a == "";    // false
b == "";    // false
a == 0;     // false
b == 0;     // false

What I was expecting here, for a == false returning false is, false is coerced to Number which is 0 so the comparison will become a == 0 then for next coercion, which is of type null == 0, null should get coerced to Number which is 0. So finally it should return true for 0 == 0.

What I got from ES5 11.9.3 is

If x is null and y is undefined, return true.
If x is undefined and y is null, return true.

Mentioned about null and undefined.

The 10th point of the ES 11.9.3 section says that if your comparison doesn't come in above 9 criteria return false.

Is my comparison returning false from according to 10th point in ES5 11.9.3 or am I missing something here>

`which is of type null == 0, null should get coerced to Number which is 0`. But `null` isn't of type boolean — Code Maniac, Nov 03 '19 at 10:25
"Is my comparison returning false from according to 10th point in ES5 11.9.3" - yes, i would think so. But there is no coercion to `0`, at least not from what i could see. As you mentioned, all points fail. PS: i dont see any reason to use ES5 here (you even tagged it as ES6?!), so why not [the ES2020 spec](https://tc39.es/ecma262/#sec-abstract-equality-comparison)? — ASDFGerte, Nov 03 '19 at 10:26
What specific comparison are you asking about? You only ever talk about `a == false`, so why show all the others? — T.J. Crowder, Nov 03 '19 at 10:32
I was just curious about null comparisons, so I mentioned all null comparisons in question. — Akshay Bande, Nov 03 '19 at 10:33

T.J. Crowder · Accepted Answer · 2019-11-03T10:43:10.080

Fundamentally, yes, you're correct that you're getting false when comparing null with anything other than null or undefined because you get to the last step of the algorithm, which is return false.

If you're asking why null == false is false, the short answer is: Because null is only == null and undefined.

The long answer is in the abstract equality operation that you pointed to, here's the up to date version: https://tc39.es/ecma262/#sec-abstract-equality-comparison Steps 1-8 don't apply, but step 9 does:

If Type(y) is Boolean, return the result of the comparison x == ! ToNumber(y).

(The ! before ToNumber there is not a negation, it's a spec annotation about abrupt completions. This confuses people sometimes.)

So now we have null == 0. None of Steps 1-12 apply, so the return false in Step 13 applies.

(I should note that the "If Type(x) is Object" in Step 11 is not true when x is null, even though typeof null is "object". The Type operation isn't the same as typeof, Type(null) is Null.)

In a comment you've said:

I was just curious about null comparisons, so I mentioned all null comparisons in question.

Yes, you're right that you end up getting to the end. The only parts of the algorithm that relate to null are Steps 2 and 3 that check if one operand is null and the other operand is undefined and return true if so.

Comparing null and undefined with booleans in Javascript

1 Answers1