Interesting array with null elements JavaScript behavior

Question

Any idea why it behaves like this?

Answer 1

When == is used to compare things of different types, like an array ([null]) and false, it has specific steps it goes through, called the Abstract Equality Algorithm (quoted below) to try to coerce them to things that it can compare. (Because == is the "loose" equality operator.)

First it sees if either operand is null or undefined, but in this case neither is.

Then it sees if it's comparing a number and a string, but that's not true in this case either.

Then it seems if either operand is a boolean, which of course on of these is. Since one of them is a boolean, it does an abstract operation called ToNumber in the spec to convert the other one to a number and does another ==. (The Number function, when used as a function and not a constructor, is basically a wrapper for that abstract ToNumber operation.)

So

[null] == false

becomes

0 == false

...because ToNumber([null]) coerces its argument to a string (via ToPrimitive), getting "", and then converts that to 0.

So now == has something where one of the operands is a number. SO it converts the other one to a number as well. Number(false) is 0.

Thus, [null] == false is true.

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So what about [null, null] == false? When converting [null, null] to a string, we get ",", which ToNumber can't turn into a valid number. So it converts it to NaN. One of the fun things about NaN is that it's a number, but it's never equal to anything else. So:

[null, null] == false 

becomes

NaN == false

becomes

NaN == 0

...which is false, because (again) NaN is never equal to anything.

---

Here's that Abstract Equality Algorithm:

1. ReturnIfAbrupt(x).
2. ReturnIfAbrupt(y).

3. If Type(x) is the same as Type(y), then

   a) Return the result of performing Strict Equality Comparison x === y.

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

5. If x is undefined and y is null, return true.
6. If Type(x) is Number and Type(y) is String,
   return the result of the comparison x == ToNumber(y).
7. If Type(x) is String and Type(y) is Number,
   return the result of the comparison ToNumber(x) == y.
8. If Type(x) is Boolean, return the result of the comparison ToNumber(x) == y.
9. If Type(y) is Boolean, return the result of the comparison x == ToNumber(y).
10. If Type(x) is either String, Number, or Symbol and Type(y) is Object, then
    return the result of the comparison x == ToPrimitive(y).
11. If Type(x) is Object and Type(y) is either String, Number, or Symbol, then
    return the result of the comparison ToPrimitive(x) == y.
12. Return false.