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I was tring a few things in the console.

!5 is actually false
0 is a falsy value, so
0 == !5 is true

Okay, but when i tried this

!0 is true
5 is a truthy, so
5 == !0 should be true

But its not, the console says false. Why is this happening?

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  • 4
    5 will get converted to false ? Commented Oct 21, 2015 at 17:33
  • How does the question you linked to relate to your question? Commented Oct 21, 2015 at 17:33
  • Because (5 == !0) is equivalent to (5 == true), which is false. Commented Oct 21, 2015 at 17:36
  • @j08691 it was just for background, its the reason why I tried the things in the console. Commented Oct 21, 2015 at 17:37
  • 1
    @j08691 More specifically 5 == 1 is false. The moral of the story is don't rely on == to do the conversion that you expect it to OP, it's quite funky. Commented Oct 21, 2015 at 17:41

1 Answer 1

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5

The reason the last line is false is that the == isn't a simple boolean conversion. It usually tries to convert operands with non-matching types down to a number.

So the 5 doesn't need conversion since it's already a number but !0, which is true, does. The value true gets converted to 1, so it doesn't equal 5.

You can infer from this that 1 == !0 will be true, and indeed it is.

This is detailed in the ES5 spec in the Abstract Equality Comparison Algorithm, step 7, which says of the comparison x == y:

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

So the right-hand boolean is coerced to a number with ToNumber. In this case, ToNumber says:

The result is 1 if the argument is true.

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3 Comments

Oh! yes 1 == !0 gives true.
x == ToNumber(y) would happen only if x is a number isn't it?
@AkshendraPratap: Sort of. The algorithm is recursive. So x could start off as something else but have been converted to a number on an earlier pass.

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