Why does "[] == 0" return true, and "[]" true, and "0" false?

Remember that an array is an object and 0 is a number.

And as "user2864740" said ..

1) When you do

!![] //--- returns true
!!0 //--- returns false

      

        
        
        
      

    

You are doing the so called "ToBoolean" conversion

https://es5.github.io/#x9.2

room

The result is false if the argument is +0, -0, or NaN; otherwise, the result is correct.

Object (our [])

always true

2) But when you use == you are doing a so called "Equality comparison"

https://es5.github.io/#x11.9.3

It's a little more complicated here, but to understand what's going on you have to remember that == is doing a type coercion (so you can compare oranges to apples :))

First of all, the compiler converts [] to some primitive type.

If type (x) is either a string or a number and type (y) is an object, returns the result of the comparison x == ToPrimitive (y).

How ToPrimitive works, this is an article article :) but it's easy to remember that the closet primitive type for an array is a string. The array will be converted to an empty string.

[]. toString () === ""

So now we need to compare the empty string and the number 0

"" == 0   // true

      

        
        
        
      

    

Hmmm. The way it is. But why? Remember that when you are comparing an Equality Comparison number and a string

4. If Type (x) is Number and Type (y) is String, return the result of the comparison x == ToNumber (y).

So try converting an empty string to a number

Number("") === 0

      

        
        
        
      

    

And in the end

0 === 0

      

        
        
        
      

    

I hope this explains something :)