Why does 2 == [2] in JavaScript?

Question

You can look up the comparison algorithm in the ECMA-spec (relevant sections of ECMA-262, 3rd edition for your problem: 11.9.3, 9.1, 8.6.2.6).

If you translate the involved abstract algorithms back to JS, what happens when evaluating 2 == [2] is basically this:

2 === Number([2].valueOf().toString())

where valueOf() for arrays returns the array itself and the string-representation of a one-element array is the string representation of the single element.

This also explains the third example as [[[[[[[2]]]]]]].toString() is still just the string 2.

As you can see, there's quite a lot of behind-the-scene magic involved, which is why I generally only use the strict equality operator ===.

The first and second example are easier to follow as property names are always strings, so

a[[2]]

is equivalent to

a[[2].toString()]

which is just

a["2"]

Keep in mind that even numeric keys are treated as property names (ie strings) before any array-magic happens.

It is because of the implicit type conversion of == operator.

[2] is converted to Number is 2 when compared with a Number. Try the unary + operator on [2].

> +[2]
2

var a = [0, 1, 2, 3];
a[[2]] === a[2]; // this is true

On the right side of the equation, we have the a[2], which returns a number type with value 2. On the left, we are first creating a new array with a single object of 2. Then we are calling a[(array is in here)]. I am not sure if this evaluates to a string or a number. 2, or "2". Lets take the string case first. I believe a["2"] would create a new variable and return null. null !== 2. So lets assume it is actually implicitly converting to a number. a[2] would return 2. 2 and 2 match in type (so === works) and value. I think it is implicitly converting the array to a number because a[value] expects a string or number. It looks like number takes higher precedence.

On a side note, I wonder who determines that precedence. Is because [2] has a number as it's first item, so it converts to a number? Or is it that when passing an array into a[array] it tries to turn the array into a number first, then string. Who knows?

var a = { "abc" : 1 };
a[["abc"]] === a["abc"];

In this example, you are creating an object called a with a member called abc. The right side of the equation is pretty simple; it is equivalent to a.abc. This returns 1. The left side first creates a literal array of ["abc"]. You then search for a variable on the a object by passing in the newly created array. Since this expects a string, it converts the array into a string. This now evaluates to a["abc"], which equals 1. 1 and 1 are the same type (which is why === works) and equal value.

[[[[[[[2]]]]]]] == 2; 

This is just an implicit conversion. === wouldn't work in this situation because there is a type mismatch.

For the == case, this is why Doug Crockford recommends always using ===. It doesn't do any implicit type conversion.

For the examples with ===, the implicit type conversion is done before the equality operator is called.

[0] == false // true
if ([0]) { /* executes */ } // [0] is both true and false!

That's interesting, it's not that [0] is both true and false, actually

[0] == true // false

It is javascript's funny way of processing if() operator.

A array of one item can be treated as the item itself.

This is due to duck typing. Since "2" == 2 == [2] and possibly more.

To add a little detail to the other answers... when comparing an Array to a Number, Javascript will convert the Array with parseFloat(array). You can try it yourself in the console (eg Firebug or Web Inspector) to see what different Array values get converted to.

parseFloat([2]); // 2
parseFloat([2, 3]); // 2
parseFloat(['', 2]); // NaN

For Arrays, parseFloat performs the operation on the Array's first member, and discards the rest.

Edit: Per Christoph's details, it may be that it is using the longer form internally, but the results are consistently identical to parseFloat, so you can always use parseFloat(array) as shorthand to know for sure how it will be converted.