how come ![] is not true?

Question

Was just playing around with nodejs and chrome's console when I tested this:

[] == true // false
![] == true // false
!![] == true // true

How come? Isn't it wrong?

@ColeJohnson—considering the `!` operator converts the LHS to boolean and the RHS is already boolean, the results will be identical. And for the second two, the two algorithms ([Abstract Equality](http://es5.github.com/#x11.9.3) and [Strict Equality](http://es5.github.com/#x11.9.6) Comparison Algorithm) do exactly the same steps, so there is zero benefit to using one over the other. There may be good reason to do `[] == 0` and not `[] === 0` since the former may be true or false (depending on the members in the arra), but the later will always be false. — RobG, Jun 14 '12 at 03:43

score 7 · Accepted Answer · answered Jun 14 '12 at 02:34

7

See the ECMAScript standard:

11.4.9 Logical NOT Operator ( ! )

The production UnaryExpression : ! UnaryExpression is evaluated as follows:

Let expr be the result of evaluating UnaryExpression.

Let oldValue be ToBoolean(GetValue(expr)).

If oldValue is true, return false.

Return true.

9.2 ToBoolean

The abstract operation ToBoolean converts its argument to a value of type Boolean according to Table 11:

undefined → false

null → false

Boolean → The result equals the input argument (no conversion).

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

The result is false if the argument is the empty String (its length is zero); otherwise the result is true.

Object → true

An array is an Object.

answered Jun 14 '12 at 02:34

Kijewski

25,517
12
101
143

but, weirdly enough, empty objects are neighter true or false D: – Hugo Mota Jun 14 '12 at 02:40
@hugo_leonardo: Empty objects are always `true` when evaluated as a Boolean. – Jun 14 '12 at 02:42
`a = {}; a == true; a == false;` both comparisons returned false in my console... – Hugo Mota Jun 14 '12 at 02:44
3

@hugo_leonardo: The `==` operator does not cast its operands to boolean values – Jun 14 '12 at 02:46
@hugo_leonardo just as am not i am says: `!` takes the boolean value (as per 2nd bullet point of 11.4.9). `==` does not do such a thing (see section 11.9.3). – Kijewski Jun 14 '12 at 02:49
@amnotiam I guess this explains it ~ so the only one doing the casting is the `!` operator. Thank you (: – Hugo Mota Jun 14 '12 at 02:54
2

@hugo_leonardo: Study the *Abstract Equality Comparison Algorithm* http://es5.github.com/#x11.9.3 You'll see that when the two operands are of different types, the conversion is almost always a *toNumber* conversion. An Object gets a *toPrimitive* conversion. In the case of an Array, I believe it first gets *toString*, followed by the *toNumber*. An empty array is therefore converted to `""`, then to `0`. A boolean gets converted *toNumber*, so `false` is `0`. `0 == 0` – Jun 14 '12 at 02:55
...yes, the `!` takes it straight to Boolean, which is `true`, but then negated to `false`, so `false == false`. – Jun 14 '12 at 02:56
@amnotiam— in the first example, the RHS is convereted using *toNumber* but the LHS is an object, so it uses *toPrimitive*, which will be to an empty string so it becomes `'' == 1` which evaluates to false. Note that `[1] == true` returns true as it's converted to `'1' == 1`. – RobG Jun 14 '12 at 03:54
@RobG: Right, I wasn't differentiating between an Object and an Array. I was just showing that in the case of an empty Array object, it's converted toString `""`, then ultimately toNumber as the conversions continue. – Jun 14 '12 at 11:40

Cole Tobin · Answer 2 · 2012-06-14T02:33:43.593

2

It has to do with how the browser handles implicit conversions in JS.

As [] is an empty array, it evaluates to false:

[] == true
false == true

However, noting it, will turn the object into a boolean with a value of false:

![] == true
false == true

However, boolean([]) will return true.

However, noting that, will turn it into a boolean with the value of !false:

!![] == true
!false == true
true == true

This is why implicit conversions arn't recomended unless needed:

"1" == true
true == true

"1" === true
false

edited Jun 14 '12 at 02:33

answered Jun 14 '12 at 02:28

Cole Tobin

9,206
15
49
74

it's the same on node.js (edit: well, off course its the same engine. but it's the same on IE's console too) – Hugo Mota Jun 14 '12 at 02:31
@zneak that was a mistake, fixed – Cole Tobin Jun 14 '12 at 02:31
`Boolean([])` -> true, try it. – FatalError Jun 14 '12 at 02:32
An empty array does not evaluate to false: Boolean([]) // true – Trevor Jun 14 '12 at 02:33
but if `Boolean([])` evaluates to true (I expected that too, because it's an object), how come `[] == true` evaluates to false? – Hugo Mota Jun 14 '12 at 02:34
3

@hugo_leonardo, because implicit casts and explicit casts are not the same. Welcome to Javascript, where casts are not isomorphic. – zneak Jun 14 '12 at 02:35
Wow, that's confusing. `[] == false` evaluates to true, but `[] || 1` evaluates to `[]`. Sometimes I just don't get javascript... – FatalError Jun 14 '12 at 02:38
1

@FatalError: There's no way around the fact that the `==` algorithm is a little complex. The first thing to always remember is that `==` is not doing a Boolean evaluation of its operands. When people assume that, it becomes understandably confusing. – Jun 14 '12 at 02:47

score 0 · Answer 3 · answered Jun 14 '12 at 02:37

[] == true is false because [] is not equal to true, just like "some string" is also not equal to true.

![] == true is false because [] evaluates to a true value when used in a conditional statement:

if([]) console.log('[]');
if(![]) console.log('![]');
// the result will be '[]' because [] will evaluate to true
// in a conditional even though it doesn't equal true

Another thing that may help you to think about it is ![] == false will be true.

!![] == true is true because !! converts anything to a true or false value based on whether it would be true or false in a conditional statement. So if(obj) and if(!!obj) will always have the same result.

I personally use the terms "equal" for `==` and "identical" for `===`, when a distinction between the two is important. Just my 2 cents. — zneak, Jun 14 '12 at 05:07

how come ![] is not true?

3 Answers3

11.4.9 Logical NOT Operator ( ! )

9.2 ToBoolean