CODEWITHSUNDEEP
javascript
Paul S.
Paul S.

Reputation: 66334

When is (true == x) === !!x false?

JavaScript has different equality comparison operators

It also has a logical NOT ! and I've tended to think of using a double logical NOT, !!x, as basically the same as true == x.

However I know this is not always the case, e.g. x = [] because [] is truthy for ! but falsy for ==.

So, for which xs would (true == x) === !!x give false? Alternatively, what is falsy by == but not !! (or vice versa)?

Upvotes: 2

Views: 1126

Answers (2)

I Hate Lazy
I Hate Lazy

Reputation: 48761

"So, for which xs would (true == x) === !!x give false?"

Any x where its Boolean conversion is not the same as its conversion by the Abstract Equality Comparison Algorithm.

An example is a string with only whitespace:

var x = " ";

Its Boolean conversion is true (as is the case with any non-empty string), but its == comparison is false because a string with only white space will be converted to the number 0, and the true value will be converted to the number 1, and those values are not equal.

x == true; // false
!!x;       // true

or to show the ultimate values the == is comparing:

Number(true) == Number(x);
 //      1   ==        0

1 == 0; // false

and to show the result of !!x, it would be equivalent to this:

Boolean(x);  // true

So your original expression could crudely be seen as the following:

   var x = " ";

   (Number(true) == Number(x)) === Boolean(x);
// (         1   ==        0 ) ===        true
//                   ( false ) ===        true

   false === true; // false

I say "crudely" because this certainly doesn't capture all the detail of the algorithm linked above, and won't be the same for all values provided to the operands.

To understand how == treats its operands, you really need to study the algorithm a bit.

Upvotes: 8

SLaks
SLaks

Reputation: 887449

Loose equality has nothing to do with truthiness.

The rules for loose equality basically involve comparing the results of the .valueOf() function for each object.
For more details, see the spec.

Upvotes: -1

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