What does a tilde do when it precedes an expression?

Question

var attr = ~\'input,textarea\'.indexOf( target.tagName.toLowerCase() )
           ? \'value\'
           : \'innerHTML\'

I saw it in an answer, and

Answer 1

Using it before an indexOf() expression effectively gives you a truthy/falsy result instead of the numeric index that's directly returned.

If the return value is -1, then ~-1 is 0 because -1 is a string of all 1 bits. Any value greater than or equal to zero will give a non-zero result. Thus,

if (~someString.indexOf(something)) {
}

will cause the if code to run when "something" is in "someString". If you try to use .indexOf() as a boolean directly, then that won't work because sometimes it returns zero (when "something" is at the beginning of the string).

Of course, this works too:

if (someString.indexOf(something) >= 0) {
}

and it's considerably less mysterious.

Sometimes you'll also see this:

var i = ~~something;

Using the ~ operator twice like that is a quick way to convert a string to a 32-bit integer. The first ~ does the conversion, and the second ~ flips the bits back. Of course if the operator is applied to something that's cannot be converted to a number, you get NaN as a result. (edit — actually it's the second ~ that is applied first, but you get the idea.)