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

The ~ is Bitwise NOT Operator, ~x is roughly the same as -(x+1). It is easier to understand, sort of. So:

~2;    // -(2+1) ==> -3

Consider -(x+1). -1 can perform that operation to produce a 0.

In other words, ~ used with a range of number values will produce a falsy (coerce to false from 0) value only for the -1 input value, otherwise, any other truthy value.

As we know, -1 is commonly called a sentinel value. It is used for many functions that return >= 0 values for success and -1 for failure in C language. Which the same rule of return value of indexOf() in JavaScript.

It is common to check of presence/absence of a substring in another string in this way

var a = "Hello Baby";

if (a.indexOf("Ba") >= 0) {
    // found it
}
if (a.indexOf("Ba") != -1) { 
    // found it
}

if (a.indexOf("aB") < 0) { 
    // not found
}
if (a.indexOf( "aB" ) == -1) { 
    // not found
}

However, it would be more easily to do it through ~ as below

var a = "Hello Baby";

~a.indexOf("Ba");         // -7   -> truthy
if (~a.indexOf("Ba")) {   // true
    // found it
}

~a.indexOf("aB");         // 0    -> falsy
!~a.indexOf("aB");        // true
if (!~a.indexOf( "aB" )) {  // true
    // not found
}

You Don't Know JS: Types & Grammar by Kyle Simpson