What does ~~ (“double tilde”) do in Javascript?

Question

I was checking out an online game physics library today and came across the ~~ operator. I know a single ~ is a bitwise NOT, would that make ~~ a NOT of a NOT, which would

Answer 1

Converting Strings to Numbers

console.log(~~-1);    // -1
console.log(~~0);     // 0
console.log(~~1);     // 1
console.log(~~"-1");  // -1
console.log(~~"0");   // 0
console.log(~~"1");   // 1
console.log(~~true);  // 1
console.log(~~false); // 0

~-1 is 0

if (~someStr.indexOf("a")) {
  // Found it
} else  {
  // Not Found
}

source