what is generator value when done javascript code example

Example: javascript generator function

function* idMaker() {
  var index = 0;
  while (true)
    yield index++;
}

var gen = idMaker();

console.log(gen.next().value); // 0
console.log(gen.next().value); // 1
console.log(gen.next().value); // 2
console.log(gen.next().value); // 3
// ...