deps/v8/src/js/array.js
function ArrayFindIndex(predicate, thisArg) {
  CHECK_OBJECT_COERCIBLE(this, "Array.prototype.findIndex");

  var array = TO_OBJECT(this);
  var length = TO_INTEGER(array.length);

  return InnerArrayFindIndex(predicate, thisArg, array, length);
}
function ArrayFindIndex(predicate, thisArg) {
  CHECK_OBJECT_COERCIBLE(this, "Array.prototype.findIndex");

  var array = TO_OBJECT(this);
  var length = TO_INTEGER(array.length);

  return InnerArrayFindIndex(predicate, thisArg, array, length);
}
ES6 draft 07-15-13, section 15.4.3.24