code: function StringSubstr(start, length) {
    CHECK_OBJECT_COERCIBLE(this, "String.prototype.substr");
    var s = TO_STRING(this);
    var size = s.length;
    start = TO_INTEGER(start);
    length = IS_UNDEFINED(length) ? size : TO_INTEGER(length);
  
    if (start < 0) start = MaxSimple(size + start, 0);
    length = MinSimple(MaxSimple(length, 0), size - start);
  
    if (length <= 0) return '';
    return %_SubString(s, start, start + length);
  }
docs: ecma262/#sec-string.prototype.substr