code: function StringConcat(other /* and more */) {  // length == 1
    "use strict";
    CHECK_OBJECT_COERCIBLE(this, "String.prototype.concat");
    var s = TO_STRING(this);
    var len = arguments.length;
    for (var i = 0; i < len; ++i) {
      s = s + TO_STRING(arguments[i]);
    }
    return s;
  }
docs: ECMA-262, section 15.5.4.6