Reflect.js
function ToPropertyKey(argument) {
            var key = ToPrimitive(argument, 3 /* String */);
            if (IsSymbol(key))
                return key;
            return ToString(key);
        }
function ToPropertyKey(argument) {
            var key = ToPrimitive(argument, 3 /* String */);
            if (IsSymbol(key))
                return key;
            return ToString(key);
        }
function ToPropertyKey ( argument ) { var key = ToPrimitive ( argument , 3 /* String */ ) ; if ( IsSymbol ( key ) ) return key ; return ToString ( key ) ; }
7.1.14 ToPropertyKey(argument) https://tc39.github.io/ecma262/#sec-topropertykey
7 . 1 . 14 ToPropertyKey ( argument ) https : // tc39 . github . io / ecma262 / #sec - topropertykey