SOURCE

var str = 'abc';
str += 1;
var test = typeof(str); //  test == 'string'
// console.log(str)
if (str.length == 6) {
    test.sign = "typeof的返回结果可能为String";
    // new String(test).sign = 'xxx' -->  delete
} 
// new String(test).sign = 'xxx'; -->  delete
console.log(test.sign);