JS运算符typeof 相关问题

[jirengu]

以下代码输出什么 ？why

    var str = new String("Hello");

    var result = typeof(str instanceof String);
    alert(result); //What is the output of the alert? 

    result = typeof typeof(str instanceof String);
    alert(result); //What is the output of the alert? 

    result = typeof typeof typeof(str instanceof String);
    alert(result); //What is the output of the alert?   

[Panda-HJN]

依次返回的是

//boolean
//string
//string

str是String的一个实例,所以str instanceof String返回的是 true
true是一个布尔值,所以typeof true返回的是 "boolean"

typeof用于判断数据类型,以字符串的形式返回数据类型. "boolean"是个字符串,所以 typeof "boolean" 返回的是"string"

typeof "string" 返回的是 "string"

[oceanpad]

boolean
string
string

str is String, so str instanceof String is true(Boolean). Then typeof Boolean is Boolean, return 'boolean'. typeof's result is String, so typeof typeof ... all is string.