js问题记录(二) -- 关于typeof,instanceof,Object.prototype.toString.call()

1、关于typeof

typeof(item):返回item的数据类型

js有基本数据类型number,string,null,undefined,boolean,symbol等，其余为引用类型object的变形

由下示例可以看出：
typeof():可以检测出：string,function,undefined,number,boolean等类型
但不可以检测：null,Array,Date,RegExp等对象的类型
代码示例：

    <script>
        var str = '123',
            arr = [],
            fun = function() {},
            n = null,
            un = undefined,
            number = 123,
            flag = true,
            date = new Date(),
            reg = new RegExp();
        console.log(typeof(str)); //string
        console.log(typeof(arr)); //object
        console.log(typeof(fun)); //function
        console.log(typeof(n)); //object
        console.log(typeof(un)); //undefined
        console.log(typeof(number)); //number
        console.log(typeof(flag)); //boolean
        console.log(typeof(date)); //object
        console.log(typeof(reg)); //object
    </script>

2、关于instanceof

A instanceof B：判断A是否是B的实例，返回boolean类型；

原理：检查一个对象A的原型链中是否有构造函数B的prototype属性

代码示例：

    <script>
        console.log(arr instanceof Array); //true
        console.log(str instanceof String); //false
        str = new String(123);
        console.log(str instanceof String); //true
        console.log(date instanceof Date); //true
        console.log(reg instanceof RegExp); //true
    </script>

3、关于Object.prototype.toString.call()

Object上的toString()返回当前this的详细信息

代码示例：

    <script>
        console.log(Object.prototype.toString.call(str)); //[object String]
        console.log(Object.prototype.toString.call(arr)); //[object Array]
        console.log(Object.prototype.toString.call(n)); //[object Null]
        console.log(Object.prototype.toString.call(un)); //[object Undefined]
        console.log(Object.prototype.toString.call(number)); //[object Number]
        console.log(Object.prototype.toString.call(flag)); //[object Boolean]
        console.log(Object.prototype.toString.call(date)); //[object Date]
        console.log(Object.prototype.toString.call(reg)); //[object RegExp]

        let type = Object.prototype.toString.call(str);
        type = type.slice(8, type.length - 1);
        console.log(type);//String
    </script>