js基本数据类型及数组对象判断

// 数据类型的判断
        var num=123;
        var bool=true;
        var str=‘qwe‘;
        var nu=null;
        var und=undefined;
        var arr=[];
        var obj={};
        // typeof(‘表达式‘)
        // typeof ‘变量名‘
        console.log(typeof(num));//number
        console.log(typeof bool);//boolean
        console.log(typeof str);//string
        console.log(typeof nu);//object
        console.log(typeof und);//undefined
        console.log(typeof arr);//object
        console.log(typeof obj);//object
        // 需要注意，在使用typeof时，判断null和Array的结果都为object
        
        // 判断对象和数组
        console.log(arr.length)//0
        console.log(obj.length)//undefined

        console.log(arr instanceof Array)//true
        console.log(obj instanceof Object)//true

        console.log(Object.prototype.toString.call(arr))//[object Array]
        console.log(Object.prototype.toString.call(obj))//[object Object]

        console.log(arr.constructor===Array);//true
        console.log(obj.constructor===Object);//true