js中的数据类型、非运算、等值判断

1 js的数据类型

//console.log(typeof arr)，输出变量的类型
var arr;                        //undefined

arr = function foo() {          //function
// body...
}

arr = [];               //object
arr[0] = 1;             //object
arr = null;             //object
arr = new Date();       //object
arr = "test";           //string
arr = 1;                //number

2 js空值的非运算与等值判断

var obj1;
var obj2 = null;
var obj3 = "";
var obj4 = [];

//非运算
console.log(!obj1);         //true
console.log(!obj2);         //true
console.log(!obj3);         //true
console.log(!obj4);         //false,空数组应该判断长度
console.log(obj4.length);   //0

//等值判断
console.log(obj1 == obj2);      //true, undefinded = null
console.log(obj1 === obj2);     //false, undefinded不严格等于null
console.log(obj2 == null);      //true, null = null
console.log(obj3 == "");        //true
console.log(obj4 == []);        //false, 空数组not equal []

3 array和object的等值判断

var arr1 = [1, 2, 3];
var arr2 = [1, 2, 3];

console.log(arr1 == arr2);          //false
console.log(arr1 == [1, 2, 3]);     //false

var obj1 = {"a" : "1"};
var obj2 = {"a" : "1"};

console.log(obj1 == obj2);          //false
console.log(obj1 == {"a" : "1"});   //false