1、关于for in
for in : 遍历对象中的可枚举的属性
例子1:for in 遍历对象的键为String类型,所以调用时用Object[key]形式,而不用Object.key形式
<script>
var obj = {
name: "zxq",
age: 22,
sex: "男"
}
for (var key in obj) {
console.log(key + ", " + obj[key]); //正确
console.log(key + ", " + obj.key); //错误,找名为key的属性,undefined
console.log(typeof(key)); //string
}
</script>
例子2:for in 只能遍历属性为可枚举的属性,对于数组或字符串中不可枚举的属性或方法而言
<script>
var obj = {
name: "zxq",
age: 22,
sex: "男"
}
obj.say = function() {
console.log('this is say');
}
console.log(Object.getOwnPropertyDescriptor(obj, 'say')); //可枚举{writable: true, enumerable: true, configurable: true, value: ƒ}
Object.defineProperty(obj, 'final', {
value: 'flase',
enumerable: false
}); //设置不可枚举的属性
for (var key in obj) {
console.log(key + ", " + obj[key]);
}
console.log(obj);
</script>
例子2截图:
2、关于sort()
数组的排序方法,默认排序的顺序是根据字符串UniCode码,参数为可选的比较函数
例子1:没有参数时
<script>
var arr = [1, 2, 3, 12];
arr.sort();
console.log(arr); //[1, 12, 2, 3]
arr = [1, 2, 4, [2, 1],
[2, 2],
[3, 1, 2]
];
arr.sort();//会将值转为字符串,根据unicode编码排序
console.log(arr);
</script>
例子1截图:
例子2:有参数时,根据比较函数的返回值来排序,具体排序算法?(先相邻比较,在二分比较)[待学习确定]
<script>
var arr = [1, 2, 12, 5, 4];
arr.sort(function(a, b) {
console.log('a: ' + a + ' , ' + 'b: ' + b);
console.log('a-b = ' + (a - b));
return a - b;
});
console.log(arr);
arr = [1, 2, [3, 4],
[3, 2],
[4, 5, 6], 4
];
arr.sort(function(a, b) {
console.log('a: ' + a + ' , ' + 'b: ' + b);
console.log('a-b = ' + (a - b)); //存在NaN,console.log(Boolean(NaN));//false
return a - b;
})
console.log(arr);
</script>
例子2截图:
3、prototype和_proto_
实例对象的__proto__指向它的原型,原型链概念为按照__proto__一级一级向上查找属性或方法
prototype是构造函数中的一个指针,指向构造函数的原型对象
例子1:实例用__proto__来访问它的原型,构造函数用prototype来访问原型对象
<script>
function Fun(name) {
this.name = name
}
var a = new Fun('a');
var b = new Fun('b');
console.log(a.__proto__.constructor);
console.log(b);
console.log(Fun.prototype == a.__proto__); //true
console.log(Fun.prototype == b.__proto__); //true
</script>
例子2:常用方法,hasOwnProperty:判断对象本身的名称和属性 isPrototypeOf: object1.isPrototypeOf(object2),判断object1是否在object2的原型链上
<script>
class Fun {
constructor(name) {
this.name = name;
}
print() {
console.log('this name is ' + this.name);
}
}
class Son extends Fun {
constructor(name, age) {
super(name);
this.age = age;
}
}
var a = new Fun('zxq');
var b = new Son('zzz', 22);
console.log(a);
console.log(b);
console.log(a.hasOwnProperty('name')); //true
console.log(Fun.isPrototypeOf(Son)); //true
</script>