JS的数据类型/判断方法/栈与堆/深浅拷贝

console.log(2 instanceof Number);                    // false
console.log(true instanceof Boolean);                // false 
console.log('str' instanceof String);                // false  
console.log([] instanceof Array);                    // true
console.log(function(){} instanceof Function);       // true
console.log({} instanceof Object);                   // true    
console.log(undefined instanceof Undefined);// 报错
console.log(null instanceof Null);//报错

console.log((2).constructor === Number); //true
console.log((true).constructor === Boolean); //true
console.log(('str').constructor === String); //true
console.log(([]).constructor === Array); //true
console.log((function() {}).constructor === Function); //true
console.log(({}).constructor === Object); //true

function Fn(){}; //原来是方法
Fn.prototype=new Array(); //改变原型为数组
var f=new Fn();
console.log(f.constructor===Fn);    // false
console.log(f.constructor===Array); // true

var a = Object.prototype.toString;
 
console.log(a.call(2));
console.log(a.call(true));
console.log(a.call('str'));
console.log(a.call([]));
console.log(a.call(function(){}));
console.log(a.call({}));
console.log(a.call(undefined));
console.log(a.call(null));

[object Number]
[object Boolean]
[object String]
[object Undefined]
[object Null]
[object Array]
[object Function]
[object Object]

var a = 2
var b = a
a = 1
console.log(a, b) // 1 ,2 ——a,b指向栈里不同数据 

var a = {c: 2}
var b = a
a.c = 1
console.log(a.c,b.c) //1，1 —— a,b指向堆里同份数据

/**
    数组的浅拷贝
**/
//1、基本 =
var arr1 = [1, 2, 3]
var arr2 = arr1
arr1[0]=100
console.log(arr1,arr2) // [ 100, 2, 3 ] [ 100, 2, 3 ]

//2、slice
var arr3 = [1, 2, 3]
var arr4 = arr3.slice(-1) // 取数组最后一个元素
arr3[2] = 100
console.log(arr3,arr4) // [ 1, 2, 100 ] [ 3 ]
//看起来修改旧数组不改变新数组，像是深拷贝了
//但是！！！
var arr5 = [1, 2, 3, {b: 4}]
var arr6 = arr5.slice(-1)
arr5[3].b = 100
console.log(arr5, arr6) //[ 1, 2, 3, { b: 100 } ] [ { b: 100 } ]
// 如果数组里元素是个引用类型，那么旧数组里这个元素被改变，会影响新数组
// 所以slice()方法是浅拷贝

//3、concat 同上理

//4、遍历
var arr7 = [1,2,3,{b:4}]
var arr8 = []
for (var i = 0; i < arr7.length; i ++) {
    arr8.push(arr7[i])
}
arr7[3].b = 100
console.log(arr7, arr8) // [ 1, 2, 3, { b: 100 } ] [ 1, 2, 3, { b: 100 } ]

// 1、 对象浅拷贝 - Object.assign
function shallowCopy4(origin) {
    return Object.assign({},origin)
}

//2、 对象浅拷贝 - 扩展运算符
// 扩展运算符（...）用于取出参数对象的所有可遍历属性，拷贝到当前对象之中
function shallowCopy5(origin) {
    return {
        ...origin
    }
}

function deepClone1(origin) {
    return JSON.parse(JSON.stringify(arr));
}