Shallow copy, deep copy (implementation mode)

Both shallow and deep copies are only for reference data types , Shallow copy copies only pointers to an object , Instead of copying the object itself , new
Old objects still share the same memory ; But deep copy creates as like as two peas , The new object does not share memory with the original object ,
Modifying the new object does not change to the original object ;

difference ： A shallow copy copies only the first level properties of an object 、 Deep copy can recursively copy the properties of an object ;

Implement shallow copy method

Object.assign Method

var obj = {
     a: 1, b: 2 }
var obj1 = Object.assign({
    },obj); 
boj1.a = 3; console.log(obj.a) // 3

for in Method

//  Copy only the shallow copy of the first layer 
function simpleCopy(obj1) {
    
	 var obj2 = Array.isArray(obj1) ? [] : {
    }; 
	 for (let i in obj1) {
    
	 	 obj2[i] = obj1[i];
	 }
	 return obj2; 
}
var obj1 = {
     a: 1, b: 2, c: {
     d: 3 } }
var obj2 = simpleCopy(obj1); 
obj2.a = 3; 
obj2.c.d = 4;
alert(obj1.a); // 1 
alert(obj2.a); // 3 
alert(obj1.c.d); // 4 
alert(obj2.c.d); // 4

Implement the deep copy method

Use recursion to copy all hierarchical attributes

function deepClone(obj){
     
	let objClone = Array.isArray(obj)?[]:{
    }; 
	if(obj && typeof obj==="object"){
     
		for(key in obj){
     
			if(obj.hasOwnProperty(key)){
     
				// Judge ojb Is the child element an object , If it is , Recursive replication  
				if(obj[key]&&typeof obj[key] ==="object"){
     
					objClone[key] = deepClone(obj[key]); 
				}else{
    // If not , Simple replication  
					objClone[key] = obj[key]; 
				} 
			} 
		} 
	}
	return objClone; 
}
let a=[1,2,3,4], b=deepClone(a); 
a[0]=2; 
console.log(a,b);

Use JSON.stringify and JSON.parse Implement deep copy ：JSON.stringify Turn the object into a string , Reuse JSON.parse Turn a string into a new object ;

function deepCopy(obj1){
    
	 let _obj = JSON.stringify(obj1); 
	 let obj2 = JSON.parse(_obj); 
	 return obj2; 
} 
var a = [1, [1, 2], 3, 4]; 
var b = deepCopy(a); 
b[1][0] = 2; 
alert(a); // 1,1,2,3,4 
alert(b); // 2,2,2,3,4

Popular function libraries lodash, There are also offers _.cloneDeep For deep copy ;

var _ = require('lodash'); 
var obj1 = {
     a: 1, b: {
     f: {
     g: 1 } }, 
c: [1, 2, 3] };
var obj2 = _.cloneDeep(obj1); 
console.log(obj1.b.f === obj2.b.f); // false