How to implement deep and shallow copies of arrays and objects

Premise: The difference between primitive data types and object types when assigning values

JavaScript data types are divided into primitive data types and object types. The way they are stored in memory is different, resulting in differences in their assignment. Let’s take a chestnut respectively

  var x = 1;  var y = x;  //y获得了和x同样的值
  y = 2;
  console.log(x);  // 1

  var m = [1,2]; //m存放的是指向[1,2]这个数组对象的引用地址
  var n = m; //n也获得 [1,2]数组对象的引用地址
  n[0] = 3;
  console.log(m);  //[3,2]

From the chestnut above, we can see that when the original data type is assigned, the actual data value is given. After the assignment, the two values ​​are the same and will not affect each other. ; The object type gives the reference address of the original data, so the old and new data will affect each other, because it is essentially the same data object , such as the array in the above list

What is a shallow copy?

　As the name suggests, shallow copy is a superficial copy method; when the attribute value is an object type, only the reference to the object data is copied, resulting in the old and new data not being completely separated. will also affect each other. Take another example...

  //测试数据
  var array1 = ['a',1,true,{name:'lei',age:18}];

  //concat()  slice() 实现浅拷贝
  var array2 = array1.concat()

  //修改拷贝后的数据
  array2[0] = 'b';            //array1[0]是原始数据类型 所以是直接赋值的
  array2[3].name = 'zhang';   //array1[3]是对象数据类型 所以拷贝的是对象的引用，其实还是和原数组使用同一对象

  console.log(array1);   //  ['a',1,true,{name:'zhang',age:18}]

In the chestnut, array2 is a shallow copy object of array1. The array elements are of primitive data types and will not affect each other (array1[0] ), but array1[3] is an object type and will still affect each other.

How to implement shallow copy

1. array.concat() or array.slice() in Shangli is a special implementation of shallow copy of array Way.

2. How to implement it yourself? Wouldn’t it be enough to traverse each attribute of the object/array and then assign it to a new object? The following implementation

  //实现浅拷贝
  function shallowCopy( target ){
    if(typeof target !== 'object') return ;
    //判断目标类型，来创建返回值
    var newObj = target instanceof Array ? [] : {};

    for(var item in target){
      //只复制元素自身的属性，不复制原型链上的
      if(target.hasOwnProperty(item)){
        newObj[item] = target[item]
      }
    }

    return newObj
  }

  //测试

  var test = [1,'a',{name:'lei',age:18}];

  var copy = shallowCopy(test);
  console.log(copy[2].name);   //lei

  copy[2].name = 'zhang';
  console.log(test[2].name);   //zhang   原数据也被修改

deep copy and its implementation

From shallow copy The explanation is basically understandable. Deep copy is a 'complete' copy. After copying, the old and new data are completely separated. They no longer share the attribute values ​​of the object type and will not affect each other.

Implementation method:

1. Tricky method JSON.parse(JSON.stringify(Obj))
   

var test = [1,'a',{name:'lei',age:18}];

var copy1 = JSON.parse(JSON.stringify(test));  //特殊方式

console.log(copy1);

copy1[2].name = 'zhang'
console.log(test);   //[1,'a',{name:'lei',age:18}]  未受到影响

Note: This method cannot deep copy objects whose attribute values ​​are functions. You can try it yourself

　2. Implement deep copy

　Having implemented shallow copy, think about it It should be that when assigning the object type attribute value, the result is not complete separation, so we need to modify the way of copying the object type attribute value, and call a deep copy on it again, thus realizing the deep copy, as follows:

//实现深拷贝
function deepCopy( target ){
  if(typeof target !== 'object') return ;
  //判断目标类型，来创建返回值
  var newObj = target instanceof Array ? [] : {};

  for(var item in target){
    //只复制元素自身的属性，不复制原型链上的
    if(target.hasOwnProperty(item)){
      newObj[item] = typeof target[item] == 'object' ? deepCopy(target[item]) : target[item]  //判断属性值类型    }
  }

  return newObj
}

//测试
var test = [1,'a',{name:'lei',age:18}];

var copy2 = deepCopy(test);
copy2[2].name = 'zhang'

console.log(test);  ////[1,'a',{name:'lei',age:18}]  未受到影响

Summary

Be sure to understand the reasons for shallow copy: when copying object type data, the reference address is copied, and the same data object is used; so the way to implement deep copy is to copy the object type attribute value Perform deep copy recursively to avoid direct assignment.

The above is the detailed content of How to implement deep and shallow copies of arrays and objects. For more information, please follow other related articles on the PHP Chinese website!