Understand JS memory life cycle, stack memory and heap memory, deep and shallow copy in JS series (2) on one piece of paper

Write at the front: Under the javascript column, this is a series I am about to start writing, mainly in the era of rampant frameworks, even though I am at work A framework is used, but for interviews and technical advancement, the foundation of basic JS knowledge is the icing on the cake, and it is also a piece of knowledge that must be learned. Although car drivers do not need to know much about cars, they only need to master the common functions of cars. But if you know cars, you can drive better, and by the same token. Of course, an article will not just talk about one knowledge point. Generally, related knowledge points will be connected in series. While recording your own learning, you will share your own learning and encourage each other! If you can, please also please give me a like, your likes will also make me work harder to update!

Overview

• Eating time: 6-12 minutes
• Difficulty: Easy, don’t run, watch before leaving

JS memory life cycle

• Allocating memory

• Memory reading and writing

• Release memory

Stack memory and heap memory

JS data types

Before talking about stack memory and heap memory, everyone should We all know that JS is divided into two data types:

• Basic data type

  String, Number, Boolean, null, undefined, Symbol (fixed size, volume Lightweight, relatively simple)

• Reference data type

  Object, Array, Function (The size is not necessarily certain, it takes up a large space, and it is relatively complex )

Memory storage mechanism

var a=true;      //布尔型，基本数据类型var b='jack';    //字符型，基本数据类型var c=18;        //数值型，基本数据类型var d={name:'jack'};   //对象，引用数据类型var d=[0,1,2,3,4,5];   //数组，引用数据类型复制代码

It is precisely because of the different data types that their storage methods are also different, just like the poor and the rich in real life People’s residences are completely different (distant). Let’s take a look at a picture first:

You can see that a , b , c are all basic Data types, d and e are both reference data types. They have essential differences in storage methods. The values ​​of basic data types are stored in stack memory, while The value of the reference data type is stored in the heap memory. The stack memory only stores its reference in the stack memory (that is, its address in the heap memory). Just like its name, the reference data type

Memory access mechanism

The above is about storage. Next, let’s talk about variable access. Basic data types can directly access the value of variables from the stack memory, while reference data types must first access the variables from the stack memory. Find its corresponding reference address in the stack memory, and then take this reference address and search it in the heap memory to get the value of the variable

deep-shallow copy

• shallow Copy

  I have already told you about the difference in storage between basic data types and reference data types. So, the deep and shallow copy mentioned next, I believe you often encounter it in interview questions, the old way , first look at a piece of code

  var name='jack';var obj={  age:24};var nameCopy=name;var objCopy=obj;
  
  nameCopy='bob';
  objCopy.age=15;console.log(name);    //jackconsole.log(obj.age);     //15复制代码

  You will find that name is not affected, and our naming is to modify objCopy.age, why is it still affected? obj.age, this is because of the problem of dark and shallow copying. Let’s take a look at the picture below

# #The reason why this happens is that for basic types and reference types in JS, when we copy the reference type, what is copied is the reference address of the object, so when executing

var objCopy=obj; , the reference address of obj is copied to objCopy, so the two objects actually point to the same object, that is, the change objCopy also changes the value of obj. We call this situation shallow copy. It only copies the reference of the object and does not open a new one. The memory is short-handed and the copy space is too shallow. (Shallow copy will only occur with reference types)

• Deep copy
  Let’s look at the next piece of code

  var name='jack';var obj={  age:24};var nameCopy=name;var objCopy=JSON.parse(JSON.stringify(obj));
  
  nameCopy='bob';
  objCopy.age=15;console.log(name);    //jackconsole.log(obj.age);     //24复制代码

  It can be found that after the conversion through

  JSON.parse(JSON.stringify(obj)), the shallow copy no longer exists. This wave is deep copy, deep copy development A new heap memory address is obtained, and the reference of the object points to the newly opened memory address. It is completely independent from the previously copied object. It is self-reliant and copied deeply. Learn the skills and master the skills. a feeling of.

• 另外实现深拷贝的方法（更多方式请自行百度）

  var objCopy=Object.assign({},obj);   //对象深拷贝的方法 Object.assign
  var arrayCopy=array.concat();       //数组深拷贝的方法  concat()  （数组无嵌套对象或者数组可用）
  var arrayCopy=array.slice();       //数组深拷贝的方法  slice()   （数组无嵌套对象或者数组可用）
  JSON.parse(JSON.stringify(array))     //顺带提下，JSON.parse(JSON.stringify())   数组和对象通用复制代码

  接着上面的数组容易踩坑的地方 ，来看一个例子

  var array = [{name: 'jack'}, ['old']];var arrCopy = array.concat();
  arrCopy[0].name='new';console.log(array); // [{name: 'new'}, ['old']]console.log(arrCopy); // [{name: 'new'}, ['old']]复制代码

  可以清楚地看到(数组无嵌套对象或者数组可用的情况下用 concat 和 slice 才有效)

系列目录

• 一张纸搞懂JS系列（1）之编译原理，作用域，作用域链，变量提升，暂时性死区

• 一张纸搞懂JS系列（2）之JS内存生命周期，栈内存与堆内存，深浅拷贝

• 一张纸搞懂JS系列（3）之垃圾回收机制，内存泄漏，闭包

更多相关免费学习推荐：javascript（视频）

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