Home > Article > Web Front-end > 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!
Before talking about stack memory and heap memory, everyone should We all know that JS is divided into two data types:
String, Number, Boolean, null, undefined, Symbol (fixed size, volume Lightweight, relatively simple)
Object, Array, Function (The size is not necessarily certain, it takes up a large space, and it is relatively complex )
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
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
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
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)
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.
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)
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