[front end -- JavaScript] knowledge point (IV) -- memory leakage in the project (I)

Hundred years of solitude 2021-08-08 17:56:37
end javascript knowledge point iv

This is my participation 8 The fourth of the yuegengwen challenge 5 God , Check out the activity details :8 Yuegengwen challenge **

Recently, in the process of writing small programs , Used a lot of console.log, At first, it was for the convenience of joint commissioning , Never deleted , Cause the page to jam from time to time , I began to think it would be good to refresh the page , But products and tests won't be allowed , Then I checked the problem , Just found that there are many kinds of... In the page Memory leak , Think of the previous interview, this question was also tested many times , There has been no summary , Take the opportunity to summarize today, or remind yourself to pay more attention to .

What is memory leak ?

First of all, we have to know what a memory leak is , There is a garbage collection mechanism in the engine , Mainly for some objects that are no longer used in the program , Clean up and recycle to free up memory , But in fact, the garbage collection mechanism does not recycle all objects that are no longer used . Therefore, we should actively avoid some operations that are not conducive to the engine's garbage collection in the code , These object memories that are not reclaimed in time , We call it the Memory leak (Memory Leak).

Common memory leaks

Let's take a look at some common memory leaks :

Improper closure

Most people think that closures are nested inside functions and return A function , I looked through the descriptions in several books :

  • JavaScript Advanced programming : A closure is a function that has access to variables in the scope of another function .
  • JavaScript Authoritative guide : From a technical point of view , be-all JavaScript Functions are all closures , They are all objects , Are associated with scope chains .
  • You don't know JavaScript: When a function can remember and access its lexical scope , There's a closure , Even if the function is executed outside the current lexical scope .

Closures cover a wide range , Now let's look at a common example and look at closures :

function fnOne(){
  let test = new Array(1000).fill('www')
  return function(){
let fn1Child = fnOne()
 Copy code 

Here because return There is a function in the function of fnOne Medium test Variable references ,test Will not be recycled , This causes a memory leak .

How to solve it ? Just after the function call , Just leave the external reference empty :

function fnOne(){
  let test = new Array(1000).fill('www')
  return function(){
return test
let fn1Child = fnOne();
fn1Child = null;
 Copy code 

Reduce the use of closures , Improper use of closures may cause memory leaks .

Implicit global variables

stay JavaScript in , Garbage collection is automatic , But for global variables , It is difficult for the garbage collector to determine when these variables are not needed , So global variables are not recycled , At this time, a memory leak occurs :

function fn(){
  //  There is no declaration, which creates an implicit global variable test1
  test1 = new Array(1000).fill('isboyjc1')
  //  Internal function this Point to window, Made implicit global variables test2
  this.test2 = new Array(1000).fill('isboyjc2')
 Copy code 

The timer of forgetting

let someResource = getData()
setInterval(() => {
  const node = document.getElementById('Node')
 if(node) {
    node.innerHTML = JSON.stringify(someResource))
}, 1000)
 Copy code 

In the code , Put the obtained data into... Every second node In nodes , But in setInterval Not before the end , The variables in the callback function and the callback function itself cannot be recycled . You need to use clearInterval To clean up this timer , Can be recycled someResource.

setTiemout The same is true , Don't need time , Clean up in time .

Forgotten event listener

In the code, we will use the event listener , Mount related functions in the component , When components are not actively cleared during destruction , The variables and functions are considered necessary , It won't be recycled , These internally referenced variables store a lot of data , It will cause the page to occupy too much memory , Memory leak .

Forgotten listener mode

Whether it's Vue still React, The listener pattern enables some message communication Very common , such as EventBus, When we implement the listener mode, mount the relevant functions in the component , Do not actively remove components when they are destroyed , It will also not be recycled . At this time, we are going to beforeDestroy  Timely removal in the component destruction life cycle .

Uncleaned Console Output

Finally, it is common in the joint commissioning console.log 了 , We can see the data output in the browser console , Memory leaks are also caused when objects are output .

Almost finished with the common memory leaks , The next article will talk about how to check whether there are... In the location code Memory leak .

本文为[Hundred years of solitude]所创,转载请带上原文链接,感谢

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