How to solve the garbage collection mechanism and common memory leak problems in JS elevation-JS Tutorial-php.cn

This article mainly introduces how to solve the garbage collection mechanism and common memory leak problems in JS elevation. It has certain reference value. Now I share it with you. Friends in need can refer to it

Preface

The reason for this is because I wanted to understand the memory leak mechanism of closures, and then I remembered the analysis of the garbage collection mechanism in "JS Advanced Programming". I didn't understand it very well before, but I will understand it when I look back after a year. , write to share with everyone. If you like it, you can like/follow and support it.

Memory life cycle:

Allocate the memory you need:

Due to string, object There is no fixed size. Every time a js program creates a string or object, the program will allocate memory to store that entity.

Do something with the allocated memory.
Release it when no longer needed Return:

When strings and objects are no longer needed, the memory occupied by them needs to be released. Otherwise, all available memory in the system will be consumed, causing the system to crash. This is the meaning of the garbage collection mechanism.

The so-called memory leak refers to: due to negligence or error, the program fails to release memory that is no longer used, resulting in a waste of memory.

Garbage collection mechanism:

In languages such as C and C, memory needs to be managed manually, which is also the source of many unnecessary problems. Fortunately, in the process of writing js, memory allocation and memory recycling are completely automatically managed, and we don't have to worry about this kind of thing.

Principle of the garbage collection mechanism:

The garbage collector will periodically find variables that are no longer used at fixed time intervals, and then release the memory they occupy.

What is a variable that is no longer used?

Variables that are no longer used are variables that have ended their life cycle and are local variables. Local variables only exist during the execution of the function. When the function ends, there are no other references (closures). Then the variable will be marked for recycling.

The life cycle of global variables will not end until the browser unloads the page, which means global variables will not be treated as garbage collection.

Mark clearing: the currently adopted garbage collection strategy

Working principle:

When a variable enters the environment (for example, declaring a variable in a function), mark the variable as "Enter the environment", when a variable leaves the environment, it is marked as "leaving the environment". The memory marked "leaving the environment" is recycled.

Workflow:

The garbage collector will mark all variables stored in the memory during operation.
Remove the tags of variables in the environment and variables referenced by variables in the environment.
Those
variables that still have tags are considered variables to be deleted.
Finally, the garbage collector will perform the final step of memory clearing,
destroy those marked values and reclaim the memory space they occupy.

As of 2008, IE, Chrome, Fireofx, Safari, Opera

all use the mark-and-sweep garbage collection strategy, but the garbage collection time intervals are different. There are different.

Reference counting omitted: obsolete garbage collection strategy

Circular reference: a technique for tracking each value referenced

In older browsers (yes, IE again), BOM and DOM objects below IE9 are implemented in the form of COM objects using C.

COM's garbage collection mechanism uses a reference counting strategy. This mechanism can never release memory when a circular reference occurs.

    var element = document.getElementById('something');
    var myObject = new Object();
    myObject.element = element; // element属性指向dom
    element.someThing = myObject; // someThing回指myObject 出现循环引用(两个对象一直互相包含 一直存在计数)。

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The solution is to manually cut off the links when we don't use them:

     myObject.element = null; 
     element.someThing = null;

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Elimination:

IE9 removes BOM and DOM objects Converted to a real js object, avoiding the use of this garbage collection strategy, eliminating the main cause of common memory leaks below IE9.

There is a performance issue with a confusing statement under IE7. Let’s take a look:

256 variables, 4096 object (or array) literals or 64KB strings, Reaching any critical value triggers the garbage collector to run.
If a js script keeps so many variables throughout its life cycle, the garbage collector will keep running frequently, causing serious performance problems.

IE7 has fixed this problem.

What situations can cause memory leaks?

Although there is a garbage collection mechanism, when we write code, some situations will still cause memory leaks. If we understand these situations and pay attention to avoid them when writing programs, our programs will be more robust. .

Unexpected global variables:

We mentioned above that

global variables will not be treated as garbage collection. Sometimes the following situation occurs in our coding:

    function foo() {
     this.bar2 = '默认绑定this指向全局' // 全局变量=> window.bar2
      bar = '全局变量'; // 没有声明变量 实际上是全局变量=>window.bar
    }
    foo();

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当我们使用默认绑定，this会指向全局，this.something也会创建一个全局变量，这一点可能很多人没有注意到。

解决方法：在函数内使用严格模式or细心一点

    function foo() {
      "use strict"; 
      this.bar2 = "严格模式下this指向undefined"; 
      bar = "报错";
    }
    foo();

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当然我们也可以手动释放全局变量的内存：

    window.bar = undefined
    delete window.bar2

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被遗忘的定时器和回调函数

当不需要setInterval或者setTimeout时，定时器没有被clear，定时器的回调函数以及内部依赖的变量都不能被回收，造成内存泄漏。

var someResource = getData();
setInterval(function() {
    var node = document.getElementById('Node');
    if(node) {
        node.innerHTML = JSON.stringify(someResource));
        // 定时器也没有清除
    }
    // node、someResource 存储了大量数据 无法回收
}, 1000);

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解决方法：在定时器完成工作的时候，手动清除定时器。

闭包：

闭包可以维持函数内局部变量，使其得不到释放，造成内存泄漏。

    function bindEvent() {
      var obj = document.createElement("XXX");
      var unused = function () {
          console.log(obj,'闭包内引用obj obj不会被释放');
      };
      // obj = null;
    }

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解决方法：手动解除引用，obj = null。

循环引用问题

就是IE9以下的循环引用问题，上文讲过了。

没有清理DOM元素引用：

    var refA = document.getElementById('refA');
    document.body.removeChild(refA); // dom删除了
    console.log(refA, "refA");  // 但是还存在引用 能console出整个p 没有被回收

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不信的话，可以看下这个dom。

解决办法：refA = null;