1. Introduction

Javascript in browsers has an automatic GC:Garbage Collecation, which means that the execution environment is responsible for managing the memory used during code execution. The idea is that the garbage collector periodically (periodically) finds variables that are no longer in use and then frees their memory. However, this process is not real-time because it is expensive and stops responding to other operations during GC, so the garbage collector periodically executes it at regular intervals.

Variables that are no longer used are variables that end their life cycle. Of course, they can only be local variables. Global variables do not end their life cycle until the browser unloads the page. Local variables exist only during the execution of a function, which allocates space on the stack or heap for local variables to store their values, and then uses them in the function until the function ends, whereas the external function does not end because of the inner function.

Or on the code description:

function fn1() {
    var obj = {name: 'hanzichi'.age: 10};
}
function fn2() {
    var obj = {name:'hanzichi'.age: 10};
    return obj;
}

var a = fn1();
var b = fn2();
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Let’s look at how the code is executed. First, we define two functions, named fn1 and fn2 respectively. When fn1 is called, fn1’s environment will open up a memory to store the object {name: ‘hanzichi’, age: 10}, and when the call ends, the fN1 environment, then the block memory will be automatically released by the GARBAGE collector in the JS engine; When fn2 is called, the returned object is referred to by the global variable B, so the block is not freed.

Here comes the question: Which variable is useless? So the garbage collector must keep track of which variables are useless, and mark those that are no longer useful, so they can be reclaimed in the future. The strategy used to tag useless variables may vary by implementation, and there are usually two implementations: tag clearing and reference counting. Reference counting is less common, and tag clearing is more common.

2. Mark clearing

The most common garbage collection method in JS is tag cleanup. When a variable enters the environment, for example, declaring a variable in a function, mark the variable as “entering the environment.” Logically, you can never free up memory occupied by variables that enter the environment, because they may be used whenever the execution stream enters the corresponding environment. When a variable leaves the environment, it is marked as “out of the environment.”

function test(){
var a = 10 ;             // Get tagged and enter the environment
var b = 20 ;             // Get tagged and enter the environment
}
test();                     // A and B are marked out of the environment and recycled.
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At run time, the garbage collector marks all variables stored in memory (of course, it can be marked in any way). It then removes the tags (closures) of variables in the environment and those referenced by variables in the environment. Variables tagged after this point are considered to be ready for deletion because they are no longer accessible to variables in the environment. Finally, the garbage collector completes the memory cleanup, destroying the tagged values and reclaiming the memory space they occupy. So far, JS implementations of IE9+, Firefox, Opera, Chrome, and Safari have all used a marked sweep garbage collection strategy or similar strategies, with different garbage collection intervals.

3. Reference count

The meaning of reference counting is to keep track of how many times each value is referenced. When a variable is declared and a reference type value is assigned to the variable, the number of references to the value is 1. If the same value is assigned to another variable, the number of references to the value is increased by one. Conversely, if a variable containing a reference to that value obtains another value, the number of references to that value is reduced by one. When the number of references to this value goes to zero, there is no way to access the value, and the memory space it occupies can be reclaimed. This way, the next time the garbage collector runs, it frees up memory for values that are referenced zero times.

function test(){
    var a = {} ;         // The number of references to a is 0
    var b = a ;         // The number of references to a increases by 1, which equals 1
    var c =a;           // The number of references to a is increased by 1, which is 2
    var b ={};          // The number of references to a is reduced by 1 to 1
}
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Netscape Navigator3 was the first browser to use a reference counting strategy, but it soon ran into a serious problem: circular references. A circular reference is when object A contains A pointer to object B, and object B contains A reference to object A.

function fn() {
    var a = {};
    var b = {};
    a.pro = b;
    b.pro = a;
}
fn();
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Code above cited a and b are 2, fn () after the execution, the two objects have to leave the environment, under the tag removal way is no problem, but under the reference counting strategy, because of a and b citations is not 0, so will not reclaimed by the garbage collector memory, if fn the function is called a lot of, can cause memory leaks. On IE7 and IE8, the memory went straight up.

As we know, there are some objects in IE that are not native JS objects. For example, objects in the memory leak DOM and BOM are implemented as COM objects using C++, and the garbage collection mechanism of COM objects uses reference counting strategy. Therefore, the COM objects accessed by JS are still based on the reference counting strategy, even though the JS engine of IE adopts the tag clearing strategy. In other words, whenever COM objects are involved in IE, there is a problem with circular references.

var element = document.getElementById("some_element");
var myObject = new Object(a); myObject.e = element; element.o = myObject;Copy the code

This example creates a circular reference between a DOM element (Element) and a native JS object (myObject). Where the variable myObject has an attribute e pointing to the Element object; The element variable also has an attribute o that refers back to myObject. Because of this circular reference, even if the DOM in the example is removed from the page, it will never be recycled.

Here’s an example:

  • Yellow means directly referenced by a JS variable, in memory
  • Red refers to the js variable referenced indirectly, as shown in the figure above. RefB is referenced indirectly by refA, so that even if the refB variable is empty, it will not be recycled
  • The child element refB due toparentNodeAs long as it is not deleted, all of its parent elements (shown in red) are not deleted

Another example:

window.onload=function outerFunction(){
    var obj = document.getElementById("element");
    obj.onclick=function innerFunction(){};
};
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This code looks fine, but obj references document.getelementById (‘ Element ‘), and the onclick method of document.getelementById (‘ Element ‘) references variables in the external environment, Including OBJ, of course. Isn’t that hidden? (Newer browsers remove Node events when they are removed, but older browsers, especially Internet Explorer, have this bug)

Solutions:

The easiest way to do this is to manually unreference the loop yourself, as the previous function did

myObject.element = null;
element.o = null;

window.onload=function outerFunction(){
    var obj = document.getElementById("element");
    obj.onclick=function innerFunction(){};
    obj=null;
};
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Setting a variable to NULL means severing the connection between the variable and the value it previously referenced. The next time the garbage collector runs, it removes these values and reclaims the memory they occupy.

Note that IE9+ does not have a Dom memory leak problem caused by circular references, either because Microsoft has optimized it or because Dom recycling has changed

4. Memory management

4.1 When is garbage collection triggered?

The garbage collector runs periodically, and if the amount of memory allocated is very high, the collection can be difficult, so it becomes a worthwhile consideration to determine the garbage collection interval. IE6 garbage collection is based on memory allocation to run, when there are 256 variables in the environment, 4096 objects, 64K string any one of the circumstances will trigger garbage collector work, looks very scientific, do not need to call a period of time, sometimes it will not be necessary, so on demand call is not very good? But if the environment has so many variables and so on all the time, and now the script is so complex that it’s normal, then the result is that the garbage collector is always working and the browser can’t play.

Microsoft made adjustment in IE7, the trigger condition is no longer a fixed, but the dynamic modification, the initial value is the same as the IE6, if the garbage collector memory allocation amount less than 15% of the program memory, that most of the memory cannot be recycled, and the recycling trigger condition is too sensitive, at that time the street condition doubled, If more than 85% of the memory is recovered, most of the memory should have been cleaned up long ago, so put the trigger condition back. This makes garbage collection a lot more functional

4.2 Reasonable GC scheme

1. Basic programmes

The basic GC scheme for Javascript engines is simple GC: Mark and sweep:

  1. Iterate over all accessible objects.
  2. Reclaim objects that are no longer accessible.

2. GC defects

As with other languages, javascript’s GC strategy is not immune to the problem of stopping responding to other operations when GC is performed for safety reasons. And Javascript GC in 100ms or even above, for the general application is ok, but for JS games, animation on coherence requirements are relatively high application, it is troublesome. This is where the new engine needs to be optimized: to avoid long pauses in response caused by GC.

3. GC optimization strategy

Uncle David mainly introduced two optimizations, and these are the most important two optimizations:

  1. Generation GC this is consistent with the Idea of Java collection strategy and is the main use of V8. The purpose is to distinguish between “temporary” and “permanent” objects; Recycle more “temporary” young generation and less “persistent generation” to reduce the number of objects to traverse each time, thus reducing the time of each GC. As shown in figure:

    It should be added here that for Tenured Generation objects, there is an additional overhead of migrating them from the Young generation to the Tenured generation, and if they are referenced, the reference reference will need to be changed. Here the main content can refer toGet the simple out of NodeThe introduction of memory, very detailed ~

  2. The idea behind incremental GC is simple: “do a little bit at a time, a little bit next time, and so on.” As shown in figure:

    This scheme, although time-consuming, but more interrupts, brings the problem of frequent context switch.

    Because each scheme has its own application scenarios and disadvantages, the scheme is selected according to the actual situation in practical application.

    For example, interrupt GC at a low (object /s) ratio, and simple GC at a lower rate; If a large number of objects are “alive” for a long time, generational processing has little advantage.

5. Memory leakage in VUE

JS program memory overflow, will make a section of function body forever invalid (depending on the JS code to run to which function), usually for the program suddenly stuck or abnormal procedures.

At this time we will be on the JS program memory leak investigation, to find out which objects occupied by the memory is not released. These objects are usually the ones the developer thinks are freed, but are actually still referenced by a closure or in an array.

5.1 leak

  1. DOM/BOM objects leak
  2. There are references to DOM/BOM objects in script
  3. Js object leakage
  4. This is usually caused by closures, such as event-handling callbacks, which lead to bidirectional references between DOM objects and objects in scripts, and is a common cause of leakage

5.2 Code Concerns

  1. In the DOMaddEventLisnerFunction and derived event listeners, such as in JqueryonFunction of the VUE component instance$onFunction, an initialization function in a third-party library
  2. Event listeners for other BOM objects, such as the webSocket instance’s on function
  3. Avoid unnecessary function references
  4. If you are usingrenderFunction to avoid binding DOM/BOM events to HTML tags

5.3 How to Handle the fault

  1. If themounted/createdHooks that bind events in a DOM/BOM objectbeforeDestroyTo do the corresponding unbinding process
  2. If themounted/createdThe hook is initialized using a third party librarybeforeDestroyTo do the corresponding destruction process
  3. If a timer is used in the component, the timer must be set tobeforeDestroyTo do the corresponding destruction process
  4. Instead of using expressions in a template to bind to a particular handler, do you place the logic in the handler?
  5. If themounted/createdUsed in the hook$onThat need to be inbeforeDestroyIn the pair should be unbound$offTo deal with
  6. Some components using event bindings in templates may leak, using$onReplace the binding in the template

5.4 Process addEventListener in vUE component

A method in a created/ Mounted hook function that defines the event response function as an instance of an object. Add event listeners by calling removeEventListener in beforeDestroy to remove the corresponding event listener. Note that the previously defined response function method needs to be passed in as a second argument and then removed from the object instance with delete or with the property set to NULL /undefined. To remove listeners accurately, do not use anonymous functions or bindings of existing functions as event listeners directly

mounted() {
    const box = document.getElementById('time-line')
    this.width = box.offsetWidth
    this.resizefun = (a)= > {
      this.width = box.offsetWidth
    }
    window.addEventListener('resize'.this.resizefun)
  },
  beforeDestroy() {
    window.removeEventListener('resize'.this.resizefun)
    this.resizefun = null
  }
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5.5 Memory leaks caused by observer mode

When using the observer mode in a SPA application, if you register the observed method with the observer and do not remove it when leaving the component, you may cause double registration and memory leak.

For example: ob.addListener(” Enter “, _func) when entering a component causes a memory leak if there is no ob.removelistener (” Enter “, _func) when leaving the component beforeDestroy

For more detailed chestnut reference: Texas Hold ’em chestnut

5.6 Memory leaks caused by context binding

Sometimes there are memory leaks when using the bind/apply/call context to bind methods.

var ClassA = function(name) {
  this.name = name
  this.func = null
}

var a = new ClassA("a")
var b = new ClassA("b")

b.func = bind(function() {
  console.log("I am " + this.name)
}, a)

b.func()    // Output: I am a

a = null        / / release a
//b = null; Release b / /
//b.func = null;        // 释放b.func

function bind(func, self) {        // Simulate context binding
  return function() {
    return func.apply(self)
  }
}
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Use Chrome dev Tool > Memory > Profiles to check the number of instances of ClassA in memory. There are two instances, A and B. Although a is set to null, the closure context self of BIND in B’s method binds a, so a is released, but b/b.func is not released, and the closure self persists and holds a reference to A.

Online posts are mostly different in depth, even some inconsistent, the following article is a summary of the learning process, if you find mistakes, welcome to comment out ~

Reference:

  1. Learn about javascript garbage collection and memory management
  2. App performance optimization
  3. Vue Web App Memory Leaks – Debugging and analysis
  4. Fix JavaScript memory leaks

Recommended reading:

  1. The 34 golden Rules for Optimizing web page performance at Yahoo
  2. Analyzing Javascript memory Collection (GC) with Chrome Developer Tools
  3. JS Memory leak detection method – Chrome Profiles
  4. Javascript typical memory leak and Chrome troubleshooting methods

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