What is JavaScript garbage collection?

 

What is JavaScript garbage collection? What's important for a web programmer to understand about JavaScript garbage collection, in order to write better code?

Eric Lippert wrote a detailed blog post about this subject a while back (additionally comparing it to VBScript). More accurately, he wrote about JScript, which is Microsoft's own implementation of ECMAScript, although very similar to JavaScript. I would imagine that you can assume the vast majority of behaviour would be the same for the JavaScript engine of Internet Explorer. Of course, the implementation will vary from browser to browser, though I suspect you could take a number of the common principles and apply them to other browsers.

Quoted from that page:

JScript uses a nongenerational mark-and-sweep garbage collector. It works like this:

  • Every variable which is "in scope" is called a "scavenger". A scavenger may refer to a number, an object, a string, whatever. We maintain a list of scavengers -- variables are moved on to the scav list when they come into scope and off the scav list when they go out of scope.

  • Every now and then the garbage collector runs. First it puts a "mark" on every object, variable, string, etc – all the memory tracked by the GC. (JScript uses the VARIANT data structure internally and there are plenty of extra unused bits in that structure, so we just set one of them.)

  • Second, it clears the mark on the scavengers and the transitive closure of scavenger references. So if a scavenger object references a nonscavenger object then we clear the bits on the nonscavenger, and on everything that it refers to. (I am using the word "closure" in a different sense than in my earlier post.)

  • At this point we know that all the memory still marked is allocated memory which cannot be reached by any path from any in-scope variable. All of those objects are instructed to tear themselves down, which destroys any circular references.

    • The main purpose of garbage collection is to allow the programmer not to worry about memory management of the objects they create and use, though of course there's no avoiding it sometimes - it is always beneficial to have at least a rough idea of how garbage collection works.

    There are a few particular points of which to be aware. The Apple developer site has some guidelines on the matter. Two important ones from there:

  • Use delete statements. Whenever you create an object using a new statement, pair it with a delete statement. This ensures that all of the memory associated with the object, including its property name, is available for garbage collection. The delete statement is discussed more in “Freeing Objects.”
  • Use the var keyword. Any variable created without the var keyword is created at the global scope and is never eligible for garbage collection, presenting the opportunity for a memory leak.

    • I would imagine that the practices should apply to all JavaScript engines (in different browsers), though because this is from an Apple site, they may be somewhat specific to Safari. (Perhaps someone could clarify that?)

    Hope that helps.

    Beware of circular references when DOM objects are involved:

    Memory leak patterns in JavaScript

    Keep in mind that memory can only be reclaimed when there are no active references to the object. This is a common pitfall with closures and event handlers, as some JS engines will not check which variables actually are referenced in inner functions and just keep all local variables of the enclosing functions.

    Here's a simple example: 

    function init() {
    var bigString = new Array(1000).join('xxx');
    var foo = document.getElementById('foo');
    foo.onclick = function() {
        // this might create a closure over `bigString`,
        // even if `bigString` isn't referenced anywhere!
    };
}

    A naive JS implementation can't collect bigString as long as the event handler is around. There are several ways to solve this problem, eg setting bigString = null at the end of init() ( delete won't work for local variables and function arguments: delete removes properties from objects, and the variable object is inaccessible - ES5 in strict mode will even throw a ReferenceError if you try to delete a local variable!).

    I recommend to avoid unnecessary closures as much as possible if you care for memory consumption.

    To the best of my knowledge, JavaScript's objects are garbage collected periodically when there are no references remaining to the object. It is something that happens automatically, but if you want to see more about how it works, at the C++ level, it makes sense to take a look at the WebKit or V8 source code

    Typically you don't need to think about it, however, in older browsers, like IE 5.5 and early versions of IE 6, and perhaps current versions, closures would create circular references that when unchecked would end up eating up memory. In the particular case that I mean about closures, it was when you added a JavaScript reference to a dom object, and an object to a DOM object that referred back to the JavaScript object. Basically it could never be collected, and would eventually cause the OS to become unstable in test apps that looped to create crashes. In practice these leaks are usually small, but to keep your code clean you should delete the JavaScript reference to the DOM object.

    Usually it is a good idea to use the delete keyword to immediately de-reference big objects like JSON data that you have received back and done whatever you need to do with it, especially in mobile web development. This causes the next sweep of the GC to remove that object and free its memory.

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