What is JavaScript garbage collection?

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.

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