How do I create a memory leak in JavaScript?
You're not keeping the element you've created around and referenced anywhere - that's why you're not seeing the memory usage increase. Try attaching the element to the DOM, or store it in an object, or set the onclick to be a different element that sticks around. Then you'll see the memory usage skyrocket. The garbage collector will come through and clean up anything that can no longer be referenced.
Basically a walkthrough of your code:
- create element (el)
- create a new function that references thatelement
- set the function to be the onclick of that element
- overwrite the element with a new element
Everything is centric around the element existing. Once there isn't a way to access the element, the onclick can't be accessed anymore. So, since the onclick can't be accessed, the function that was created is destroyed.. and the function had the only reference to the element.. so the element is cleaned up as well.
Someone might have a more technical example, but that's the basis of my understanding of the javascript garbage collector.
Edit: Here's one of many possibilities for a leaking version of your script:
<html><body></body><script>var i, el;var createdElements = {};var events = [];function attachAlert(element) { element.onclick = function() { alert(element.innerHTML); };}function reallyBadAttachAlert(element) { return function() { alert(element.innerHTML); };}for (i = 0; i < 1000000; i++) { el = document.createElement('div'); el.innerHTML = i; /** posibility one: you're storing the element somewhere **/ attachAlert(el); createdElements['div' + i] = el; /** posibility two: you're storing the callbacks somewhere **/ event = reallyBadAttachAlert(el); events.push(event); el.onclick = event;}</script></html>So, for #1, you're simply storing a reference to that element somewhere. Doesn't matter that you'll never use it - because that reference is made in the object, the element and its callbacks will never go away (or at least until you delete the element from the object). For possibility #2, you could be storing the events somewhere. Because the event can be accessed (i.e. by doing events[10]();) even though the element is nowhere to be found, it's still referenced by the event.. so the element will stay in memory as well as the event, until it's removed from the array.
update: Here is a very simple example based on the caching scenario in the Google I/O presentation:
/*This is an example of a memory leak. A new property is added to the cacheobject 10 times/second. The value of performance.memory.usedJSHeapSizesteadily increases.Since the value of cache[key] is easy to recalculate, we might want to freethat memory if it becomes low. However, there is no way to do that...Another method to manually clear the cache could be added, but manuallyadding memory checks adds a lot of extra code and overhead. It would benice if we could clear the cache automatically only when memory became low.Thus the solution presented at Google I/O!*/(function(w){ var cache = {} function getCachedThing(key) { if(!(key in cache)) { cache[key] = key; } return cache[key]; } var i = 0; setInterval(function() { getCachedThing(i++); }, 100); w.getCachedThing = getCachedThing})(window);Because usedJSHeapSize does not update when the page is opened from the local file system, you might not see the increasing memory usage. In that case, I have hosted this code for you here: https://memory-leak.surge.sh/example-for-waterfr
This Google I/O'19 presentation gives examples of real-world memory leaks as well as strategies for avoiding them:
- Method
getImageCached()returns a reference to an object, also caching a local reference. Even if this reference goes out of the method consumer's scope, the referenced memory cannot be garbage collected because there is a still a strong reference inside the implementation ofgetImageCached(). Ideally, the cached reference would be eligible for garbage collection if memory got too low. (Not exactly a memory leak, but a situation where there is memory that could be freed at the cost of running the expensive operations again). - Leak #1: the reference to the cached image. Solved by using weak references inside
getImageCached(). - Leak #2: the string keys inside the cache (Map object). Solved by using the new
FinalizationGroupAPI.
Please see the linked video for JS code with line-by-line explanations.
More generally, "real" JS memory leaks are caused by unwanted references (to objects that will never be used again). They are usually bugs in the JS code. This article explains four common ways memory leaks are introduced in JS:
- Accidental global variables
- Forgotten timers/callbacks
- Out of DOM references
- Closures
An interesting kind of JavaScript memory leak documents how closures caused a memory leak in the popular MeteorJS framework.
I tried to do something like that and got exception out of memory.
const test = (array) => { array.push((new Array(1000000)).fill('test'));};const testArray = [];for(let i = 0; i <= 1000; i++) { test(testArray);}