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MutationObserver vs. Proxy to detect .textContent changes
<!--
This demo shows two ways to detect changes to a DOM node `.textContent`, one
using a `MutationObserver` and the other using an ES2015 `Proxy`.
From testing, a `Proxy` appears to be 6-8x faster than using a MO in Chrome 50.
**Update**: removing the `Proxy` altogether speeds up the MO to be inline with the Proxy.
This has something to do with how the browser queues/prioritizes Proxies over MO.
Why is this useful? One could imagine creating a primative data-binding system
around the `Proxy` approach or using it to [polyfill `Object.observer()`](https://gist.github.com/ebidel/1b553d571f924da2da06).
Run it: http://jsbin.com/dukuluwufa/edit?html,output
-->
<p><button onclick="update()">update .textContent</button></p>
<p>[MO]'d node content: <span id="source"></span></p>
<p>[Proxy]'d node content: <span id="source-proxied"></span></p>
<output></output>
<script>
let start, finish, start2, finish2;
// Watch accesses/sets on a DOM element property.
function watchPropsOn(el, prop, callback=null) {
return new Proxy(el, {
set(target, propKey, value, receiver) {
if (prop === propKey) {
let finish2 = performance.now();
out.innerHTML += `Proxy took: ${finish2 - start2}ms<br>`;
}
console.log(`Proxy set .${propKey} to ${value}`);
target[propKey] = value;
}
});
}
function observe(target) {
// create an observer instance
let observer = new MutationObserver(mutations => {
let finish = performance.now();
out.innerHTML += `MutationObserver took: ${finish - start}ms<br>`;
mutations.forEach(mutation => {
if (mutation.addedNodes.length) {
console.log(`MutationObserver observed childList as ${target.textContent}`)
}
});
});
observer.observe(target, {childList: true});
}
function update() {
out.innerHTML = '';
start = start2 = performance.now();
proxy.textContent = source.textContent = Math.random();
}
let source = document.querySelector('#source');
let proxiedSource = document.querySelector('#source-proxied');
let out = document.querySelector('output');
observe(source); // setup MO
let proxy = watchPropsOn(proxiedSource, 'textContent'); // setup proxy.
</script>
@ebidel

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ebidel commented Apr 18, 2016

@cmbankester

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cmbankester commented Apr 19, 2016

I think the mutation observer update gets added to the job queue after the proxy update does, which makes it seem like the proxy is faster. For instance, if you remove the proxy altogether and change the textContent on source, does the mutationObserver take less time?

@ebidel

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ebidel commented Aug 12, 2016

You're right that removing the Proxy altogether speeds up the MO. It's more in line with the Proxy. 0.20ms MO vs. 0.125ms Proxy on first run.

@jimmont

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jimmont commented Nov 5, 2016

In Chrome MO doesn't fire if the value is unchanged but in Safari it always does, along with the Proxy. As I recall MO is async and the performance measurement follows after the DOM updates so it makes sense (to me) it would appear slower regardless. Is this valid and accurate? If so it doesn't seem to matter what the performance is but instead which approach better fits the problem being solved. Does this seem a valid conclusion to make?

@snuggs

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snuggs commented Feb 11, 2017

@ebidel Thanks for this one! Shouts in my README (if you don't mind). #salute 🙏
devpunks/snuggsi@7e4ac9f#diff-04c6e90faac2675aa89e2176d2eec7d8R47

Using them in a textContent Relay MutationObserver binding. https://github.com/snuggs/snuggsi/blob/master/elements/text-content.js

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