Recently, I got this kind of warning, and this is my first time getting it:
[Violation] Long running JavaScript task took 234ms [Violation] Forced reflow while executing JavaScript took 45ms
I'm working on a group project and I have no idea where this is coming from. This never happened before. Suddenly, it appeared when someone else got involved in the project. How do I find what file/function causes this warning? I've been looking for the answer, but mostly about the solution on how to solve it. I can't solve it if I can't even find the source of the problem.
In this case, the warning appears only on Chrome. I tried to use Edge, but I didn't get any similar warnings, and I haven't tested it on Firefox yet.
I even get the error from jquery.min.js:
[Violation] Handler took 231ms of runtime (50ms allowed) jquery.min.js:2
This question is related to
javascript
google-chrome
dom
The answer is
This is not an error just simple a message. To execute this message change
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> (example)
to
<!DOCTYPE html>(the Firefox source expect this)
The message was shown in Google Chrome 74 and Opera 60 . After changing it was clear, 0 verbose.
A solution approach
I found the root of this message in my code, which searched and hid or showed nodes (offline). This was my code:
search.addEventListener('keyup', function() {
for (const node of nodes)
if (node.innerText.toLowerCase().includes(this.value.toLowerCase()))
node.classList.remove('hidden');
else
node.classList.add('hidden');
});
The performance tab (profiler) shows the event taking about 60 ms:
Now:
search.addEventListener('keyup', function() {
const nodesToHide = [];
const nodesToShow = [];
for (const node of nodes)
if (node.innerText.toLowerCase().includes(this.value.toLowerCase()))
nodesToShow.push(node);
else
nodesToHide.push(node);
nodesToHide.forEach(node => node.classList.add('hidden'));
nodesToShow.forEach(node => node.classList.remove('hidden'));
});
The performance tab (profiler) now shows the event taking about 1 ms:
And I feel that the search works faster now (229 nodes).
In order to identify the source of the problem, run your application, and record it in Chrome's Performance tab.
There you can check various functions that took a long time to run. In my case, the one that correlated with warnings in console was from a file which was loaded by the AdBlock extension, but this could be something else in your case.
Check these files and try to identify if this is some extension's code or yours. (If it is yours, then you have found the source of your problem.)
For what it’s worth, here are my 2¢ when I encountered the
[Violation] Forced reflow while executing JavaScript took <N>ms
warning. The page in question is generated from user content, so I don’t really have much influence over the size of the DOM. In my case, the problem is a table of two columns with potentially hundreds, even thousands of rows. (No on-demand row loading implemented yet, sorry!)
Using jQuery, on keydown the page selects a set of rows and toggles their visibility. I noticed that using toggle() on that set triggers the warning more readily than using hide() & show() explicitly.
For more details on this particular performance scenario, see also this article.
Forced reflow often happens when you have a function called multiple times before the end of execution.
For example, you may have the problem on a smartphone, but not on a classic browser.
I suggest using a setTimeout to solve the problem.
This isn't very important, but I repeat, the problem arises when you call a function several times, and not when the function takes more than 50 ms. I think you are mistaken in your answers.
- Turn off 1-by-1 calls and reload the code to see if it still produces the error.
- If a second script causes the error, use a
setTimeOutbased on the duration of the violation.
Adding my insights here as this thread was the "go to" stackoverflow question on the topic.
My problem was in a Material-UI app (early stages)
- placement of custom Theme provider was the cause
when I did some calculations forcing rendering of the page (one component, "display results", depends on what is set in others, "input sections").
Everything was fine until I updated the "state" that forces the "results component" to rerender. The main issue here was that I had a material-ui theme (https://material-ui.com/customization/theming/#a-note-on-performance) in the same renderer (App.js / return.. ) as the "results component", SummaryAppBarPure
Solution was to lift the ThemeProvider one level up (Index.js), and wrapping the App component here, thus not forcing the ThemeProvider to recalculate and draw / layout / reflow.
before
in App.js:
return (
<>
<MyThemeProvider>
<Container className={classes.appMaxWidth}>
<SummaryAppBarPure
//...
in index.js
ReactDOM.render(
<React.StrictMode>
<App />
//...
after
in App.js:
return (
<>
{/* move theme to index. made reflow problem go away */}
{/* <MyThemeProvider> */}
<Container className={classes.appMaxWidth}>
<SummaryAppBarPure
//...
in index.js
ReactDOM.render(
<React.StrictMode>
<MyThemeProvider>
<App />
//...
This was added in the Chrome 56 beta, even though it isn't on this changelog from the Chromium Blog: Chrome 56 Beta: “Not Secure” warning, Web Bluetooth, and CSS position: sticky
You can hide this in the filter bar of the console with the Hide violations checkbox.
A couple of ideas:
Remove half of your code (maybe via commenting it out).
Is the problem still there? Great, you've narrowed down the possibilities! Repeat.
Is the problem not there? Ok, look at the half you commented out!
Are you using any version control system (eg, Git)? If so,
git checkoutsome of your more recent commits. When was the problem introduced? Look at the commit to see exactly what code changed when the problem first arrived.
These are just warnings as everyone mentioned. However, if you're keen on resolving these (which you should), then you need to identify what is causing the warning first. There's no one reason due to which you can get force reflow warning.
Someone has created a list for some possible options. You can follow the discussion for more information.
Here's the gist of the possible reasons:
What forces layout / reflow
All of the below properties or methods, when requested/called in JavaScript, will trigger the browser to synchronously calculate the style and layout*. This is also called reflow or layout thrashing, and is common performance bottleneck.
Element
Box metricsScroll stuff
elem.offsetLeft,elem.offsetTop,elem.offsetWidth,elem.offsetHeight,elem.offsetParentelem.clientLeft,elem.clientTop,elem.clientWidth,elem.clientHeightelem.getClientRects(),elem.getBoundingClientRect()Focus
elem.scrollBy(),elem.scrollTo()elem.scrollIntoView(),elem.scrollIntoViewIfNeeded()elem.scrollWidth,elem.scrollHeightelem.scrollLeft,elem.scrollTopalso, setting themAlso…
elem.focus()can trigger a double forced layout (source)
elem.computedRole,elem.computedNameelem.innerText(source)getComputedStyle
window.getComputedStyle()will typically force style recalc (source)
window.getComputedStyle()will force layout, as well, if any of the following is true:
- The element is in a shadow tree
- There are media queries (viewport-related ones). Specifically, one of the following: (source) *
min-width,min-height,max-width,max-height,width,height*aspect-ratio,min-aspect-ratio,max-aspect-ratio
device-pixel-ratio,resolution,orientation- The property requested is one of the following: (source)
height,width*top,right,bottom,left*margin[-top,-right,-bottom,-left, or shorthand] only if the margin is fixed. *padding[-top,-right,-bottom,-left, or shorthand] only if the padding is fixed. *transform,transform-origin,perspective-origin*translate,rotate,scale*webkit-filter,backdrop-filter*motion-path,motion-offset,motion-rotation*x,y,rx,rywindow
window.scrollX,window.scrollYwindow.innerHeight,window.innerWidthwindow.getMatchedCSSRules()only forces styleForms
inputElem.focus()inputElem.select(),textareaElem.select()(source)Mouse events
mouseEvt.layerX,mouseEvt.layerY,mouseEvt.offsetX,mouseEvt.offsetY(source)document
doc.scrollingElementonly forces styleRange
range.getClientRects(),range.getBoundingClientRect()SVG
- Quite a lot; haven't made an exhaustive list , but Tony Gentilcore's 2011 Layout Triggering List pointed to a few.
contenteditable
- Lots & lots of stuff, …including copying an image to clipboard (source)
Check more here.
Also, here's Chromium source code from the original issue and a discussion about a performance API for the warnings.
Edit: There's also an article on how to minimize layout reflow on PageSpeed Insight by Google. It explains what browser reflow is:
Reflow is the name of the web browser process for re-calculating the positions and geometries of elements in the document, for the purpose of re-rendering part or all of the document. Because reflow is a user-blocking operation in the browser, it is useful for developers to understand how to improve reflow time and also to understand the effects of various document properties (DOM depth, CSS rule efficiency, different types of style changes) on reflow time. Sometimes reflowing a single element in the document may require reflowing its parent elements and also any elements which follow it.
In addition, it explains how to minimize it:
- Reduce unnecessary DOM depth. Changes at one level in the DOM tree can cause changes at every level of the tree - all the way up to the root, and all the way down into the children of the modified node. This leads to more time being spent performing reflow.
- Minimize CSS rules, and remove unused CSS rules.
- If you make complex rendering changes such as animations, do so out of the flow. Use position-absolute or position-fixed to accomplish this.
- Avoid unnecessary complex CSS selectors - descendant selectors in particular - which require more CPU power to do selector matching.
This is violation error from Google Chrome that shows when the Verbose logging level is enabled.
Example of error message:
Explanation:
Reflow is the name of the web browser process for re-calculating the positions and geometries of elements in the document, for the purpose of re-rendering part or all of the document. Because reflow is a user-blocking operation in the browser, it is useful for developers to understand how to improve reflow time and also to understand the effects of various document properties (DOM depth, CSS rule efficiency, different types of style changes) on reflow time. Sometimes reflowing a single element in the document may require reflowing its parent elements and also any elements which follow it.
Original article: Minimizing browser reflow by Lindsey Simon, UX Developer, posted on developers.google.com.
And this is the link Google Chrome gives you in the Performance profiler, on the layout profiles (the mauve regions), for more info on the warning.
If you're using Chrome Canary (or Beta), just check the 'Hide Violations' option.
Look in the Chrome console under the Network tab and find the scripts which take the longest to load.
In my case there were a set of Angular add on scripts that I had included but not yet used in the app :
<script src="//cdnjs.cloudflare.com/ajax/libs/angular-ui-router/0.2.8/angular-ui-router.min.js"></script>
<script src="//cdnjs.cloudflare.com/ajax/libs/angular-ui-utils/0.1.1/angular-ui-utils.min.js"></script>
<script src="//ajax.googleapis.com/ajax/libs/angularjs/1.3.9/angular-animate.min.js"></script>
<script src="//ajax.googleapis.com/ajax/libs/angularjs/1.3.9/angular-aria.min.js"></script>
These were the only JavaScript files that took longer to load than the time that the "Long Running Task" error specified.
All of these files run on my other websites with no errors generated but I was getting this "Long Running Task" error on a new web app that barely had any functionality. The error stopped immediately upon removing.
My best guess is that these Angular add ons were looking recursively into increasingly deep sections of the DOM for their start tags - finding none, they had to traverse the entire DOM before exiting, which took longer than Chrome expects - thus the warning.
I found a solution in Apache Cordova source code. They implement like this:
var resolvedPromise = typeof Promise == 'undefined' ? null : Promise.resolve();
var nextTick = resolvedPromise ? function(fn) { resolvedPromise.then(fn); } : function(fn) { setTimeout(fn); };
Simple implementation, but smart way.
Over the Android 4.4, use Promise.
For older browsers, use setTimeout()
Usage:
nextTick(function() {
// your code
});
After inserting this trick code, all warning messages are gone.
Source: Stackoverflow.com