The screenshot shows an example of layout thrashing, where we repeatedly read and modify DOM properties, causing a series of relayouts. Typically layout affects the whole document, so we should avoid it whenever possible.Ĭhrome will point out when DOM access in our JavaScript code triggered a reflow. Layout or Reflows can have a notable impact on the performance of our page. Layout or reflows triggered by JavaScript The main-thread section shows not only what JavaScript function was running at any given time, but also when Chrome spent time parsing code or updating the page layout. We’ll use the performance profiler in our examples to analyze JavaScript performance and view execution timings. It covers a wide range of information, including network activity, timings, main-thread tasks, and frames per second. The Performance tab shows a detailed view of page runtime performance. Alternatively, press Command+Option+I (Mac) or Control+Shift+I (Windows, Linux) to open the DevTools. Right-click an element on the page and select Inspect.Open a Chrome window in Incognito Mode (Incognito mode ensures that our performance measurements won't be contaminated by browser extensions). We can use the browser DevTools to debug JavaScript Performance problems. How to debug Javascript performance issues We'll cover memory leaks an a future blog post. But how we write our JavaScript code still impacts its efficiency. Browsers have an intelligent garbage collection mechanism that cleans up after our code. Memory leaks are an underrated aspect of JavaScript performance. We'll show you what causes a reflow and how you can optimize your DOM manipulations to avoid them. While some of the properties won’t affect performance, others will cause the browser to synchronously recalculate the page layout, before continuing to execute the JavaScript code. The DOM API allows us to read and modify the content and styling of each element. This step requires geometric information like width and height. Layout is when the browser calculates where it needs to paint DOM elements. Running JavaScript can have a significant effect on the overall performance of your website.Īs this happens on the browser main thread, JavaScript execution can freeze the UI and prevent the user from interacting with the page. Where possible this is done lazily, to avoid spending time on code that will never run. We can separate JavaScript Performance issues into four different types:īefore browsers can run our code they first need to parse and compile it.
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