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+> * 原文地址：[Performant JavaScript Best Practices](https://levelup.gitconnected.com/performant-javascript-best-practices-c5a49a357e46)
+> * 原文作者：[John Au-Yeung](https://medium.com/@hohanga)
+> * 译文出自：[掘金翻译计划](https://github.com/xitu/gold-miner)
+> * 本文永久链接：[https://github.com/xitu/gold-miner/blob/master/TODO1/performant-javascript-best-practices.md](https://github.com/xitu/gold-miner/blob/master/TODO1/performant-javascript-best-practices.md)
+> * 译者：
+> * 校对者：
+
+# Performant JavaScript Best Practices
+
+![Photo by [Jason Chen](https://unsplash.com/@ja5on?utm_source=medium&utm_medium=referral) on [Unsplash](https://unsplash.com?utm_source=medium&utm_medium=referral)](https://cdn-images-1.medium.com/max/5760/0*UyQ42ciE79LF-bK4)
+
+Like any program, JavaScript programs can get slow fast if we aren’t careful with writing our code.
+
+In this article, we’ll look at some best practices for writing fast JavaScript programs.
+
+## Reduce DOM Manipulation With Host Object and User’s Browser
+
+DOM manipulation is slow. The more we do, the slower it’ll be. Since DOM manipulation is synchronous, each action is done one at a time, holding the rest of our program.
+
+Therefore, we should minimize the number of DOM manipulation actions that we're doing.
+
+The DOM can be blocked by loading CSS and JavaScript. However, images aren’t blocking render so that they don’t hold up our page from finishing loading.
+
+However, we still want to minimize the size of our images.
+
+Render blocking JavaScript code can be detected with Google PageSpeed Insights, which tells us how many pieces of render-blocking JavaScript code we have.
+
+Any inline CSS would block the rendering of the whole page. They are the styles that are scattered within our page with `style` attributes.
+
+We should move them all to their own style sheets, inside the `style` tag, and below the body element.
+
+CSS should be concatenated and minified to reduce the number of the stylesheet to load and their size.
+
+We can also mark `link` tags as non-render blocking by using media queries. For instance, we can write the following to do that:
+
+```html
+<link href="portrait.css" rel="stylesheet" media="orientation:portrait">
+```
+
+so that it only loads when the page is displayed in portrait orientation.
+
+We should move style manipulation outside of our JavaScript and put styles inside our CSS by putting styles within their own class in a stylesheet file.
+
+For instance, we can write the following code to add a class in our CSS file:
+
+```css
+.highlight {
+  background-color: red;
+}
+```
+
+and then we can add a class with the `classList` object as follows:
+
+```js
+const p = document.querySelector('p');
+p.classList.add('highlight');
+```
+
+We set the p element DOM object to its own constant so we can cache it and reuse it anywhere and then we call `classList.add` to add the `hightlight` class to it.
+
+We can also remove it if we no longer want it. This way, we don’t have to do a lot of unnecessary DOM manipulation operations in our JavaScript code.
+
+If we have scripts that no other script depends on, we can load then asynchronously so that they don’t block the loading of other scripts.
+
+We just put `async` in our script tag so that we can load our script asynchronously as follows:
+
+```html
+<script async src="script.js"></script>
+```
+
+Now `script.js` will load in the background instead of in the foreground.
+
+We can also defer the loading of scripts by using the `defer` directive. However, it guarantees that the script in the order they were specified on the page.
+
+This is a better choice if we want our scripts to load one after another without blocking the loading of other things.
+
+Minifying scripts is also a must-do task before putting our code into production. To do that, we use module bundlers like Webpack and Parcel, which so create a project and then build them for us automatically.
+
+Also, command-line tools for frameworks like Vue and Angular also do code minification automatically.
+
+## Minimize the Number of Dependencies Our App Uses
+
+We should minimize the number of scripts and libraries that we use. Unused dependencies should also be removed.
+
+For instance, if we’re using Lodash methods for array manipulation, then we can replace them with native JavaScript array methods, which are just as good.
+
+Once we remove our dependency, we should remove them from `package.json` and the run `npm prune` to remove the dependency from our system.
+
+![Photo by [Tim Carey](https://unsplash.com/@baudy?utm_source=medium&utm_medium=referral) on [Unsplash](https://unsplash.com?utm_source=medium&utm_medium=referral)](https://cdn-images-1.medium.com/max/6154/0*9Qx9V9XpyjsjvSME)
+
+## Poor Event Handling
+
+Event handling code is always slow if they’re complex. We can improve performance by reducing the depth of our call stack.
+
+That means we call as few functions as possible. Put everything in CSS style sheets if possible if we’re manipulating styles in our event handlers.
+
+And do everything to reduce calling functions like using the `**` operator instead of calling `Math.pow` .
+
+## Conclusion
+
+We should reduce the number of dependencies and loading them in an async manner if possible.
+
+Also, we should reduce the CSS in our code and move them to their own stylesheets.
+
+We can also add media queries so that stylesheets don’t load everywhere.
+
+Finally, we should reduce the number of functions that are called in our code.
+
+> 如果发现译文存在错误或其他需要改进的地方，欢迎到 [掘金翻译计划](https://github.com/xitu/gold-miner) 对译文进行修改并 PR，也可获得相应奖励积分。文章开头的 **本文永久链接** 即为本文在 GitHub 上的 MarkDown 链接。
+
+---
+
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