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The main difference between `defer` and `async` is the order of execution. |
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## Defer attribute |
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A `<script>` element with a `defer` attribute, it will continue to load the HTML page and render it while the script is being downloaded. The script is executed after the HTML page has been completely parsed. `defer` scripts maintain their order in the document. |
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```html |
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<script defer src="script1.js"></script> |
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<script defer src="script2.js"></script> |
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``` |
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In the example above, `script1.js` will be executed before `script2.js`. The browser will download both scripts in parallel, but `script1.js` will be executed after the HTML page has been parsed and `script2.js` will be executed after `script1.js` has been executed. |
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## Async attribute |
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On the other hand, A `<script>` element with an `async` attribute, it will pause the HTML parser and execute the script immediately after it has been downloaded. The HTML parsing will resume after the script has been executed. |
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```html |
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<script async src="script1.js"></script> |
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<script async src="script2.js"></script> |
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``` |
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In the example above, the browser will download both scripts in parallel, and execute them as soon as they are downloaded. The order of execution is not guaranteed. |
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To know more you can check [this diagram](https://roadmap.sh/guides/avoid-render-blocking-javascript-with-async-defer) from us that explains the difference between `defer` and `async` in a visual way. |
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As the name says, the increment operator increases the value of a variable by **1**. There are two types of increment operators: `pre-increment` and `post-increment`. |
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## Pre-increment |
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The pre-increment operator increases the value of a variable by 1 and then returns the value. For example: |
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```js |
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let x = 1; |
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console.log(++x); // 2 |
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console.log(x); // 2 |
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``` |
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## Post-increment |
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The post-increment operator returns the value of a variable and then increases the value by 1. For example: |
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```js |
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let x = 1; |
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console.log(x++); // 1 |
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console.log(x); // 2 |
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``` |
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