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Ilya Kantor 2019-12-01 14:56:11 +03:00
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1-js/11-async/01-callbacks/article.md
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@ -2,10 +2,12 @@
# Introduction: callbacks # Introduction: callbacks
```warn header="We use browser methods here" ```warn header="We use browser methods in examples here"
To demonstrate the use of callbacks, promises and other abstract concepts, we'll be using some browser methods; specifically, loading scripts and performing simple document manipulations. To demonstrate the use of callbacks, promises and other abstract concepts, we'll be using some browser methods: specifically, loading scripts and performing simple document manipulations.
If you're not familiar with these methods, and their usage in the examples is confusing, or if you would just like to understand them better, you may want to read a few chapters from the [next part](/document) of the tutorial. If you're not familiar with these methods, and their usage in the examples is confusing, you may want to read a few chapters from the [next part](/document) of the tutorial.
Although, we'll try to make things clear anyway. There won't be anything really complex browser-wise.
``` ```
Many actions in JavaScript are *asynchronous*. In other words, we initiate them now, but they finish later. Many actions in JavaScript are *asynchronous*. In other words, we initiate them now, but they finish later.
@ -18,13 +20,15 @@ Take a look at the function `loadScript(src)`, that loads a script with the give
```js ```js
function loadScript(src) { function loadScript(src) {
// creates a <script> tag and append it to the page
// this causes the script with given src to start loading and run when complete
let script = document.createElement('script'); let script = document.createElement('script');
script.src = src; script.src = src;
document.head.append(script); document.head.append(script);
} }
``` ```
It appends to the document the new, dynamically created, tag `<script src="…">`. The browser loads and executes it. It appends to the document the new, dynamically created, tag `<script src="…">` with given `src`. The browser automatically starts loading it and executes when complete.
We can use this function like this: We can use this function like this: