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# Hello, world!
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The tutorial that you're reading is about core JavaScript, which is platform-independent. Later on, you'll learn about Node.js and other platforms that use it.
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This part of the tutorial is about core JavaScript, the language itself. Later on, you'll learn about Node.js and other platforms that use it.
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But we need a working environment to run our scripts and, since this book is online, the browser is a good choice. We'll keep the amount of browser-specific commands (like `alert`) to a minimum so that you don't spend time on them if you plan to concentrate on another environment (like Node.js). We'll focus on JavaScript in the browser in the [next part](/ui) of the tutorial.
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@ -46,7 +46,7 @@ The `<script>` tag contains JavaScript code which is automatically executed when
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The `<script>` tag has a few attributes that are rarely used nowadays but can still be found in old code:
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The `type` attribute: <code><script <u>type</u>=...></code>
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: The old HTML standard, HTML4, required a script to have a `type`. Usually it was `type="text/javascript"`. It's not required anymore. Also, the modern HTML standard, HTML5, totally changed the meaning of this attribute. Now, it can be used for JavaScript modules. But that's an advanced topic; we'll talk about modules in another part of the tutorial.
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: The old HTML standard, HTML4, required a script to have a `type`. Usually it was `type="text/javascript"`. It's not required anymore. Also, the modern HTML standard, HTML5, totally changed the meaning of this attribute. Now, it can be used for JavaScript modules. But that's an advanced topic; we'll talk about modules in another part of the tutorial.
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The `language` attribute: <code><script <u>language</u>=...></code>
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: This attribute was meant to show the language of the script. This attribute no longer makes sense because JavaScript is the default language. There is no need to use it.
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The lifecycle of an HTML page has three important events:
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- `DOMContentLoaded` -- the browser fully loaded HTML, and the DOM tree is built, but external resources like pictures `<img>` and stylesheets may be not yet loaded.
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- `load` -- the browser loaded all resources (images, styles etc).
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- `beforeunload/unload` -- when the user is leaving the page.
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- `load` -- not onyl HTML is loaded, but also all the external resources: images, styles etc.
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- `beforeunload/unload` -- the user is leaving the page.
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Each event may be useful:
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- `DOMContentLoaded` event -- DOM is ready, so the handler can lookup DOM nodes, initialize the interface.
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- `load` event -- additional resources are loaded, we can get image sizes (if not specified in HTML/CSS) etc.
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- `load` event -- external resources are loaded, so styles are applied, image sizes are known etc.
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- `beforeunload` event -- the user is leaving: we can check if the user saved the changes and ask them whether they really want to leave.
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- `unload` -- the user almost left, but we still can initiate some operations, such as sending out statistics.
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