Create BlogCategories “js”

src/data/categories/js.yaml

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+id: JS
+title: JavaScript
+description: >-
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+  ## [A high-level definition](https://developer.mozilla.org/en-US/docs/Learn/JavaScript/First_steps/What_is_JavaScript#a_high-level_definition "Permalink to A high-level definition")
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+  JavaScript is a scripting or programming language that allows you to implement complex features on web pages — every time a web page does more than just sit there and display static information for you to look at — displaying timely content updates, interactive maps, animated 2D/3D graphics, scrolling video jukeboxes, etc. — you can bet that JavaScript is probably involved. It is the third layer of the layer cake of standard web technologies, two of which ([HTML](https://developer.mozilla.org/en-US/docs/Learn/HTML) and [CSS](https://developer.mozilla.org/en-US/docs/Learn/CSS)) 
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+  The core client-side JavaScript language consists of some common programming features that allow you to do things like:
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+  * Store useful values inside variables. In the above example for instance, we ask for a new name to be entered then store that name in a variable called `name`.
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+  * Operations on pieces of text (known as "strings" in programming). In the above example we take the string "Player 1: " and join it to the `name` variable to create the complete text label, e.g. "Player 1: Chris".
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+  * Running code in response to certain events occurring on a web page. We used a `click` event in our example above to detect when the label is clicked and then run the code that updates the text label.
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+  * And much more!
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