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Let's have a look at this interpreted on-the-fly compile thing.

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How is the Javascript code in our browser

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executed and not just in our browser but typically in any environment where you run Javascript? Let's

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say you write your Javascript code and you want it to have some effect on the web page

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if we talk about the browser as the environment where we run our script. Then you have one important

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thing built into any environment where you want to run Javascript code

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and that's a Javascript engine.

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It's built into the browser as I said, there in Chrome for example, in the Chrome browser,

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it's v8,

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that's the name of the engine, in Firefox

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the name would be spider monkey

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and of course other browsers also either reuse these engines or have their own engines.

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Now the job of the engine is to parse code, so parse, read and understand your Javascript code,

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then on the fly compile it to machine code because machine code executes faster,

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so it reads your code but it does not necessarily execute it like that but instead, it now takes that code

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and compiles it to code which is faster to execute by the machine and then it executes that machine

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code.

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This all happens in the browser with the help of the Javascript engine

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and then when that code is executed, we have that effect on our web page.

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Now important, modern engines have a lot of optimizations there,

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they might start executing your uncompiled code and then compile the code whilst they're also already

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executing it to get started executing faster and then switch to the compiled code dynamically and so on,

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so we have a lot of optimizations going on here and we will dig a bit deeper into what the Javascript

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engine is

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exactly

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in a separate module in the course, for now this is all we need to know, the browser has a built-in tool

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that takes our code, compiles it, optimizes it and executes it

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and also a bit more technical side note, all of that happens on a single thread.

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Now this is very technical but you might know that in a computer, you have certain tasks that are executed,

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for example the browser you opened is a task or actually might consist of multiple different tasks

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that are executed together.

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The Javascript code is also yet another task which your operating system in the end has to take care

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of

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and this runs on a single thread.

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Of course we have multi-threading on modern machines so that they can do multiple tasks at the same time

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but the Javascript code execution runs on one single thread there,

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so the Javascript code execution always happens in one single thread on your operating system.

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Right now this doesn't really matter,

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it will become more important later in the course though when I will come back to this concept of single

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threading and which implications this has for Javascript.

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But with that we know what interpreted means, what compilation means and how the Javascript code is executed.

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What does dynamic and weakly type mean then?