What are ambient declarations

Question

I've been seeing a lot using articles mentioning ambient declarations. For example this article. What are they? Can somebody provide an example? Is an ambient declaration a declaration of a type created outside existing typescript files but used in these files? Are all declarations ambient?

As I understand ambient declarations don't produce any javascript code and are defined using declare keyword. Is this the only case of ambient declarations or there are others?

Answer 1

Yes, ambient declarations let you tell the compiler about existing variable/functions/etc.

For example let's say that in your web page you're using a library that adds a global variable, let's say that it's name is ON_READY and it is a reference to a function.
You need to assign a function to it so you'll do something like:

ON_READY = () => {
    console.log("ready!");
    ...
};

The compiler will complain that:

Cannot find name 'ON_READY'

So you use an ambient declaration to inform the compiler that this variable exists and what it's type is:

declare var ON_READY: () => void;

Now it won't complain about not finding it.

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Edit

When using the declare keyword it is always ambient, just like it says in the article you linked to:

The declare keyword is used for ambient declarations where you want to define a variable that may not have originated from a TypeScript file

Non-ambient declarations are just normal variable/function declarations:

let x: number;
const y = "string";
var a = () => { console.log("here"); }