When do you use declare in TypeScript?

Question

In TypeScript, why do you sometimes need to use declare for declaring a variable, and sometimes you don't (the same question is true for functions, …)?

To give an example: When (and why) do I use

declare var foo: number;

if

let foo: number;

would do the same (at least it seems to me as if it did the same, i.e. they both declare a variable called foo of type number). What's the difference?

Answer 1

You never use declare to declare a variable. You just use it to let TypeScript know that the variable exists, even though it's not declared in the code (for instance, because it's a global declared in other code, or because you're going to combine the JavaScript output of tsc with another file that declares the variable). Or put it another way: It only declares it for the TypeScript compiler, not for the JavaScript runtime.

If you use the playground to compile your declare var foo: number;, it literally outputs nothing for that declaration; example.

In contrast, let foo: number; (or var foo: number;) is a variable declaration; example.