What is declare var in Node.js?

Question

in this nodejs code,

declare var process: NodeJS.Process;
declare var global: NodeJS.Global;
declare var console: Console; 
declare var __filename: string;
declare var __dirname: string;

that...

What's the difference between 'declare var' and 'var'?

When I look up on the googling, I get the word runtime.

wiki says runtime is an operation while a computer program is running....

but i can't understand.

and line 1, what does it mean by ":" after "process" and then "NodeJS.Process"?

Is that mean "process" is equal "NodeJS.Process"?

also line 4, what does it mean by ":" after "__filename" and then "string"?

Is that mean "__filename" is equal "string"?

thanks you.

Answer 1

When you use:

var process: NodeJS.Process;

You are creating a variable named process (with no value defined) and telling the TypeScript compiler to enforce the NodeJS.Process type for assignments.

When you add declare:

declare var process: NodeJS.Process;

You are telling the TypeScript compiler that there is already a variable named process with the type NodeJS.Process. This is useful when you have variables introduced by sources that the compiler is not be aware of.

See Declaration Files in the TypeScript handbook.