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 .