How can I declare a function from another file in Typescript?

Question

You can do this (assuming title and message are both supposed to be strings):

interface alertWinInterface{
    (title:string, message: string):any;
}

declare var alertWin: alertWinInterface;

You could put this in the same file, or put it in a separate ambient definitions file (.d.ts) which you import:

/// <reference path="myDefinitions.d.ts" />

Or, you could just import the other file that has the actual function definition, but you won't get static typing support.

This approach seems to work for me:

declare function alertWin(title: string, message: string) : void;

And as with Matt's solution you put it in a definition file and then reference it.

You just need to tell the tools and compiler where to find your function, by adding a reference to the top of your file:

/// <reference path="fileWithFunction.ts" />

Additionally, all of your parameters are currently typed as any, you can type them explicitly if you wish.

function alertWin(title: string, message: string) : void {
   //.......
   //.......
}