TypeScript-All-In-One(A.R)

Creating a namespace

Let’s take a look at the first version of a declaration file for our ErrorHelper JavaScript class in the file named globals.d.ts:

declare module ErrorHelper {
 function containsErrors(response: any): boolean; // checks if response contains any errors
 function trace(message: any): void; // logs a message for debugging purposes
}

This module declaration is acting as a namespace, meaning that we need to reference each of these functions by their fully qualified names, that is, ErrorHelper.containsErrors, and ErrorHelper.trace.

Defining interfaces

Note that even though we have declared the two functions that are available on the ErrorHelper class, we are still missing some crucial information about them.

// This code defines TypeScript interfaces and a module for error handling

interface IResponse {
 responseText: IFailureMessage;
}
interface IFailureMessage {
 failure: boolean | string;
 errorMessage?: string;
}
declare module ErrorHelper {
  function containsErrors(response: IResponse): boolean; // checks if response contains any errors
  function trace(message: IResponse | string): void; // logs a message for debugging purposes
}