Check out example codes for "typescript declare global". It will help you in understanding the concepts better.

Code Example 1

ts// Type definitions for [~THE LIBRARY NAME~] [~OPTIONAL VERSION NUMBER~]
// Project: [~THE PROJECT NAME~]
// Definitions by: [~YOUR NAME~] <[~A URL FOR YOU~]>

/*~ If this library is callable (e.g. can be invoked as myLib(3)),
 *~ include those call signatures here.
 *~ Otherwise, delete this section.
declare function myLib(a: string): string;
declare function myLib(a: number): number;

/*~ If you want the name of this library to be a valid type name,
 *~ you can do so here.
 *~ For example, this allows us to write 'var x: myLib';
 *~ Be sure this actually makes sense! If it doesn't, just
 *~ delete this declaration and add types inside the namespace below.
interface myLib {
  name: string;
  length: number;
  extras?: string[];

/*~ If your library has properties exposed on a global variable,
 *~ place them here.
 *~ You should also place types (interfaces and type alias) here.
declare namespace myLib {
  //~ We can write 'myLib.timeout = 50;'
  let timeout: number;

  //~ We can access 'myLib.version', but not change it
  const version: string;

  //~ There's some class we can create via 'let c = new myLib.Cat(42)'
  //~ Or reference e.g. 'function f(c: myLib.Cat) { ... }
  class Cat {
    constructor(n: number);

    //~ We can read 'c.age' from a 'Cat' instance
    readonly age: number;

    //~ We can invoke 'c.purr()' from a 'Cat' instance
    purr(): void;

  //~ We can declare a variable as
  //~   'var s: myLib.CatSettings = { weight: 5, name: "Maru" };'
  interface CatSettings {
    weight: number;
    name: string;
    tailLength?: number;

  //~ We can write 'const v: myLib.VetID = 42;'
  //~  or 'const v: myLib.VetID = "bob";'
  type VetID = string | number;

  //~ We can invoke 'myLib.checkCat(c)' or 'myLib.checkCat(c, v);'
  function checkCat(c: Cat, s?: VetID);

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