How to define global function in TypeScript?

Question

I want to define a global function that is available everywhere, without the need to import the module when used.

This function aims to replace the safe navigation operator (?) available in C#. For the sake of readability, I don't want to prefix the function with a module name.

Global.d.ts:

declare function s<T>(someObject: T | null | undefined, defaultValue?: T | null | undefined) : T; 

Global.tsx:

///<reference path="Global.d.ts" />  export function s<T>(object: T | null | undefined, defaultValue: T | null = null = {} as T) : T {     if (typeof object === 'undefined' || object === null)         return defaultValue as T;     else         return object; } 

App.tsx (root TypeScript file):

import 'Global'; 

Other TSX file (method usage):

s(s(nullableVar).member).member; //Runtime error 

This compiles fine, however, in the browser this throws 's is not a function'.

Answer 1

You're defining the type for the compiler, but not actually attaching it to the global namespace — window in the browser, global in node. Instead of exporting it from the module, attach it. For isomorphic use, use something like...

function s() { ... }  // must cast as any to set property on window const _global = (window /* browser */ || global /* node */) as any _global.s = s 

You can also ditch the .d.ts file and declare the type in the same file using declare global, e.g.

// we must force tsc to interpret this file as a module, resolves // "Augmentations for the global scope can only be directly nested in external modules or ambient module declarations." // error export {}  declare global {   function s<T>(someObject: T | null | undefined, defaultValue?: T | null | undefined) : T; }  const _global = (window /* browser */ || global /* node */) as any _global.s = function<T>(object: T | null | undefined, defaultValue: T | null = null) : T {   if (typeof object === 'undefined' || object === null)     return defaultValue as T;   else     return object; }