Export declaration file for JavaScript function in TypeScript

Question

I'm new to TypeScript and I want to know how to write declaration file for custom JavaScript function. I tried this, but it give me error "Could not find a declaration file for module './main'." and don't give any IntelliSense in index.ts file.

main.js

var maths={ sum:function (a,b){ console.log(a+b)}}

module.exports=maths;

index.ts

import * as mt from "./main"
mt.sum(2,5);

type.d.ts

 declare module 'main' {
export function sum(a:number,b:number):void }

`

Answer 1

This import

import * as mt from "./main"

is looking for either main.ts or main.d.ts in the same directory as index.ts.

So one thing you can try is to rename type.d.ts to main.d.ts and remove declare module 'main' from it, leave just export function sum at the top level. When imported that way, module name is taken from the file name. Also, you might have to remove reference to type.d.ts from your project (or tsconfig.json) if you have one.

ALternatively, you can try to change the import to

import * as mt from "main"

so that imported module name matches exactly the name given in declare module.