Question

I have a simple TypeScript (ts) which needs a function from a JavaScript file. How can I import that js file into ts file? Do I have to create a ts file for that js file to be able to use it in my ts file?

Answer 1

The easiest way is to just declare the function you’re using:

File1.js

function greet() { return "Hello!"; }

File2.ts

declare function greet(): string;

/* ... later ... */
var hi = greet();

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If your scenario is more complex (i.e. multiple files referencing File1.js, or there are many functions in File1.js that would clutter up File2.ts), you can make a File1.d.ts file and reference that from File2.ts:

File1.d.ts

function greet(): string;

File2.ts

/// <reference path="File1.d.ts" />

/* ... later ... */
var hi = greet();