NodeJS How to import JS file into TypeScript

Question

I'm new with TypeScript. I'm currently learning NodeJS Loopback 4 framework which use Typescript language. And my question is how to import some function, class which has been exported in JS file into my TS file. After search several way but it still doesn't work with me. Here's example:

  // /src/index.ts
    import {plus} from './lib/test';

    console.log(plus(1,2));

// /src/lib/test.js
    export function plus(x, y) {
      return x + y;
    }

I also try using definition typescript like this

// /src/lib/test.d.ts
export declare function plus(x: number, y: number): number;

But still got error when import this function in index.ts file

Error: Cannot find module './lib/test' at Function.Module._resolveFilename (module.js:543:15)

Possible duplicate of How to import js-modules into TypeScript file? — maazadeeb
– maazadeeb, Commented Jan 23, 2019 at 6:13
@MaazSyedAdeeb Sorry, I've followed that post and many post about this but it still got error when I try to import it — Quoc Van Tang
– Quoc Van Tang, Commented Jan 23, 2019 at 6:24
How about you when you import { plus } from './lib/test.js'? Notice the js extension at the end — maazadeeb
– maazadeeb, Commented Jan 23, 2019 at 14:19
It can't. It will be consider to test.ts or test.d.ts and ignore js extension — Quoc Van Tang
– Quoc Van Tang, Commented Jan 23, 2019 at 14:24
I just found that I must put allowJs true in compilerOptions to make js file be compiled. By the way, thank you for your support — Quoc Van Tang
– Quoc Van Tang, Commented Jan 23, 2019 at 16:40

Christopher Slater · Accepted Answer · 2019-01-23 15:56:57Z

9

It looks like the tsconfig.json doesn't have 'allowJs' enabled because it exports declarations.

Is there a reason you are not wanting this to be a typescript file? If you change test.js to test.ts, by making it a .ts should allow it to be recognised in your index file.

UPDATE

Full chat history to get to this point can be found here. Repository used to test found here

OK so easy solution as @maaz-syed-adeeb mentions will work:

import { path } from './lib/test.js'

The reason the extension is important is because the definition file takes priority over a javascript file in a typescript environment. That is why the module import was blowing up.

To avoid specifying the .js extension you can also setup your directory structure like this:

src
|- index.ts
|- lib
  |- test.js
  |- test.d.ts
  |- index.[js|ts]

in ./lib/index file export all from test:

//./src/lib/index.[js|ts]
export * from './test'

and then import all from lib:

// ./src/index.ts
import { path } from './lib'
// or
import { path } from './lib/test.js'

If you are using a blend of javascript and typescript (say you are moving over to typescript with an existing code base), you will need to update your tsconfig.json to include so you don't get the warnings in your IDE:

{
  "compilerOptions": {
    "allowJs": true,
    "declaration": false
  }
}

This is so your javascript files will be transpiled to your destination directory along with the typescript files.

edited Jan 23, 2019 at 15:56

answered Jan 23, 2019 at 6:30

Christopher Slater

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9 Comments

Quoc Van Tang Over a year ago

thanks for your reply. My intention is to import JS function into TS, in real case that maybe in previous I have many function write with JS language and I want to reuse it in my TS Project (loopback 4).

Quoc Van Tang Over a year ago

I've recognize that when I put this { allowJs: true, declaration: false} in my tsconfig.json file then it works but maybe the test.d.ts file may not be required in this case

Christopher Slater Over a year ago

I have actually created a copy of loopback 4 and without the compilerOptions: { allowJs: true, declaration: false} and it works with your example. So I think you can safely disregard that suggestion

Christopher Slater Over a year ago

So just to be clear, I am running npmr run build then npm run start

Quoc Van Tang Over a year ago

That's weird. You mean that you create a new app using lb4 app command than add a js and d.ts file like above, then import it from index.ts and it does not throw any error. For me it produce "Cannot find module './lib/test' " error

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