Use JavaScript class in TypeScript

Question

I have a problem introducing TypeScript to our JavaScript project. First I want to use TypeScript only in my part of the code, leaving the JavaScript untouched.

Now I try to use a JavaScript class in my TypeScript code, but I don't find a solution in the last days.

The head of my TypeScript class with import of the JavaScript:

import { BaseLogic } from "../baseLogic";
export class ClaimLogic extends BaseLogic {
...

The JavaScript class ("baseLogic.js"):

module.exports = class BaseLogic {
    constructor(meta, logger) {
    ...

My *.d.ts file ("baseLogic.d.ts"):

export class BaseLogic {
    meta: any;
    log: any;

    constructor(meta: any, logger: any)
}

The head of the compiled JavaScript:

const baseLogic_1 = require("../baseLogic");
class ClaimLogic extends baseLogic_1.BaseLogic {
...

As you see in the compiled JavaScript baseLogic_1.BaseLogic is used. This results in following error:

TypeError: Class extends value undefined is not a constructor or null

With only baseLogic_1 after the extends keyword in the JavaScript file all is fine.

I have no idea about a solution and hope you can help me!

possible duplicate: stackoverflow.com/questions/40463441/… — DDRamone
– DDRamone, Commented Aug 18, 2017 at 8:40

felixmosh · Accepted Answer · 2017-08-18 09:00:11Z

3

Your import suppose to be import * as BaseLogic from "../baseLogic";.

In that way you will get the Class that you put on module.exports.

edited Aug 18, 2017 at 9:00

answered Aug 18, 2017 at 8:42

felixmosh

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3 Comments

JulianG Over a year ago

With this import I musst use "extends BaseLogic.BaseLogic" in the TypeScript code. If not, I got the error "is not a constructor or function type"

felixmosh Over a year ago

No, you will get the Class, so you should extend BaseLogic

JulianG Over a year ago

Sadly the TypeScript compiler is of another opinion

Roman · Accepted Answer · 2017-08-18 08:41:15Z

1

The codesnipet in baseLogic.js exports the class.

module.exports = class BaseLogic {
  constructor(meta, logger) {
  ...
}

You try to access with class ClaimLogic extends baseLogic_1.BaseLogic an object that includes the class BaseLogic

Solution

import BaseLogic from '../baseLogic'
// or:  const BaseLogic = require("../baseLogic");

class ClaimLogic extends BaseLogic {
  ...
}

answered Aug 18, 2017 at 8:41

Roman

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8 Comments

JulianG Over a year ago

Thank you for your answer, but with your solution I got following error: "../baseLogic has no default export"

Roman Over a year ago

can you try it again with import {BaseLogic} from '../baseLogic'

JulianG Over a year ago

Now the error in the TypeScript file is gone, but it's lead to the same compiled JS file.

JulianG Over a year ago

I think there is a problem with the *.d.ts file, but I don't now

Roman Over a year ago

do you try to import the java- oder typescript file?

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