Carlos Vergara Carlos Vergara - 2 months ago 90
TypeScript Question

Building a class registry: Cannot use 'new' with an expression whose type lacks a call or construct signature

I have this setup:

//./things/Base.ts
export default class Base{
constructor(){
console.log("HI I AM A BASE THING");
}
}

//things.ts
import Base = require('./things/Base');

export = {
defaultThing: Base
};

//entry.ts
import Things = require('./things');

new Things.defaultThing();


What I'm trying to do is build a dictionary with the keys I want for classes of a given type, letting me change the underlying implementation without touching the consuming code. This fails with the following message

λ tsc entry.ts
entry.ts(3,1): error TS2351: Cannot use 'new' with an expression whose type lacks a call or construct signature.


Why is this and what's the proper idiom?

Answer

import Base = require(...) does not mix well with export default class Base.

If you add console.dir(Base) to things.ts, you will see that Base is actually a module there, not a class:

{ __esModule: true, default: [Function: Base] }

If you change that import in things.ts to

import Base from './things/Base';

then your example starts working.

The explanation is given in the typescript language specification:

An import require declaration of the form

    import m = require("mod");

is equivalent to the ECMAScript 2015 import declaration

    import * as m from "mod";

That es6 form always imports m as a module, even if it contains default export.

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