Typescript definitions, ES6 class, and constructors

Question

I have a TypeScript project in which I'd like to use libmarkov from npm. It provides an ES6-importable class called Generator. You use it via new Generator('some text').

In my local project, I created a file typedefs/libmarkov.d.ts:

export class Generator {
    constructor(text: string);
    generate(depth: number);
}

I use typings to install it: typings install --save file:./typedefs/libmarkov.d.ts

However, this: let generator = new Generator('Foo Bar Baz');

...generates this compiler error:

Error:(5, 21) TS2351: Cannot use 'new' with an expression whose type lacks a call or construct signature.

I could change my constructor: constructor(text: string) {};

...but this gives:

Error:(2, 31) TS1183: An implementation cannot be declared in ambient contexts.

If it matters, I'm targeting TS 2.0.

If I replace with export, I still get the error about new. If I then add {} at the end of constructor, I still get the new error, but also get Error:(2, 31) TS1183: An implementation cannot be declared in ambient contexts. — Paul Everitt
– Paul Everitt, Commented Sep 12, 2016 at 16:15

Bruno Grieder · Accepted Answer · 2016-09-12 16:22:11Z

7

Since this is a js library, I suspect you load it with something like

import {Generator} from "libmarkov"

In which case your external module definition must look like

declare module "libmarkov" {

    export class Generator {
        constructor(text: string);
        generate(depth: number);
    }
}

EDIT The definition is wrong; libmarkov seems to use a default export.

declare module "libmarkov" {

    export default class Generator {
        constructor(text: string);
        generate(depth: number);
    }
}

And the import would be

import Generator from 'libmarkov'

edited Sep 12, 2016 at 16:22

answered Sep 12, 2016 at 15:49

Bruno Grieder

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6 Comments

Paul Everitt Over a year ago

Yes, that's what results under typings/modules/libmarkov/index.d.ts after I run typings. I guess typings wraps my libmarkov.d.ts in the declare module. However, I'm still stuck with the error about new.

Bruno Grieder Over a year ago

The definition is wrong; libmarkov seems to use a default export. See edit above

Paul Everitt Over a year ago

Here's a question that might help me understand ambient/global etc. For this scenario (external ES6 module in node_modules), after I run my typings, should libmarkov wind up in typings/globals or typings/modules? Meaning, do I do typings --global --save or just typings -save?

Paul Everitt Over a year ago

Hmmm, not quite there. The commonjs that TypeScript generates says (to paraphrase) var libmarkov_1 = require('libmarkov'); new libmarkov_1.default('Foo Bar zBaz');. But .default is undefined.

Bruno Grieder Over a year ago

Try const Generator = require('libmarkov'); new Generator('Foo Bar')

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