TypeScript: How to write definition when I extend a module?

Question

I extend Chai with a helper in my TypeScript test.

import * as chai from 'chai';

chai.use((_chai) => {
  let Assertion = _chai.Assertion;
  Assertion.addMethod('sortedBy', function(property) {
    // ...
  });
});

const expect = chai.expect;

In the same file test case makes use of this method:

expect(tasks).to.have.been.sortedBy('from');

Compiler gives error that "Property 'sortedBy' does not exist on type 'Assertion'".

How can I add declaration of sortedBy to Chai.Assertion?

I've tried to add module declaration, just like other Chai plugin modules do, but it doesn't work.

declare module Chai {
  interface Assertion {
    sortedBy(property: string): void;
  }
}

I don't want to make the helper an individual module, because it's trivial.

@DanRevah I'm using tsd. My question is how to define my extension to the module. The method 'sortedBy' is not included in chai typings. — aleung
– aleung, Commented Feb 29, 2016 at 10:55

Amid · Accepted Answer · 2016-02-29 16:47:10Z

4

Try the following:

Extend chai in the chaiExt.ts like this:

declare module Chai 
{
    export interface Assertion 
    {
        sortedBy(property: string): void;
    }
}

Consume in chaiConsumer.ts:

import * as chai from 'chai';
//...
chai.expect(tasks).to.have.been.sortedBy('from');

[EDIT]

If you are using 'import' - you turn your file into the external module and declaration merging is not supported: link

edited Feb 29, 2016 at 16:47

answered Feb 29, 2016 at 15:00

Amid

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3 Comments

aleung Over a year ago

I know this way works. But I'm looking for a way to extend and declare it just in-place in my test file.

Amid Over a year ago

If you will use 'import' - you turn your file into the external module and declaration merging is not supported there: github.com/Microsoft/TypeScript/issues/2821

aleung Over a year ago

Thanks. The link explains why my declaration does't work. Let me spend more time to look into it.

thitemple · Accepted Answer · 2016-02-29 15:11:43Z

0

Your code is correct. Modules and interfaces are opened by default on TS so you can redeclare and augment them.

Generally, what I do in cases like this is: I create a globals.d.ts file in the same folder as my project so the .d.ts will be automatically loaded, and then I'd add the type definition as you did.

declare module Chai {
  interface Assertion {
    sortedBy(property: string): void;
  }
}

answered Feb 29, 2016 at 15:11

thitemple

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Comments

Tomasz Pluskiewicz · Accepted Answer · 2024-04-22 10:53:37Z

0

When I wanted to extend chai assertions, I needed to declare the interface slightly differently

declare global {
  namespace Chai {
    interface Assertion {
      sortedBy(property: string): void;
    }
  }
}

answered Apr 22, 2024 at 10:53

Tomasz Pluskiewicz

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