How can I add expect() to the global namespace with Typescript?

Question

I am able to make the linter recognize global.expect as a variable with the following

// types.d.ts
namespace NodeJS {
  interface Global {
    expect: any
  }
}

However, it won't autocomplete any of expect() methods. I figure it's cause it's any.

I'm trying to type it to the expect imported type but it's not working, Here's what I've tried.

import expectExport from 'expect';

namespace NodeJS {
  interface Global {
    expect: expectExport
  }
}

What am I missing?

Where *is* expect coming from? Have you installed the types for that package? — jonrsharpe, Apr 07 '20 at 18:12
Does this answer your question? [Create a global variable in TypeScript](https://stackoverflow.com/questions/38906359/create-a-global-variable-in-typescript) — Shlang, Apr 07 '20 at 20:44
@Shlang I tried that, but it's particularly the expect library that I don't understand how the types are exported — Christopher Francisco, Apr 07 '20 at 22:32
I missed the point you want to use it as `global.expect` not just `expect`. — Shlang, Apr 07 '20 at 22:52

score 1 · Accepted Answer · answered Apr 07 '20 at 21:13

TL;DR

Do the following in global-expect.ts:

import expectExports from "expect";

declare global {
    namespace NodeJS {
        interface Global {
            expect: typeof expectExports;
        }
    }
}

global.expect = expectExports;

Make sure you import/execute the file somewhere at your application's entry point, e.g. index.ts:

import "./global-expect";

Then you can use this anywhere:

global.expect(...); // (property) NodeJS.Global.expect: <...>(actual: ...) => Matchers<...>

Explanation

the NodeJS namespace is in the global scope so it must be declared that way inside of a module:
```
declare global {
  namespace NodeJS {
    ...
  }
}
```
the type expect property in the Global interface must be a type of the "expect" exports - not the instance of it. So it must be declared like this:
```
expect: typeof expectExports;
```

then you actually have to set the property:

import expectExports from "expect";

...

global.expect = expectExports;

score 0 · Answer 2 · answered Apr 07 '20 at 23:04

A bit simpler way to accomplish it is to augment NodeJS namespace in some d.ts file

declare namespace NodeJS {
    interface Global {
        expect: typeof import("expect")
    }
}

It also may be handy to use expect as a global variable, so you can make a global.d.ts file near to tsconfig.json and put declaration there

// global.d.ts
declare const expect: typeof import("expect");

How can I add expect() to the global namespace with Typescript?

2 Answers2

TL;DR

Explanation