Subsequent variable declarations must have the same type. any

Question

I'm using ava (no link, since I'm not allowed to use more than 2 😕) for testing and want to type ava's test context. It's typed as any in ava's definition file.

What I specifically want is that the typescript compiler knows that t.context is of the type {foo: number} in the following test:

import test from 'ava'

test.beforeEach((t) => {
  t.context = { foo: 5 }
})

test('Is context typed', (t) => {
  // uncaught typo
  t.is(t.context.fooo, 5)
})

I tried to use declaration merging to do this, but it fails with TS2403: Subsequent variable declarations must have the same type. Variable 'context' must be of type 'any', but here has type '{ foo: number; }'. :

declare module 'ava' {
    interface ContextualTestContext {
      context: {
        foo: number,
      }
    }
}

test.beforeEach((t) => {
  t.context = { foo: 5 }
})

test('Is context typed', (t) => {
  // uncaught ypo
  t.is(t.context.fooo, 5)
})

Is there a way to do this without casting the context all the time like so:

interface IMyContext {
  foo: number
}

test.beforeEach((t) => {
  t.context = { foo: 5 }
})

test('Is context typed', (t) => {
  const context = <IMyContext> t.context
  // caught typo
  t.is(context.fooo, 5)
})

Answer 1

Typing the context will be possible with the next version of ava. Then you can do something like this:

import * as ava from 'ava';

function contextualize<T>(getContext: () => T): ava.RegisterContextual<T> {
    ava.test.beforeEach(t => {
        Object.assign(t.context, getContext());
    });

    return ava.test;
}

const test = contextualize(() => {
    return { foo: 'bar' };
});

test.beforeEach(t => {
    t.context.foo = 123; // error:  Type '123' is not assignable to type 'string'
});

test.after.always.failing.cb.serial('very long chains are properly typed', t => {
    t.context.fooo = 'a value'; // error: Property 'fooo' does not exist on type '{ foo: string }'
});

test('an actual test', t => {
    t.deepEqual(t.context.foo.map(c => c), ['b', 'a', 'r']); // error: Property 'map' does not exist on type 'string'
});

If you acquire your context asynchronously you need to change the type signature of contextualize accordingly:

function contextualize<T>(getContext: () => Promise<T>): ava.RegisterContextual<T> {
    ava.test.beforeEach(async t => {
        Object.assign(t.context, await getContext());
    });

    return ava.test;
}

const test = contextualize(() => {
    const db = await mongodb.MongoClient.connect('mongodb://localhost:27017')

    return { db }
});

Otherwise the TypeScript compiler will think t.context is a Promise, although it isn't