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Rasmus Schultz
Rasmus Schultz

Posted on

minimal DI container in TypeScript

I tried to come up with the simplest possible, useful, async DI container I could think of, and here's the result:

function Container(provider) {
    const cache = {};

    const container = function (name) {
        if (!cache[name]) {
            cache[name] = provider[name](container);
        }

        return cache[name];
    }

    return container;
}
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Totally trivial - you provide an object (provider) where each property is a function that receives the container and returns a Promise that resolves to a service.

Here's a simple use-case:

class UserCache { }

class UserService {
    constructor(private cache: UserCache) { }
}
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And usage:

const container = Container({
    "cache": async c => new UserCache(),
    "user-service": async c => new UserService(await c("cache"))
});

// and a simple test:

container("user-service").then(service => {
    console.log(service);

    container("user-service").then(same_service => {
        console.log(service === same_service); // true
    });
});
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You can try it here.

This is actually surprisingly powerful - for example, you can bootstrap multiple service-providers like this:

const container = Container({
    ...UserServices,
    ...WebServices,
});
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What I can't figure out, is how to make it type-safe - I know I'll need generics and keyof, but I could really use some help in this area.

Any takers? :-)

Top comments (2)

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mindplay profile image
Rasmus Schultz • Edited on

It's checking the keys - so far so good:

function Container<P extends { [name: string]: { (c: any): Promise<any> } }>(provider: P) {
    const cache: { [name in keyof P]?: Promise<any> } = {};

    const container = function(name: keyof P) {
        if (!cache[name]) {
            cache[name] = provider[name](container);
        }

        return cache[name];
    }

    return container;
}

Now to figure out the generic return-type...

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mindplay profile image
Rasmus Schultz

Someone has been helpful on StackOverflow already - it's partially type-hinted now!

stackoverflow.com/questions/511764...

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