Ginject [ʤɪnject] is a non-intrusive and typesafe dependency injection library for Node.js and JavaScript, powered by TypeScript.

Ginject empowers developers designing decoupled applications and frameworks. Ginject's main goal is increasing the developer experience by offering a tiny, yet powerful API, keeping dependencies in central module definitions and by using TypeScript's type system to restrain runtime challenges.

The concept of ginject's central module definition is inspired by Google Guice. However, ginject is going further by lifting the API to the functional level.

Despite its simplicity, ginject is powerful enough to cover all features provided by Inversify. Direct support for classes and constructors, property injection, rebinding dependencies and dependency cycle detection are just a few of the features worth mentioning.

| | ginject | inversify | |------------------|:----------:|:-----------:| | minified | | | | minzipped | | | | typesafe | ✅ | ❌ | | requirements | none | decorators | | style | functional | imperative | | API surface area | tiny | non-trivial |

Quickstart

The first step is to add ginject to your application.

npm i ginject

Bascially, the only thing needed is to define modules of factories and finally call inject. The resulting container provides concrete instances.

import { inject } from 'ginject';

// create an inversion of control container
const container = inject({
    hi: () => 'Hi',
    sayHi: () => (name: string) => `${container.hi} ${name}!`
});

// prints 'Hi Ginject!'
console.log(container.sayHi('Ginject'));

API

Terminology

The inject function is turning modules into a container. A module is a plain vanilla JS object, composed of nested groups and dependency factories. Factories may return any JS value, e.g. constants, singletons and providers. Unlike Inversify, there is no need to decorate classes.

import { inject, Module } from 'ginject';

// Defining a _context_ of dependencies
type Context = {
    group: {
        value: Value // any JS type, here a class
    }
}

// A _module_ contains nested _groups_ (optional) and _factories_
const module: Module<Context> = {
    group: {
        // a factory of type Factory<Context, Value>
        value: (ctx: Context) => new Value(ctx)
    }
};

// A _container_ of type Container<Module<Context>> = Context
const container = inject(module);

// Values can be obtained from the container
const value = container.group.value;

Context

A container provides each factory with a parameter called context.

type C = {
    value: string
}

const container = inject({
    factory: (ctx: C) => () => ctx.value
});

The context of type C provides a value that can't be resolved. The inject call is type-checked by TS the way that the completeness of the arguments is checked.

Such missing dependencies need to be provided by adding additional modules to the inject call.

const container = inject({
    factory: (ctx: C) => () => ctx.value
}, {
    value: () => '🍸'
});

Now the compiler is satisfied and we can start using the container.

// prints 🍸
console.log(container.factory());

You might have noticed that the container automatically calls the factory and injects itself as the context. The use-site receives the value.

Eager vs lazy initialization

A dependency container.group.value is lazily initialized when first accessed on the container. Turn a factory eager to initialize the dependency at the time of the inject call.

A use case for eager initialization would be to ensure that side effects take place during the initialization of the container.

import { eager, inject, Module } from 'ginject';

type C = {
    gin: string
}

const module: Module<C> = {
    gin: eager(() => {
        const gin = '🍸';
        console.log('Gin mixed');
        return gin;
    })
}

const ctr = inject(module);

console.log('App started');

ctr.gin

In the eager case, the output is

Gin mixed
App started

In the lazy case, the output is

App started
Gin mixed

Please note that eager factories overwrite lazy factories vice versa when rebinding them.

Rebinding dependencies

The main advantage of dependency injection arises from the fact that an application is able to rebind dependencies. That way the structure of a system can be fixated while the behavior can be changed.

The main vehicle for rebinding dependencies is the inject function which receives a variable amount of modules.

The behavior of an application can be enhanced by overwriting existing functionality using additional modules.

type C = {
    readonly print: () => void
    eval: (a: number, b: number) => number
}

const module_0: Module<C> = {
    print: (ctx) => () => {
        console.log(ctx.eval(1, 1));
    },
    eval: () => (a, b) => a + b
};

const ctr = inject(module_0, {
    eval: () => (a: number, b: number) => a * b
});

// = 1
ctr.print();

Cyclic Dependencies

Asynchronous Factories