@owja/ioc

This library implements dependency injection for javascript.

Features

• Similar syntax to InversifyJS
• Can be used without decorators
• Less Features but straight forward
• Can bind dependencies as classes, factories and static values
• Supports binding in singleton scope
• Cached - Resolves only once in each dependent class by default
• Cache can switched off directly at the inject decorator
• Made with unit testing in mind
• Supports dependency rebinding and container snapshots and restores
• Lightweight - Just around 750 Byte gzip and 650 Byte brotli compressed
• Does NOT need reflect-metadata which size is around 50 kb
• 100% written in Typescript

The Container API

Creating a container

The container is the place where all dependencies get bound to. It is possible to have multiple container in our project in parallel.

import {Container} from "@owja/ioc";
const container = new Container();

Binding

Binding a class

This is the default way to bind a dependency. The class will get instantiated when the dependency gets resolved.

container.bind<ServiceInterface>(symbol).to(Service);

Binding a class in singleton scope

This will create only one instance of Service

container.bind<ServiceInterface>(symbol).to(Service).inSingletonScope();

Binding a factory

Factories are functions which will get called when the dependency gets resolved

container.bind<ServiceInterface>(symbol).toFactory(() => new Service());
container.bind<string>(symbol).toFactory(() => "just a string");

A factory can configured for singleton scope too. This way will only executed once.

container.bind<ServiceInterface>(symbol).toFactory(() => new Service()).inSingletonScope();

Binding a value

This is always like singleton scope, but it should be avoid to instantiate dependencies here. If they are circular dependencies, they will fail.

container.bind<ServiceInterface>(symbol).toValue(new Service()); // Bad, should be avoid
container.bind<string>(symbol).toValue("just a string");
container.bind<() => string>(symbol).toValue(() => "i am a function");

Rebinding

This is the way how we can rebind a dependency while unit tests. We should not need to rebind in production code.

container.rebind<ServiceMock>(symbol).toValue(new ServiceMock());

Removing

Normally this function is not used in production code. This will remove the dependency from the container.

container.remove(symbol);

Getting a dependency

Getting dependencies without @inject decorators trough container.get() is only meant for unit tests. This is also the internal way how the @inject decorator and the functions wire() and resolve() are getting the dependency.

container.get<Interface>(symbol);

To get a dependency without @inject decorator in production code use wire() or resolve(). Using container.get() directly to getting dependencies can result in infinite loops with circular dependencies when called inside of constructors. In addition container.get() does not respect the cache.

Important Note: You should avoid accessing the dependencies from any constructor. With circular dependencies this can result in a infinite loop.

Snapshot & Restore

This creates a snapshot of the bound dependencies. After this we can rebind dependencies and can restore it back to its old state after we made some unit tests.

container.snapshot();

container.restore();

The inject Decorator

To use the decorator you have to set experimentalDecorators to true in your tsconfig.json.

First we have to create a inject decorator for each container:

import {createDecorator} from "@owja/ioc";
export const inject = createDecorator(container);

Then we can use the decorator to inject the dependency.

class Example {
    @inject(symbol)
    readonly service!: Interface;
    
    method() {
        this.service.doSomething();
    }
}

The wire() Function

If we do not want to use decorators, we can use the wire function. It does the same like the inject decorator and we have to create the function first like we do with inject.

import {createWire} from "@owja/ioc";
export const wire = createWire(container);

Then we can wire up the dependent to the dependency.

class Example {
    readonly service!: Interface;
    
    constructor() {
        wire(this, "service", symbol);
    }
    
    method() {
        this.service.doSomething();
    }
}

Notice: With wire() the property, in this case service, has to be public.

The resolve() Function

A second way to resolve a dependency without decorators is to use resolve(). To use resolve() we have to create the function first.

import {createResolve} from "@owja/ioc";
export const resolve = createResolve(container);

Then we can resolve the dependency in classes and even functions.

class Example {
    private readonly service = resolve<Interface>(symbol);
    
    method() {
        this.service().doSomething();
    }
}

function Example() {
    const service = resolve<Interface>(symbol);
    service().doSomething();
}

Notice: We access the dependency trough a function. The dependency is not assigned directly to the property/constant. If we want direct access we can use container.get() but we should avoid using get() inside of classes because we then loose the lazy dependency resolving/injection behavior and caching.

The symbol

Symbols are used to identify our dependencies. A good practice is to keep them in one place.

export const TYPE = {
    "Service" = Symbol("Service"),
    // [...]
}

Symbols can be defined with Symbol.for() too. This way they are not unique. Remember Symbol('foo') === Symbol('foo') is false but Symbol.for('foo') === Symbol.for('foo') is true

export const TYPE = {
    "Service" = Symbol.for("Service"),
    // [...]
}

Since 1.0.0-beta.3 we use the symbol itself for indexing the dependencies. Prior to this version we indexed the dependencies by the string of the symbol.

Usage

Step 1 - Installing the OWJA! IoC library

npm install --save-dev @owja/ioc

Step 2 - Create symbols for our dependencies

Now we create the folder services and add the new file services/types.ts:

export const TYPE = {
    "MyService" = Symbol("MyService"),
    "MyOtherService" = Symbol("MyOtherService"),
};

Step 3 - Creating a container

Next we need a container to bind our dependencies to. Let's create the file services/container.ts

import {Container, createDecorator} from "@owja/ioc";

import {TYPE} from "./types";

import {MyServiceInterface, MyService} from "./service/my-service";
import {MyOtherServiceInterface, MyOtherService} from "./service/my-other-service";

const container = new Container();
const inject = createDecorator(container);

container.bind<MyServiceInterface>(TYPE.MyService).to(MyService);
container.bind<MyOtherServiceInterface>(TYPE.MyOtherService).to(MyOtherService);

export {container, TYPE, inject};

Step 4 - Injecting dependencies

Lets create a example.ts file in our source root:

import {container, TYPE, inject} from "./services/container";
import {MyServiceInterface} from "./service/my-service";
import {MyOtherServiceInterface} from "./service/my-other-service";

class Example {
    @inject(TYPE.MyService)
    readonly myService!: MyServiceInterface;
    
    @inject(TYPE.MyOtherSerice)
    readonly myOtherService!: MyOtherServiceInterface;
}

const example = new Example();

console.log(example.myService);
console.log(example.myOtherSerice);

If we run this example we should see the content of our example services.

The dependencies (services) will injected on the first call. This means if you rebind the service after accessing the properties of the Example class, it will not resolve the new service. If you want a new service each time you call example.myService you have to add the NOCACHE tag:

import {container, TYPE, inject} from "./services/container";
import {NOCACHE} from "@owja/ioc";

// [...]

class Example {
    @inject(TYPE.MyService, NOCACHE)
    readonly myService!: MyServiceInterface;
    
    @inject(TYPE.MyOtherSerice, NOCACHE)
    readonly myOtherService!: MyOtherServiceInterface;
}

// [...]

Unit testing with IoC

We can snapshot and restore a container for unit testing. We are able to make multiple snapshots in a row too.

import {container, TYPE} from "./services/container";

beforeEach(() => {
    container.snapshot();
});

afterEach(() => {
    container.restore();
}

test("can do something", () => {
    container.rebind<MyServiceMock>(TYPE.MySerice).toValue(new MyServiceMock());
    const mock = container.get<MyServiceMock>(TYPE.MySerice);
});

Development

Current state of development can be seen in our Github Projects.

Inspiration

This library is highly inspired by InversifyJS but has other goals:

1. Make the library very lightweight (less than one kilobyte)
2. Implementing less features to make the API more straight forward
3. Always lazy inject the dependencies
4. No meta-reflect required

License

License under Creative Commons Attribution 4.0 International

Copyright © 2019 Hauke Broer