ChevronJS

A TypeScript IoC library for lazy dependency injection.

Introduction

Chevron is a TypeScript IoC library for lazy dependency injection inspired by the AngularJS Module API and Spring's DI System. Unlike similar libraries like InversifyJS, Ana and others, Chevron does not depend on reflection metadata or compile time tooling; The trade-off by not using those is that less type validation and Quality of Life features are possible.

Not suitable for production, use InversifyJS instead of this.

Docs

Usage

npm install chevronjs

Basic usage:

import { Chevron } from "chevronjs";

// Create a new chevron instance.
const chevron = new Chevron<null>();

type LoggingNoop = () => void;

const myFunction: LoggingNoop = () => {
    console.log("Hello world!");
};

// Register the myFunction variable as a plain injectable.
chevron.registerInjectable<LoggingNoop, LoggingNoop>(myFunction);

// Retrieve injectable (could also be done using `chevron.getInjectableInstance("myFunction")`.
const myFunctionInstance = chevron.getInjectableInstance<LoggingNoop>(
    myFunction
);

Custom names can be set like this:

import { Chevron } from "chevronjs";

const chevron = new Chevron<null>();

type LoggingNoop = () => void;

const myFunction: LoggingNoop = () => {
    console.log("Hello world!");
};
chevron.registerInjectable<LoggingNoop, LoggingNoop>(myFunction, {
    // A custom name can either be a string or another nameable value like a function.
    name: "myCoolName",
});

const myFunctionInstance = chevron.getInjectableInstance<LoggingNoop>(
    "myCoolName"
);

Factories

A core feature are factories, which describe the process how injectable instances are created. The default factory for this is DefaultFactory.IDENTITY which means that values are not modified during instantiation. When dealing with dependencies though (Chapter "Dependencies"), it is essential to have a factory which allows usage of those dependencies: DefaultFactory.CLASS allows class-like injectables to receive their dependencies as constructor parameters and DefaultFactory.FUNCTION allows injectables that are plain functions to simply receive them as arguments.

import { Chevron, InjectableClassInitializer, DefaultFactory } from "chevronjs";

const chevron = new Chevron<null>();

class MyClass {
    private readonly modifier: number;

    public constructor() {
        this.modifier = 2;
    }

    public getDouble(n: number): number {
        return n * this.modifier;
    }
}

chevron.registerInjectable<MyClass, InjectableClassInitializer<MyClass, void>>(
    MyClass,
    {
        // Use the "CLASS" factory to instantiate the value as class
        factory: DefaultFactory.CLASS(),
    }
);

const myClassInstance = chevron.getInjectableInstance<MyClass>(MyClass);

import { Chevron, DefaultFactory } from "chevronjs";

const chevron = new Chevron<null>();

type MathUnaryOperation = (val: number) => number;
const multiply: MathUnaryOperation = (val: number) => val * 2;

const myFunction: () => MathUnaryOperation = () => multiply;
chevron.registerInjectable<MathUnaryOperation, () => MathUnaryOperation>(
    myFunction,
    {
        // Use the "FUNCTION" factory to instantiate the value as a function
        factory: DefaultFactory.FUNCTION(),
    }
);

const myFunctionInstance = chevron.getInjectableInstance<MathUnaryOperation>(
    myFunction
);

Custom factories can also be used to modify values during instantiation:

import { Chevron } from "chevronjs";

const chevron = new Chevron<null>();

const myInjectable = 16;
chevron.registerInjectable<number, number>(myInjectable, {
    factory: (val: number) => val * 2,
    name: "val",
});

const myFunctionInstance = chevron.getInjectableInstance<number>("val");

Dependencies

When an injectable relies on others in order to be instantiated, you can declare those as its dependencies:

import { Chevron, InjectableClassInitializer, DefaultFactory } from "chevronjs";

const chevron = new Chevron<null>();

type MathFn = (a: number) => number;
const doublingFn: MathFn = (a: number) => a * 2;

chevron.registerInjectable<MathFn, MathFn>(doublingFn);

class MyClass {
    public constructor(private readonly doublingFnAsDep: MathFn) {}

    public getDouble(n: number): number {
        return this.doublingFnAsDep(n);
    }
}

/*
 * Register injectable with dependency - we could also use `["doublingFn"]`.
 * We want MyClass to be instantiated by constructing it through the CLASS factory,
 * where we will have the dependencies as constructor parameters.
 */
chevron.registerInjectable<MyClass, InjectableClassInitializer<MyClass>>(
    MyClass,
    {
        dependencies: [doublingFn],
        factory: DefaultFactory.CLASS(),
    }
);

// When retrieving, all dependencies will be resolved first.
const myClassInstance = chevron.getInjectableInstance<MyClass>(MyClass);

All dependencies and sub-dependencies will be resolved and instantiated if needed when retrieving an injectable that needs to be instantiated.

Scopes

Injectables can have scopes defining when new instances are created and reused; By default, DefaultScopes.SINGLETON is used, meaning only a single instance of each injectable will be created which will be reused for every retrieval. There is also DefaultScopes.PROTOTYPE which is the opposite, creating a new instance for every single request.

import {
    Chevron,
    InjectableClassInitializer,
    DefaultFactory,
    DefaultScope,
} from "./src/main";

const chevron = new Chevron<null>();

class MyClass {}

chevron.registerInjectable<MyClass, InjectableClassInitializer<MyClass, void>>(
    MyClass,
    {
        factory: DefaultFactory.CLASS(),
        scope: DefaultScope.PROTOTYPE(),
    }
);

const myClassInstance1 = chevron.getInjectableInstance<MyClass>(MyClass);
const myClassInstance2 = chevron.getInjectableInstance<MyClass>(MyClass);

Scopes can be also be used to provide for example session based instances:

import {
    Chevron,
    DefaultFactory,
    InjectableClassInitializer,
} from "./src/main";

interface SessionContext {
    sessionId: string;
}

const chevron = new Chevron<SessionContext>();

class MySession {}

chevron.registerInjectable<
    MySession,
    InjectableClassInitializer<MySession, void>
>(MySession, {
    factory: DefaultFactory.CLASS(),
    // Define a custom scope to create scopes based on the property `sessionId` of the context.
    scope: (context: SessionContext | null) => {
        if (context == null) {
            return "DEFAULT";
        }
        return context.sessionId;
    },
});

// Injectable retrieval can pass optional context data to influence scoping.
const mySessionInstanceFoo = chevron.getInjectableInstance<MySession>(
    MySession,
    {
        sessionId: "123",
    }
);
const mySessionInstanceBar = chevron.getInjectableInstance<MySession>(
    MySession,
    {
        sessionId: "987",
    }
);
const mySessionInstanceBarAgain = chevron.getInjectableInstance<MySession>(
    MySession,
    { sessionId: "987" }
);

Note that if a scope function returns null, a new instance, that will not be re-used, will be created.

TypeScript Decorators

Chevron provides also provides TypeScript Decorators for its API. Keep in mind that decorators are an experimental TypeScript feature and might not be fully stabilized yet.

import { Chevron, Injectable } from "./src/main";

const chevron = new Chevron<null>();

// Same as chevron.registerInjectable(Foo, { factory: DefaultFactory.CLASS() });
@Injectable<Foo>(chevron)
class Foo {
    public getFoo(): string {
        return "foo";
    }
}

@Injectable<FooBar>(chevron, {
    dependencies: [Foo],
})
class FooBar {
    public constructor(private readonly foo: Foo) {}

    public getFooBar(): string {
        return this.foo.getFoo() + "bar";
    }
}

const fooBarInstance = chevron.getInjectableInstance<FooBar>(FooBar);