@loopback/context

This module provides facilities to manage artifacts and their dependencies using Context in your Node.js applications. It can be used independent of the LoopBack framework.

Overview

The @loopback/context package exposes TypeScript/JavaScript APIs and decorators to register artifacts, declare dependencies, and resolve artifacts by keys. The Context also serves as an IoC container to support dependency injection.

Context and Binding are the two core concepts. A context is a registry of bindings and each binding represents a resolvable artifact by the key.

Bindings can be fulfilled by a constant, a factory function, a class, or a provider.
Bindings can be grouped by tags and searched by tags.
Binding scopes can be used to control how a resolved binding value is shared.
Bindings can be resolved synchronously or asynchronously.
Provide @inject and other variants of decorators to express dependencies.
Support Constructor, property, and method injections.
Allow contexts to form a hierarchy to share or override bindings.
Track bindings and injections during resolution to detect circular dependencies.

Installation

npm install --save @loopback/context

Basic use

Locate an artifact by key

const Context = require('@loopback/context').Context;
const ctx = new Context();
ctx.bind('hello').to('world'); // BindingKey='hello', BindingValue='world'
const helloVal = ctx.getSync('hello');
console.log(helloVal); // => 'world'

The binding can also be located asynchronously:

const helloVal = await ctx.get('hello');
console.log(helloVal); // => 'world'

Dependency injection using context

import {Context, inject} from '@loopback/context';
const ctx = new Context();

// bind 'greeting' to 'Hello' as the constant value
ctx.bind('greeting').to('Hello');

class HelloController {
  constructor(
    // injecting the value bound to `greeting` using context
    @inject('greeting') private greeting: string,
  ) {}

  greet(name) {
    return `${this.greeting}, ${name}`;
  }
}

// Bind 'HelloController' to class HelloController
ctx.bind('HelloController').toClass(HelloController);

async function hello() {
  // Get an instance of HelloController
  const helloController = await ctx.get<HelloController>('HelloController');
  // helloController now has the `greeting` property injected with `Hello`
  console.log(helloController.greet('John')); // => Hello, John
}

hello();

For additional information, please refer to the Context page.

Examples

To learn more about advanced features, check out standalone examples at @loopback/example-context.

Use the following command to download the example project to try out:

lb4 example context

Contributions

Tests

Run npm test from the root folder.

Contributors

See all contributors.

License

MIT