Dependency Injection

Dependency injection (DI) is a technique in which a component is given the objects it depends on, usually through its constructor, instead of creating them internally. The code that wires things together is separate from the code that uses them.

Why it matters

When a class builds its own database client or HTTP client, it is tightly coupled to it and hard to test. With DI you can pass in a real client in production and a fake in a test, or swap implementations without editing the class. Frameworks such as Spring, ASP.NET Core and Angular include DI containers that build and supply dependencies automatically, but the idea works with plain constructors and needs no framework.

Example

class Checker {
  constructor(http) { this.http = http; }
  async isUp(url) {
    const res = await this.http.get(url);
    return res.status < 400;
  }
}

// production
new Checker(realHttpClient);
// test
new Checker({ get: async () => ({ status: 200 }) });

Common mistakes

• Treating a DI container as required and hiding dependencies behind global lookups.
• Injecting too many dependencies, a sign the class does too much.
• Confusing it with dependency management through a package manager.

Related

• Mocking
• Unit test
• Dependency
• Singleton

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