Field Injection (ARCH3200)

Key typescript.quality.field-injection · Severity warning · Category quality

Field/property injection hides dependencies, breaks immutability, and makes the class hard to instantiate without the DI container, lowering testability. Dependencies should be taken through the constructor.

Incorrect

class InvoiceService {
  @Inject() private mailer!: Mailer;

  send() { this.mailer.deliver(); }
}

Correct

class InvoiceService {
  constructor(private readonly mailer: Mailer) {}

  send() { this.mailer.deliver(); }
}

Fix

Inject dependencies through readonly constructor parameters instead of field annotations.