Overview

The Adapter pattern (also called Wrapper) bridges two interfaces that were never meant to talk to each other. Instead of rewriting the original class — which may be third-party, legacy, or simply expensive to change — you build a thin object that exposes the interface your code wants and delegates internally to the one that already exists. It is the textbook way to plug a square peg into a round hole without modifying either side.

When to use it

• You need to use an existing class but its interface doesn't match the one your code expects.
• You're integrating a third-party SDK or legacy module that you can't (or shouldn't) modify.
• You want to isolate the rest of the system from a volatile external API behind a stable internal contract.

Example

interface Logger {
  log(message: string): void;
}

class ThirdPartyLogger {
  writeMessage(level: string, payload: { msg: string }) {
    console.log(`[${level}] ${payload.msg}`);
  }
}

class ThirdPartyLoggerAdapter implements Logger {
  constructor(private readonly external: ThirdPartyLogger) {}

  log(message: string): void {
    this.external.writeMessage("info", { msg: message });
  }
}

const logger: Logger = new ThirdPartyLoggerAdapter(new ThirdPartyLogger());
logger.log("App started");