Problems

• How to use an old layer code for a new layer without effect to the old code?

Adapter pattern

• Create an adapter to connect an old layer to use in the new context.

Steps:

• Determine an old layer
• Create a new layer with input is an old instance
• Using the new layer in new context.

Example:

1. Old layer

class OldSystem {
  oldRequest() {
    return "Old system request";
  }
}

2. New layer

interface Target {
  request(): string;
}

class SystemAdapter implements Target {
  constructor(private oldSystem: OldSystem) {}
  request(): string {
    return this.oldSystem.oldRequest();
  }
}

3. Implementation of the new layer

const oldSystem = new OldSystem();
console.log(oldSystem.oldRequest()); // Old system request

const newSystem = new SystemAdapter(oldSystem);
console.log(newSystem.request()); // Old system request

Conclusions

• Keep the old layer (no need to modify it)
• Easier to expand the old layer
• Can reuse code