Allow setters to return

Search Terms

Setters cannot return a value
decorators

Suggestion

By default JavaScript does nothing with the return value of a setter, but it isn't an error.

Allow setters to return as the default behaviour and add flag to turn errors on.

Use Cases

• Decorators that override the setter and use the original setters return value.
• Methods that directly call the setter function and expect a return.

Examples

A live example would be @computed from ember-decorators, which matches the old behaviour from ember.

Code example using legacy decorators (same would be possible with stage 2 decorators):

function saved (target: {}, key: string | symbol, descriptor: PropertyDescriptor) {
  const savedStore = new WeakMap();

  const oldSet = descriptor.set;
  if (!oldSet) {
    throw new Error('Must be used on setter only');
  }
  const oldGet = descriptor.get;
  return {
    ...descriptor,
    get () {
      let response = savedStore.get(this);
      if (!response && oldGet) {
        response = oldGet.call(this);
      }
      return response;
    },
    set (value: any) {
      const result = oldSet.call(this, value);
      savedStore.set(this, result);
    }
  };
}

class Foo {
  @saved
  get square () {
    // default value of 1 if none is saved
    return 1;
  }
  set square (value: number) {
    return value * value;
  }
}

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript / JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. new expression-level syntax)

まず、issue で説明されている setter と decorator の動作を確認します。これには、legacy decorator の例と、その中での PropertyDescriptor.set の使用も含まれます。要求されているデフォルトの戻り値の動作を、提案されている opt-in のエラーフラグおよび既存の JavaScript の動作と比較します。言語の動作とフラグのセマンティクスが仕様化され、適切な compiler テストでカバーされていれば完了です。