Optional generic type

Issue

I have the following logging method:

  private logData<T, S>(operation: string, responseData: T, requestData?: S) {
    this.logger.log(operation + ' ' + this.url);
    if (requestData) {
      this.logger.log('SENT');
      this.logger.log(requestData);
    }
    this.logger.log('RECEIVED');
    this.logger.log(responseData);
    return responseData;
  }

The requestData is optional, I want to be able to call logData without having to specify the S type when I don't send the requestData to the method: instead of: this.logData<T, any>('GET', data), I want to call this.logData<T>('GET', data).

Is there a way to achieve this?

Solution

As per TypeScript 2.2 (you can try it in the TS Playground), calling this.logData("GET", data) (with data of type T) gets inferred succesfully as this.logData<T, {}>("GET", data).

The overload suggested by David Bohunek can be applied if the inference fails with the TS version you use. Anyway, ensure that the second signature is before declared and then defined, otherwise it would not participate in the available overloads.

// Declarations
private logData<T>(operation: string, responseData: T);
private logData<T, S>(operation: string, responseData: T, requestData?: S);
// Definition
private logData<T, S>(operation: string, responseData: T, requestData?: S) {
    // Body
}

Answered By - FstTesla