Is it possible to retrieve the second computed type in an overloaded method using TypeScript?

Looking for a solution to receive the second calculated type in an overload method using TypeScript

  type V1 = 'v1';
  type V2 = 'v2';
  type Versions = V1 | V2;

  async analyze(test: 'v1', data: number): Promise<void>;
  async analyze(test: 'v2', data: string): Promise<void>;
  async analyze(test: Versions, data: string | number): Promise<void> {
    switch (test) {
      case 'v1':
        return this.checkNumber(data); // data is number
      case 'v2':
        return this.checkString(data); // data is string
    }
  }

  async checkString(data: string): Promise<void> {
    console.log(data);
  }

  async checkNumber(data: number): Promise<void> {
    console.log(data);
  }

The current workaround involves adding conditional typing within the switch statement:

type Data<Type> = Type extends V1 ? number : string;

async analyze(test: 'v1', data: number): Promise<void>;
async analyze(test: 'v2', data: string): Promise<void>;
async analyze(test: Versions, data: Data<typeof test>): Promise<void> {
  switch (test) {
    case 'v1':
      return this.checkNumber(data as Data<typeof test>); // data is number
    case 'v2':
      return this.checkString(data as Data<typeof test>); // data is string
  }
}

Are there any alternative ways to achieve this?

Answer №1

Due to the fact that the return type of your method remains constant regardless of input types (always a Promise<void>), you can simplify the code by utilizing a discriminated union with tuple types as rest parameters and then destructure it into the test and data parameters. To put it simply, here is the implementation:

async evaluate(
    ...[test, data]: [test: 'v1', data: number] | [test: 'v2', data: string]
): Promise<void> {
    switch (test) {
        case 'v1':
            return this.verifyNumber(data);
        case 'v2':
            return this.validateString(data);
    }
}

The parameter names on the left side of the colon represent destructured parameters, while those on the right side are tuple labels. The compiler understands that when 'v1' is used for test, number should be used for data</code, and when <code>'v2' is used for test, string should be used for data.

For callers, functions accepting unions of rest tuples appear similar to overloads from the caller's perspective:

async callFunction() {
    // IntelliSense displays 2 overloads:
    // 1/2 analyze(test: "v1", data: number): Promise<void>
    // 2/2 analyze(test: "v2", data: string): Promise<void>

    this.evaluate("v2", ""); // works fine
    this.evaluate("v1", ""); // results in an error
}

Feel free to explore and experiment further through this Playground link!

Answer №2

It is generally not advisable to mix data types, but as a last resort, you can utilize the "any" datatype.

async sampleFunction(parameter: any) : Promise<void> {
...
}

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