Overloading method throws Duplicate error in Typescript

Question

I have the next piece of code:

public async insert(data: iFlower | iFlower[]): Promise<iFlower> | Promise<iFlower[]> {
 await this.insert(data);
}

private async insert(data: iFlower): Promise<iFlower>{
 ....
 return data;
}

private async insert(data: iFlower[]): Promise<iFlower[]> {
 ....
 return data;
}

iFlower is:

export interface iFlower {
   color: string;
   number: string;
}

I get the following errors: The return type of an async function or method must be the global Promise<T> type. Duplicate function implementation. 'insert' is declared but its value is never read.

Is it because iFlower is an interface?

Nicholas Tower · Accepted Answer · 2019-11-06 14:26:04Z

1

When you do an overload in typescript, the only thing you provide multiples of is the types. The actual implementation is just a single function. So, you'll do a series of types for the function, then one implementation which is compatible with all those types, as in:

private async insert(data: iFlower): Promise<iFlower>;
private async insert(data: iFlower[]): Promise<iFlower[]>;
private async insert(data: iFower | iFlower[]): Promise<iFlower> | Promise<iFlower[]> {
  // Your code here. Maybe something like:
  if (Array.isArray(data)) {
    ... 
    return data;
  } else {
    ... 
    return data;
  }
}

edited Nov 6, 2019 at 14:26

answered Nov 6, 2019 at 13:10

Nicholas Tower

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