class MyStore {
    data: Record<string, string> = {};
    getKey(key: string, defaultValue?: string): string | undefined {
        return this.data[key] ?? defaultValue;
    }
}

const store = new MyStore();

const value1 = store.getKey("test");
const value2 = store.getKey("test", "def");

1. Now value1 has type of string | undefined. It is right, no problem there.
2. But value2 has also same type of value1. How to update signature of getKey() so that when default value is provided, there will be no undefined. Like value2 should be of type string only.

CodePudding user response：

Easiest way (which also plays nicely with intellisense) is with function overloads:

class MyStore {
  data: Record<string, string> = {};
  getKey(key: string): string | undefined
  getKey(key: string, defaultValue: string): string
  getKey(key: string, defaultValue?: string): string | undefined {
    return this.data[key] ?? defaultValue;
  }
}

Playground Link

You could also work this out with generics for a considerably less readable solution:

class MyStore {
  data: Record<string, string> = {};
  getKey<T extends string | undefined>(key: string, defaultValue?: T): 
      T extends string ? string : string | undefined {
    return this.data[key] ?? defaultValue;
  }
}

Playground Link

CodePudding user response：

Using a generic should work but your return value should not contain string anyway. Since, your Records values are of type any.

getKey<T = undefined>(key: string, defaultValue?: T): string | T {
        return this.data[key] ?? defaultValue;
}

If you are planning on using only strings for the values. I suggest updating the type of the record.

data: Record<string, string> = {};