Removing 'undefined' from a return type in Typescript when a default value is given: A guide

class StorageUnit {
    records: Record<string, string> = {};
    getEntry(key: string, defaultValue?: string): string | undefined {
        return this.records[key] ?? defaultValue;
    }
}

const unit = new StorageUnit();

const entry1 = unit.getEntry("test");
const entry2 = unit.getEntry("test", "def");
  1. The variable entry1 is of type string | undefined, which is expected behavior.
  2. However, entry2 also has the same type as entry1. How can we modify the signature of getEntry() so that when a default value is provided, there is no undefined? For example, entry2 should be of type string only.
typescript

Answer №1

The simplest method (that also integrates well with intellisense) is using function overloads:

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

Playground Link

You could also achieve this using generics and conditional types for a solution that is less readable:

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

Answer №2

It is recommended to use a generic in this scenario, however, ensure that your return value does not include any string. This is because the values of your Record are currently set as type any.

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

If you plan on restricting the values to only strings, consider updating the type of the record.

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

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