Partial type for reduce initialValue

Search Terms

reduce initialvalue partial type assertion

Suggestion

Add a signature to the type for Array.reduce<U> to allow the initialValue to be a Partial<U>

Use Cases

Many cases where the reduce function is used to build an object that will eventually conform to U, but starts with an empty accumulator.

Examples

Simple example where the initialValue does not satisfy U but the final result will:

['one'].reduce<{ one: string }>((acc, i) => ({ [i]: i, ...acc }), {})

This yields

Argument of type '{}' is not assignable to parameter of type '{ one: string; }'.
Property 'one' is missing in type '{}' but required in type '{ one: string; }'.

Calling the function without an initialValue causes problems because the initialValue needs to be an object for this to work.

The below type signature would fix this issue:

interface Array<T> {
  reduce<U>(callbackfn: (previousValue: U, currentValue: T, currentIndex: number, array: T[]) => U, initialValue: Partial<U>): U;
}

Checklist

My suggestion meets these guidelines:

• [?] This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

Start by reviewing the Array.reduce signature described in the issue and the example using an empty object as initialValue. Check how the proposed Partial parameter affects type safety and existing reduce overloads. Done means the proposal is resolved with an agreed type-system design and corresponding validation.