export type ReducerFunction<ValueType, ReducedValueType = ValueType> = (
	previousValue: ReducedValueType,
	currentValue: ValueType,
	index: number
) => PromiseLike<ReducedValueType> | ReducedValueType;

/**
Reduce a list of values using promises into a promise for a value.

@param input - Iterated over serially in the `reducer` function.
@param reducer - Expected to return a value. If a `Promise` is returned, it's awaited before continuing with the next iteration.
@param initialValue - Value to use as `previousValue` in the first `reducer` invocation.
@returns A `Promise` that is fulfilled when all promises in `input` and ones returned from `reducer` are fulfilled, or rejects if any of the promises reject. The resolved value is the result of the reduction.

@example
```
import pReduce from 'p-reduce';
import humanInfo from 'human-info'; // Not a real module

const names = [
	getUser('sindresorhus').then(info => info.name),
	'Addy Osmani',
	'Pascal Hartig',
	'Stephen Sawchuk'
];

const totalAge = await pReduce(names, async (total, name) => {
	const info = await humanInfo(name);
	return total + info.age;
}, 0);

console.log(totalAge);
//=> 125
```
*/
export default function pReduce<ValueType, ReducedValueType = ValueType>(
	input: Iterable<PromiseLike<ValueType> | ValueType>,
	reducer: ReducerFunction<ValueType, ReducedValueType>,
	initialValue?: ReducedValueType
): Promise<ReducedValueType>;