p-reduce-array.js

'use strict';
module.exports = (arr, reducer, initVal) => new Promise((resolve, reject) => {
	let i = 0;

	const next = total => {
		const item = arr[i];

		if (!item) {
			resolve(total);
			return;
		}

		Promise.all([total, item])
			.then(value => {
				next(reducer(value[0], value[1], i++));
			})
			.catch(reject);
	};

	next(initVal);
});