Very simple JavaScript Reduce polyfill

reduce.js

/**
 * 
 * @param {(acc: any, curr: any, index: number, list:) => any} cb 
 * @returns {any}
 */
function myReduce(cb, initialValue) {
    const arr = this;
    let acc = initialValue;
    for (let i = 0; i < arr.length; i++) {
        acc = cb(acc, arr[i], i, arr);
    }
    return acc;
}

Array.prototype.myReduce = myReduce;

const arr = [1, 2, 3, 4, 5];
const sum = arr.myReduce((acc, curr) => acc + curr, 0);
console.log(sum);


const arr2 = [1, 2, 3, 4, 5];
const sum2 = arr2.myReduce((acc, curr) => {
    return acc
        .then(() => new Promise((res) => setTimeout(() => res(curr), 1000)))
        .then(console.log);
}, Promise.resolve(0));

sum2.then(console.log);