SOURCE

Array.prototype.reduce = function (fn, intVal) {
    let arr = this
    let acc = intVal || arr[0]
    let start = intVal ? 0 :1
    for (let i = start; i < arr.length; i++) {
        const isa = arr[i]
        console.log(acc)
        acc = fn(acc, isa, i, arr)
    }
    return acc
}
let arr = [1, 2, 3]
let res = arr.reduce((a, b) => {
    return a + b
}, 2)

console.log(res)