array-iterator

for

let arr = [12, 23, 34]
arr[Symbol.iterator] = function () {
    let self = this,
        index = 0;
    return {
        next() {
            if(index > self.length-1){
                return {
                    done: true,
                    value: undefined
                }
            }
            return {
                done: false,
                value: self[index++] + 1
            }
        }
    }
}

> for(let i of arr){console.log(i)}
13
24
35