iterators.js

var obj = { foo: "bar", wut: "what" };
var arr = [1,2,3,4,5];

obj[Symbol.iterator] = function() {
    let keys = Object.keys(this);
    let nextIndex = 0;
    return {
        next() {
            return nextIndex < keys.length ?
                { value: keys[nextIndex++], done: false } :
                { value: void 0, done: true };
        }
    };
};

arr[Symbol.iterator] = function() {
    let self = this;
    let nextIndex = 0;
    return {
        next() {
            return nextIndex < self.length ?
                { value: self[nextIndex++], done: false } :
                { value: void 0, done: true };
        }
        //next: () => nextIndex < this.length ? { value: this[nextIndex++], done: false } : { value: void 0, done: true };
    };
};

function makeIterator(array){
    var nextIndex = 0;

    return {
        next: function() {
            return nextIndex < array.length ?
                {value: array[nextIndex++], done: false} :
                {done: true};
        }
    };
}

let arrIter = arr[Symbol.iterator]();
console.log('spread:', [...arrIter]);

let arrIter2 = arr[Symbol.iterator]();
for(let x of arrIter2) {
    console.log('--', x);
}

//let arrIter = makeIterator(arr);
//console.log('spread:', [...arrIter]);

//let arrIter2 = makeIterator(arr);
//for(let x of arrIter2) {
    //console.log('--', x);
/