Array.prototype[@@iterator]()

[Symbol.iterator]()

<ul id="letterResult">
</ul>

const arr = ['a', 'b', 'c'];
const eArr = arr[Symbol.iterator]();
const letterResult = document.getElementById('letterResult');
// your browser must support for..of loop
// and let-scoped variables in for loops
// const and var could also be used
for (let letter of eArr) {
  const li = document.createElement('LI');
  li.textContent = letter;
  letterResult.appendChild(li);
}

var arr = ['a', 'b', 'c', 'd', 'e'];
var eArr = arr[Symbol.iterator]();
console.log(eArr.next().value); // a
console.log(eArr.next().value); // b
console.log(eArr.next().value); // c
console.log(eArr.next().value); // d
console.log(eArr.next().value); // e

function logIterable(it) {
 if (!(Symbol.iterator in Object.getPrototypeOf(it)
 /* or "Symbol.iterator in Object.__proto__"
    or "it[Symbol.iterator]" */)) {
   console.log(it, ' is not an iterable object...');
   return;
 }

 var iterator = it[Symbol.iterator]();
  // your browser must support for..of loop
  // and let-scoped variables in for loops
  // const and var could also be used
  for (let letter of iterator) {
      console.log(letter);
  }
}

// Array
logIterable(['a', 'b', 'c']);
// a
// b
// c

// string
logIterable('abc');
// a
// b
// c

logIterable(123);
// 123 " is not an iterable object..."