JavaScript: Iterable Object

Purpose of Iteratable Object

• An iterable object can hold special values to be iterated over using for-of Loop. Items iterated over are not necessarily properties. It can be characters in string, or items in Map Object, or anything the iteratable object is designed for, such a list of prime numbers.
• An iterable object's items can be turned into an array, or function arguments, by using Spread Operator.
• An iterable object can have potentially infinite number of items, computed at the time when needed. As opposed to a array, where every item already exists and occupy memory.

What is Iterable Object

A iterable object is basically any object with a Symbol.iterator method (own property or inherited).

Here is what it means:

1. Symbol is a new type of value in JS2015.
2. The Symbol is a function object. Symbol() returns a value of type symbol.
3. The function object Symbol has many properties.
4. One of Symbol's property key is "iterator", and its value is a symbol. (that is, the type of Symbol.iterator is symbol.)
5. Objects can use the value of Symbol.iterator as a property key. For example, obj_name[Symbol.iterator] = ….
6. The value of obj_name[Symbol.iterator] must be a function that returns a Iterator.
7. Any object that has a property key of Symbol.iterator is called “iterable”.

console.log(
  Symbol.hasOwnProperty("iterator"),
);

console.log(
  (typeof Symbol.iterator) === "symbol",
);

For example, array is iterable object. Because it has inherited property key Symbol.iterator.

const aa = [3, 4, 5];

// array has an inherited property key Symbol.iterator
console.log(Reflect.has(aa, Symbol.iterator) === true);

// not its own property
console.log(
  !aa.hasOwnProperty(Symbol.iterator) === true,
);

// the property Symbol.iterator of array is inherited from Array.prototype

console.log(
  Reflect.apply(
    Object.prototype.hasOwnProperty,
    Array.prototype,
    [Symbol.iterator],
  ) === true,
);

test if a object is iterable

To test if something is iterable, check for the existence of property Symbol.iterator, using Reflect.has

// array is iterable
const aa = [3, 4, 5];
console.log(Reflect.has(aa, Symbol.iterator));

Standard Iterable Objects

Standard objects that are iterables include: String, Array, Set, Map.

console.log(
  [String, Array, Set, Map]
    .every((x) => Reflect.has(x.prototype, Symbol.iterator)) === true,
);

The Iterable Interface

Technically, any object that conforms to the Iterable Interface is called Iterable. To fully understand iterable, you need to understand JavaScript Interface.

Define Your Own Iterable Object

You can easily create your own iterable object using Generator Function .