New in JS2015.
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 an object that has (own or inherited) property
In practice, it just means you can use for-of Loop or Spread Operator or Array.from on it.
Here is what it means:
- Symbol is a new type of value in JS2015.
Symbolis a function object.
Symbol()returns a value of type symbol.
- The function object
Symbolhas many properties.
- One of
Symbol's property key is
"iterator", and its value is a symbol. (that is, the type of
- Objects can use the value of
Symbol.iteratoras a property key. For example,
obj_name[Symbol.iterator] = ….
- The value of
obj_name[Symbol.iterator]must be a function that returns a Iterator.
- Any object that has a property key of
Symbol.iteratoris 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
const aa = [3, 4, 5]; // array has an inherited property key Symbol.iterator console.log(Reflect.has(aa, Symbol.iterator)); // not its own property console.log( aa.hasOwnProperty(Symbol.iterator) === false, ); // the property Symbol.iterator of array is inherited from Array.prototype console.log( Reflect.apply( Object.prototype.hasOwnProperty, Array.prototype, [Symbol.iterator], ), );
Test If Object is Iterable
Standard Iterable Objects
Standard objects that are iterables include: String, Array, Set, Map.
const is_iterable = ((x) => (Reflect.has(x, Symbol.iterator))); console.log( is_iterable(String.prototype), is_iterable(Array.prototype), is_iterable(Set.prototype), is_iterable(Map.prototype), );
The Iterable Interface
Define Your Own Iterable Object
You can easily create your own iterable object using Generator Function .