ES6 Built-ins.md

ES6 Built-ins

Description: This is an index for the files notes of Built-ins lesson of Udacity Front End Nanodegree course.

Table of Contents:

1. symbols.
2. Object Iteration.

symbols:

• A new JS primitive data type.(number, string, boolean, null, undefined, symbol)
• is "unique, and immutable data type that is used to identify object properties".
• created by invoking Symbol() function.
• Symbol('description') function can have an optional string description that is used for debugging.
• Description is a way to describe the created symbol. It can not access the symbol or affect its value. That's why the symbols with the same description are not equal.
• syntax:

  const bowl = {
  [Symbol('apple')]: { color: 'red', weight: 136.078 },
  [Symbol('banana')]: { color: 'yellow', weight: 183.15 },
  [Symbol('orange')]: { color: 'orange', weight: 170.097 },
  [Symbol('banana')]: { color: 'yellow', weight: 176.845 }
  };
  console.log(bowl);//{Symbol(apple): {…}, Symbol(banana): {…}, Symbol(orange): {…}, Symbol(banana): {…}}
  
  # Object Iteration

Table of Contents:

1. Description.
2. Iterable Protocol.

Description:

• This will show us "How to iterate over an object".

Iterable Protocol

• Definition: define and customize the iteration behavior of objects (which are not iterables). This means that now we have the flexibility of defining and customizing this iteration behavior for objects.
• Using [Symbol.iterator]()

Iterator Protocol

• Definition: define a standard way that an object produces a sequence of values (how an object will iterate).
• Using next()

---

• So for an object to be iterable ---must implement---> iterator method(define how object will be iterated).
• Our iterator method here is [Symbol.iterator]()
• [Symbol.iterator]() is a zero argument method ---returns---> iterator object (which is an object that conforms to iterator protocol).
• On iterator object, you can call method next()
• next() method is a zero argument method that returns an object with 2 properties:
  1. value: value returned by the iterator. undefined when the iterator past the end of the iterated sequence (past the last value of the object).
  2. done: (boolean) which can be true or false. true if the iterator past the end of the iterated sequence (past the last value of the object). false if the iterator was able to produce the next value in the sequence (still there is a value or values to produce).

const digits = [0, 1, 2];
const arrayIterator = digits[Symbol.iterator]();

console.log(arrayIterator.next());//Object {value: 0, done: false}
console.log(arrayIterator.next());//Object {value: 1, done: false}
console.log(arrayIterator.next());//Object {value: 2, done: false}
console.log(arrayIterator.next());//Object {value: undefined, done: true}