ES6

let, var, const

Variables declared using the var keyword are scoped to the function in which they are created, or if created outside of any function, to the global object.

let and const are block scoped, ie. they are only accessible within the nearest set of curly braces (function, if-else block, or for-loop).

Function constructors

Example of each:

// ES5 Function Constructor
function Person(name) {
  this.name = name;
}

// ES6 Class
class Person {
  constructor(name) {
    this.name = name;
  }
}

For simple constructors, they look pretty similar.

The main difference in the constructor comes when using inheritance. If we want to create a Student class that subclasses Person and add a studentId field, this is what we have to do in addition to the above.

// ES5 Function Constructor
function Student(name, studentId) {
  // Call constructor of superclass to initialize superclass-derived members.
  Person.call(this, name);

  // Initialize subclass's own members.
  this.studentId = studentId;
}

Student.prototype = Object.create(Person.prototype);
Student.prototype.constructor = Student;

// ES6 Class
class Student extends Person {
  constructor(name, studentId) {
    super(name);
    this.studentId = studentId;
  }
}

It's much more verbose to use inheritance in ES5 and the ES6 version is easier to understand and remember.

References

Arrow functions

Simplify the syntax needed to create functions, without a need for the functionkeyword.

The this within arrow functions is also bound to the enclosing scope which is different compared to regular functions where the this is determined by the object calling it. Lexically-scoped this is useful when invoking callbacks especially in React components.

Advantage of using the arrow syntax for methods in a constructor

The value of this gets set at the time of the function creation and can't change after that. So, when the constructor is used to create a new object, this will always refer to that object.

Example: we have a Person constructor that takes a first name as an argument has two methods to console.log that name, one as a regular function and one as an arrow function:

const Person = function(firstName) {
  this.firstName = firstName;
  this.sayName1 = function() { console.log(this.firstName); };
  this.sayName2 = () => { console.log(this.firstName); };
};

const john = new Person('John');
const dave = new Person('Dave');

john.sayName1(); // John
john.sayName2(); // John

// The regular function can have its 'this' value changed, but the arrow function cannot
john.sayName1.call(dave); // Dave (because "this" is now the dave object)
john.sayName2.call(dave); // John

john.sayName1.apply(dave); // Dave (because 'this' is now the dave object)
john.sayName2.apply(dave); // John

john.sayName1.bind(dave)(); // Dave (because 'this' is now the dave object)
john.sayName2.bind(dave)(); // John

var sayNameFromWindow1 = john.sayName1;
sayNameFromWindow1(); // undefined (because 'this' is now the window object)

var sayNameFromWindow2 = john.sayName2;
sayNameFromWindow2(); // John

this can be changed for a normal function, but the context always stays the same for an arrow function.

Even if you are passing around your arrow function to different parts of your application, you wouldn't have to worry about the context changing.

This can be particularly helpful in React class components. If you define a class method for something such as a click handler using a normal function, and then you pass that click handler down into a child component as a prop, you will need to also bind this in the constructor of the parent component. If you instead use an arrow function, there is no need to also bind "this", as the method will automatically get its "this" value from its enclosing lexical context. (See this article for an excellent demonstration and sample code: https://medium.com/@machnicki/handle-events-in-react-with-arrow-functions-ede88184bbb)

References

Destructuring

Extract values of Objects or Arrays and place them into distinct variables.

Array destructuring

// Variable assignment.
const foo = ['one', 'two', 'three'];

const [one, two, three] = foo;
console.log(one); // "one"
console.log(two); // "two"
console.log(three); // "three"
// Swapping variables
let a = 1;
let b = 3;

[a, b] = [b, a];
console.log(a); // 3
console.log(b); // 1

Object destructuring

// Variable assignment.
const o = { p: 42, q: true };
const { p, q } = o;

console.log(p); // 42
console.log(q); // true

References

Very useful when coding in a functional paradigm as we can easily create copies of arrays or objects without resorting to Object.create, slice, or a library function. This language feature is used often in Redux and rx.js projects.

function putDookieInAnyArray(arr) {
  return [...arr, 'dookie'];
}

const result = putDookieInAnyArray(['I', 'really', "don't", 'like']); // ["I", "really", "don't", "like", "dookie"]

const person = {
  name: 'Todd',
  age: 29,
};

const copyOfTodd = { ...person };

ES6's rest syntax offers a shorthand for including an arbitrary number of arguments to be passed to a function. It is like an inverse of the spread syntax, taking data and stuffing it into an array rather than unpacking an array of data, and it works in function arguments, as well as in array and object destructuring assignments.

function addFiveToABunchOfNumbers(...numbers) {
  return numbers.map(x => x + 5);
}

const result = addFiveToABunchOfNumbers(4, 5, 6, 7, 8, 9, 10); // [9, 10, 11, 12, 13, 14, 15]

const [a, b, ...rest] = [1, 2, 3, 4]; // a: 1, b: 2, rest: [3, 4]

const { e, f, ...others } = {
  e: 1,
  f: 2,
  g: 3,
  h: 4,
}; // e: 1, f: 2, others: { g: 3, h: 4 }

References

Template literals

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