5 Javascript (ES6+) features that you should be using in 2019

5 Javascript (ES6+) features that you should be using in 2019

We as developers must be trying to use the best tools and features in our disposition to make our job more easy and efficient.

Here I’m going to explain 5 features of modern Javascript that are very helpful and you are going to be using all the time:

  1. Let and const keywords
  2. Arrow functions
  3. Destructuring
  4. Spread operator
  5. Template literals

It’s 2019 and every modern browser support all these features so there is no excuse!.

You can run all the example code in your browser Developer Tools


1. let and const keywords

In Javascript var was the only way to declare a variable, the problem with var is it has no scope and you can declare the same variable multiple times, that is why now we have let and const keywords.


let

let allows you to declare variables that are limited in scope to the block, statement, or expression on which it is used. This is unlike the var keyword, which defines a variable globally, or locally to an entire function regardless of block scope.[1]

Let’s see an example:

var date = new Date("2015-01-01");
var region = "US";

// Some were further in the code
if (region === "US") {
  var date = new Date();
  console.log("New date", date); // We get the current date as expected
}

console.log(date); //Expecting 2015-01-01T00:00:00.000Z but we get the current date

We developers are bad at naming and if we are working with others the problem is magnified 10x. So is no rare that we use the same name for different things.

Let’s see how this work with let

let date = new Date("2015-01-01");
let region = "US";

// Some were further in the code
if (region === "US") {
  let date = new Date();
  console.log("New date", date); // We get the current date as expected
}

console.log(date); //We get 2015-01-01T00:00:00.000Z as expected :)

For me scoping is the more important feature of let. Other features are:

  1. Redeclaration: If you declare a variable with the same name in the same function or block scope it raises a SyntaxError. Good for stop using the same name variables.
  2. Hoisting: If you use a variable before declaration you get a ReferenceError.

const

const allows us to declare a constant variable, a value that shouldn’t change in our code. Let’s see an example:

const speedOfLight=299792458; //m*s-1

try {
  speedOfLight=300;
} catch(error) {
  console.log(error); // TypeError: Assignment to constant variable.
  // Note - error messages will vary depending on browser
}

console.log(speedOfLight); // Expected output 299792458

Other features of const:

  1. Scoping: variables are also block-scoped.
  2. Immutable: The value of a constant variable cannot change.
  3. Redeclaration: As same as let a const variable cannot be redeclared and will raise a Syntax Error.

Note: Another good practice for variables is always declare variables on the top of your function or block scope, is more easy to keep track.

For this 2019 please please don’t use var anymore.


2. Arrow functions

Arrow functions (also known as fat arrow for the => symbol) has a shorter syntax that a regular function and allow us to write more concise code.

Let’s see the difference between old function expressions and arrow functions:

//ES5
let doubleNumbers = [1,2,3,4,5].map(function(number) { 
  return number*2;
});

//ES6 Arrow function
let doubleNumbers = [1,2,3,4,5].map((number) => { 
  return number*2 
});

In arrow functions, you don’t need parenthesis when you have only one argument and if a one-liner expression like this you can drop the return and the curly braces:

//ES6 Arrow function one-liner
let doubleNumbers = [1,2,3,4,5].map(number => number*2);

//ES6 Arrow function multiple arguments
handleClick((event, seconds) => {
  event.preventDefault();
  displayFireworks();
  showAnimation(seconds);
});

Arrow functions save us a lot of typing and also, in my opinion, make the code more readable.

What we lose with arrow functions is that we don’t have reference to this, arguments, super or new.target. This means that if you really need any of these arguments inside a function you should use traditional functions.

My recommendation is that you should use arrow functions as much as you can. In code, readability is key.


3. Destructuring

This is one of my favorite features of ES6.

Let’s see first an example:

// Old method
const myArray = ['apple', 'pear', 'orange', 'banana'];
let fruit1 = myArray[0];
let fruit2 = myArray[1];
let fruit3 = myArray[2];
let fruit4 = myArray[3];

//ES6 destructuring
let [fruit1, fruit2, fruit3, fruit4] = myArray; // much better isn't? 

We can use it on objects to:

let dog = {
 name: 'Toby',
 age: 3,
 breed: 'Beagle',
 features: {
   color: 'White and brown',
   favoriteToy: 'Plastic duck'
 }
}

// We can obtain the values like this with destructuring

let {name, age, breed} = dog;

// What if we want only name and age and all the other in another variable

let {name, age, ...info} = dog;

So what destructuring assignment allows us is extracting data from arrays or objects into distinct variables in an easy useful way.

I use it all the time for JSON objects.

Bonus

You can also go the other way around:

let firstName="Albert"
let lastName="Einstein"
let person = {firstName, lastName}

console.log(person.firstName); // "Albert"
console.log(person.lastName); // "Einstein"

4. Spread operator

Spread operator allows us to “spread” (duh!) or “explode” an array into its individual items.

Let’s see an example:

let first = [1,2,3];
let second = [4,5,6];

// If we do this
first.push(second);

// We get
console.log(first); // [1,2,3,[4,5,6]] that is not right

// Using the spread operator

first.push(...second);

console.log(first); // [1,2,3,4,5,6] that's what we wanted!

Using the spread operator (…) we manage to obtain each individual element without doing an iteration, this is very helpful in many situations. Let’s see another example:

let scores = [23, 45, 56];

function averageThreeScores(a, b, c) {
  let sum = a + b + c;
  return sum/3;
}

console.log(averageThreeScores(...scores)); // Result 41.333333...

Here we are using the spread operator to pass arguments to a function.

Spread operator also works with objects. As with arrays, the spread operator allows us to obtain each individual element of an object:

let name='Toby';
let age=3;
let features = {race: 'Beagle', size: 'small'};

let dog = {name, age, ...features}; // We expand the features object

console.log(dog); // {name: 'Toby', age: 3, race: 'Beagle', size: 'small'}

Spread operator also allow us to clone an object instead of using Object.assign:

let dog = {name: 'Toby', age: 3, race: 'Beagle', size: 'small'}

let puppy = {...dog, name: 'Max', age: 1}; // Clone dog object and modify its properties

console.log(puppy); // {name: 'Max', age: 1, race: 'Beagle', size: 'small'}
console.log(dog); // {name: 'Toby', age: 3, race: 'Beagle', size: 'small'}

As we can see we clone the dog object, and we changed the value of age and name without modifying the original object.


5. Template literals

We use strings everywhere, and we usually have to pass some variable to the string. Here is where Template literals come to the rescue.

Template literals are enclosed by the back-tick () character instead of double or single quotes.

Template literals can contain placeholders. These are indicated by the dollar sign and curly braces (${expression}):

let a = 5;
let b = 10;
console.log(`The sum of a and b is ${a+b} and the multiplication is ${a*b}`); 
// The sum of a and b is 15 and the multiplication is 50

We can also write multiline text like:

let name='Mike';
let age=30;
let country='Italy';

console.log(`${name} is
  ${age} years old and
  lives in ${country}
`);

// Mike is
//  30 years old and
//  lives in Italy

Here Javascript will show multiline text and will respect the spaces without the requirement of special characters such as \n.

References:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/

That is, for now, I hope that find this helpful. If you have any questions or anything that you want to add please leave a comment!


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Originally published by Olivers De Abreu at https://dev.to