📜ES6 & ES7

Basic features of modern JavaScript syntax useful to know for learning React.

let & const

Read more about let Read more about const

let and const basically replace var . You use let instead of var and const instead of var if you plan on never re-assigning this "variable" (effectively turning it into a constant therefore).

ES6 Arrow Functions

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Arrow functions are a different way of creating functions in JavaScript. Besides a shorter syntax, they offer advantages when it comes to keeping the scope of the this keyword.

Arrow function syntax may look strange but it's actually simple.

function callMe(name) {
  console.log(name);
}

which you could also write as:

const callMe = function(name) {
  console.log(name);
}

becomes:

const callMe = (name) => {
  console.log(name);
}

Important:

When having no arguments, you can use the following shortcut:

const callMe = name => {
  console.log(name);
}

When just returning a value, you can use the following shortcut:

const callMe = name => name;

That's equal to:

const callMe = name => {
  return name;
}

Exports & Imports

In React projects (and actually in all modern JavaScript projects), you split your code across multiple JavaScript files - so-called modules. You do this, to keep each file/ module focused and manageable.

To still access functionality in another file, you need export (to make it available) and import (to get access) statements.

You got two different types of exports: default (unnamed) and named exports: default => export default ...; named => export const someData = ...;

You can import default exports like this:

 import someNameOfYourChoice from './path/to/file.js';

The variable name someNameOfYourChoice is totally up to you.

Named exports have to be imported by their name:

import { someData } from './path/to/file.js';

A file can only contain one default and an unlimited amount of named exports. You can also mix the one default with any amount of named exports in one and the same file.

When importing named exports, you can also import all named exports at once with the following syntax:

import * as upToYou from './path/to/file.js';

upToYou is - well - up to you and simply bundles all exported variables/functions in one JavaScript object. For example, if you export const someData = ... (/path/to/file.js) you can access it on upToYou like this: upToYou.someData

Classes

Classes are a feature which basically replace constructor functions and prototypes. You can define blueprints for JavaScript objects with them.

Like this:

class Person {
  constructor() {
    this.name = 'Sid';
  }
}

const person = new Person();
console.log(person.name); // prints 'Sid'

In the above example, not only the class but also a property of that class (=> name ) is defined. They syntax you see there, is the "old" syntax for defining properties. In modern JavaScript projects, you can use the following, more convenient way of defining class properties:

class Person {
  name = 'Sid'
}

const person = new Person();
console.log(person.name); // prints 'Sid'

You can also define methods. Either like this:

class Person {
  name = 'Sid';
  printMyName() {
    console.log(this.name);//`this` is required to refer to the class
  }
}

const person = new Person();
person.printMyName();

Or like this:

class Person {
  name = 'Sid';
  printMyName = () => {
    console.log(this.name);
  }
}

const person = new Person();
person.printMyName();

The second approach has the same advantage as all arrow functions have: The this keyword doesn't change its reference.

You can also use inheritance when using classes:

class Human {
  species = 'human';
}

class Person extends Human {
  name = 'Sid';
  printMyName = () => {
    console.log(this.name)
  }
}

const person = new Person();
person.printMyName();
console.log(person.species);// prints 'human'

Spread & Rest Operator

The spread and rest operators actually use the same syntax: ...

Yes, that is the operator - just three dots. It's usage determines whether you're using it as the spread or rest operator.

Using the Spread Operator:

The spread operator allows you to pull elements out of an array (=> split the array into a list of its elements) or pull the properties out of an object. Here are two examples:

const oldArray = [1, 2, 3];
const newArray = [...oldArray, 4, 5];// This is now [1, 2, 3, 4, 5]

Here's the spread operator used on an object:

const oldObj = {
  name: 'Sid'
};

const newObj = {
  ...oldObj,
  age: 25
};

newObj would then be:

{
  name: 'Sid',
  age: 25
}

The spread operator is extremely useful for cloning arrays and objects. Since both are reference types (and not primitives), copying them safely (i.e. preventing future mutation of the copied original) can be tricky. With the spread operator you have an easy way of creating a (shallow!) clone of the object or array.

Using the Rest Operator:

The rest operator is used to merge a list of function arguments into an array.

function sortArgs(...args) {
  return args.sort();
}

Since args are now of type Array, they can be sorted by using the sort() built in function.

Example:

const restEg = (...args) => {
  return args.filter(el => el%2==0);
}

console.log(restEg(1,2,3,4,5,6,7,8,9,10)); //[2,4,6,8,10]

De-structuring

De-structuring allows you to easily access the values of arrays or objects and assign them to variables.

Here's an example for an array:

const arr = [1, 2, 3];
const [a, b] = arr;
console.log(a);// prints 1
console.log(b);// prints 2
console.log(arr);// prints [1, 2, 3]

And here for an object:

const myObj = {
  name: 'Sid',
  age: 25
}

const {name} = myObj;
console.log(name);// prints 'Sid'
console.log(age);// prints undefined
console.log(myObj);// prints {name: 'Sid', age: 25}

De-structuring is very useful when working with function arguments. Consider this example:

const printName = (personObj) => {
  console.log(personObj.name);
}

printName({name: 'Sid', age: 25});

Here, we only want to print the name in the function but we pass a complete person object to the function. Of course this is no issue but it forces us to call personObj.name

const printName = ({name}) => {
  console.log(name);
}

printName({name: 'Sid', age: 25}); // prints 'Sid'

We get the same result as above but we save some code. By de-structuring, we simply pull out the name property and store it in a variable/ argument named which we then can use in the function body.

Refreshers

Reference & Primitive data types

Reference data types are arrays and objects. In this type, there are pointers to the original data are copied in the memory, so a direct copy of the data isn't made.

Example:

const Person1 = {
  name: 'Sid'
}

const Person2 = Person1;
Person1.name = 'Max';

console.log(Person2.name); // 'Max'

So, any changes to the original data type will be reflected in the copy of that "referenced" array or object.

Primitive data types are boolean, string, number. In this type of data, a direct copy of the data is made. Any changes made to the original data type are not reflected in the copied data type, they are both independent of each other.

Example:

a = 'Siddharth';
b = a;
a = 'Rahul';
console.log(a); // 'Rahul'
console.log(b); // 'Siddharth'

Array Functions

• MAP This array function returns a new array.

const numbers = [1,2,3,4,5,6];
const double = numbers.map(item => item*2);
console.log(numbers); //[1,2,3,4,5,6]
console.log(double); //[2,4,6,8,10,12]

• FILTER This array function returns a new array.

const numbers = [1,2,3,4,5,6,7,8,9,10];
const even = numbers.filter(item => item % 2 == 0);
console.log(numbers); //[1,2,3,4,5,6,7,8,9,10]
console.log(even); //[2,4,6,8,10]

• REDUCE This returns a single value.

const numbers = [1,2,3,4];
const sum = numbers.reduce((accumulator, currVal, currIdx, numbers) => {
    return accumulator + currentVal;
  }, 0);
console.log(sum); //10

const newSum = numbers.reduce((acc,currVal) => {
    return acc+currVal;
  }, 5);
console.log(newSum); //15

• SLICE This returns a shallow copy of the portion of the array into a new array from "START" to "END" index that is specified in the limits. [END index is NOT INCLUDED]

Also, if the END index is not specified, the slice operation will add a default end index, that is the last index of the referred array.

const animals = ['cat', 'dog', 'cow', 'duck'];
console.log(animals.slice(2)); // ['cow', 'duck']
console.log(animals.slice(1,3)); // ['dog','cow']
console.log(animals.slice(1, animals.length)); // ['dog', 'cow', 'duck']

• SPLICE This array function changes the context of array by removing/replacing the existing elements and/or adding the new elements specified IN PLACE [same array is mutated].

const months = ['Jan', 'Mar', 'Jun'];

months.splice(1, 0, 'Feb'); //['Jan' ,'Feb', 'Mar', 'Jun']
// insert 'Feb' at index 1, replace 0 elements.

months.splice(3, 0, 'Apr', 'May');
//['Jan' ,'Feb', 'Mar', 'Apr', 'May', 'Jun']