Javascript Basics For React Beginners

I am one of those persons who learned React before properly learning the basic concepts of javascript. Because of this, in the early days of my React journey, I didn't know which part of the code is React and which part is the vanilla js. It's important to know the basic javascript concepts to understand better which part of the puzzle is React solving.

In this blog post, I will be writing about different concepts of javascript that you see yourself using very often while working with React. It's better to know these before you deep dive into learning React.

Logical AND (&&) and Logical OR (||) Operators

Logical AND (&&) Operator

Let's say we have the following expression - where

b

c

b && c

This will be evaluated to the value of

c

b

b

Notes:

• If

  b

  is falsy, then the expression

  c

  is not even going to be evaluated.
• This is called

  shortcut evaluation

  .
• This will be used quite a lot while using React.

Logical OR (||) Operator

Let's say we have the following expression - where

b

c

b || c

This will be evaluated to the value of

b

c

Note:

• Shortcut evaluation happens here too.
• If

  b

  is truthy, then the expression

  c

  is not even going to be evaluated.
• You will be using this also quite often while using React.

Template Literals

This is a new ES6 way to create strings.

Let's see an example.

Assume that you want to create the following type of strings:

• 3 blog posts were written by Bhanu Teja in a span of 2 weeks.

You will be given

count

name

span

Without using template literals

const count = 3
const user = 'Bhanu Teja'
const span = 2

const result = count + ' blog posts were written by ' 
                     + name + ' in a span of ' + span 
                     + ' weeks.'

Using template literals

const count = 3
const name = 'Bhanu Teja'
const span = 2

const result = `${count} blog posts were written by ${name} in a span of ${span} weeks.`

Template literals start and end with a

backtick(`)

${

}

Let's add another use-case to the above example.

• If we have just 1 blog post you have to use

  blog post

  instead of

  blog posts

• If the time span is just 1 week, you have to use

  week

  instead of

  weeks

  .

Without using template literals

function pluralize(text, count) {
    if (count === 1) {
        return text
    }
    return text + 's'
}

const result = count + ' ' + pluralize('blog post', count)  
                     + ' were written by ' + name
                     + ' in a span of ' + span 
                     + ' ' + pluralize('week', span) + '.'

Using template literals

function pluralize(text, count) {
    if (count === 1) {
        return text
    }
    return text + 's'
}

const result = `${count} ${pluralize('blog post', count)} were written by ${name} in a span of ${span} ${pluralize('week', span)}.`

Ternary Operator

This is a shorthand representation of the if-else statements.

This is best explained using an example.

if (condition) {
    doSomething()
} else {
    doSomethingElse()
}

The above example when written using ternary operator

condition ? doSomething() : doSomethingElse()

Syntax

condition ? expressionIfTrue : expressionIfFalse

Shorthand Property Names

const id = 2
const name = 'Bhanu'
const count = 2

// This is the normal way
const user = {
    id: id,
    blogs: count,
    name: name,
}

// Using shorthand property names
const user = {
    id,
    blogs: count,
    name,
}

If the name of the

variable

property

This is one of the things that I did not know when I was initially learning React, and you usually see this being used a lot in code and documentation.

Object Destructuring

This is a short-hand way to get the properties of an object into variables.

// we have a `user` variable that looks like this
const user = {
    name: 'Bhanu Teja',
    blogs: 3,
    timeSpan: 2.
}

// without using object destructuring
const name = user.name
const blogs = user.blogs
const timeSpan = user.timeSpan

// using object destructuring
const { name, blogs, timeSpan } = user

Note:

The name of the destructured variables should be same as the name of the object properties.

Array Destructuring

This is a short-hand way to get the elements of an array into variables.

// we have a `name` variable that looks like this
const name = [ 'Bhanu Teja', 'P']

// without using array destructuring
const firstName = name[0]
const lastName = name[1]

// using array destructuring
const [firstName, lastName] = name

Default Parameters

You often want the function parameters to take some default values if that is not passed while calling the function.

Let's see an example

function sum(a = 2, b = 5) {
    return a + b
}

sum(5, 7) // a = 5, b = 7, result = 12
sum(4) // a = 4, b = 5(default value of b), result = 9
sum() // a = 2(default a), b = 5(default b), result = 7

So, whenever you want a parameter to take a default value, simply add a

=

Optional Chaining

This is a relatively new feature of javascript.

The optional chaining operator (?.) permits reading the value of a property located deep within a chain of connected objects without having to expressly validate that each reference in the chain is valid. - Taken from MDN Docs

Consider the expression

a?.b

This expression evaluates to

a.b

a

not null

not undefined

undefined

You can even chain this multiple times like

a?.b?.c

• If

  a

  is

  undefined

  or

  null

  , then this expression evaluates to

  undefined

• Else if

  b

  is undefined or

  null

  , then this expression evaluates to

  undefined

• Else this evaluates to

  a.b.c

Syntax:

obj.val?.prop
obj.val?.[expr]
obj.arr?.[index]
obj.func?.(args)

Nullish Coalescing Operator

The nullish coalescing operator (??) is a logical operator that returns its right-hand side operand when its left-hand side operand is

null

or

undefined

, and otherwise returns its left-hand side operand. - Taken from MDN Docs

Consider the expression

a ?? b

This evaluates to

b

a

null

undefined

a

Spread Operator

This operator spreads the values of an iterable object.

Array Spread

const a = [1, 2, 3]
const b = [5, 6]

console.log(...a) // 1 2 3

// Now, if you want to have an array with values 0, 1, 2, 3, 4, 5, 6
const c = [0, ...a, 4, ...b]

console.log(c) // 0 1 2 3 4 5 6

Object Spread

const first = {a: 1, b: 2}
const second = {c: 3}


// Now to create an object {a: 1, b: 2, c: 3, d: 4}
const result = {...first, ...second, d: 4}

console.log(result) // {a: 1, b: 2, c: 3, d: 4}

• To learn more about spread operator, Check out this amazing article ES6 - Spread Operator

Rest Operator

The rest parameter syntax allows us to represent an indefinite number of arguments as an array. - Taken from MDN Docs

Function Arguments

function sum(a, b, ...rest) {
    // ...
}

sum(1, 2, 3, 4, 5, 6) // a = 1, b = 2, rest = [3, 4, 5, 6]

Usage with objects

const user = {
    name: 'Bhanu Teja',
    blogs: 3,
    span: 2,
}
const {name, ...rest} = user
console.log(name) // Bhanu Teja
console.log(rest) // { blogs: 3, span: 2}

Arrow Functions

This is a new ES6 way to write functions.

// without using arrow functions
const sum = function (a, b) {
    return a + b
}

// (or)

function sum (a, b) {
    return a + b
}

// Using arrow functions
const sum = (a, b) => {
    return a + b
}

// (or)

const sum = (a, b) => a+ b

const multiplyBy2 = (a) => a * 2

(or)

const multiplyBy2 = a => a * 2

As you can see from the above example, converting the normal function to arrow functions can be done as follows:

• Remove the

  function

  keyword.
• Add an

  =>

  after the parameters.

Note

• If the body of the function is a simple expression you can even omit the

  return

  keyword and also need not wrap it between

  {

  and

  }

• If there is only one argument, you have the option to remove parenthesis around the arguments.
• There are still some more differences between arrow functions and normal functions, Checkout the following amazing articles to know more.
• A Simple Guide To Arrow Functions
• ES6 => Arrow Functions

Array Methods

There are so many array methods, but we frequently use some of these. I will be covering the following array methods.

• map
• filter
• reduce
• sort
• includes
• slice
• splice

Array map() Method

This method creates a new array from an existing array by calling a function for every element of the array.

I always remember this as

mapping the values in an array to some other values

Let's see an example.

const names = [
    { firstName: 'Bhanu Teja', lastName: 'P' },
    { firstName: 'Florin', lastName: 'Pop'},
    { firstName: 'Brad', lastName: 'Traversy'},
]

// Let's say we have to create a new array with full names.

// First let's write a callback function which takes an array element as an argument.
function callback(name) {
    return name.firstName + ' ' + name.lastName
}

// Now let's call .map() method on the array
console.log(names.map(callback)) // ["Bhanu Teja P", "Florin Pop", "Brad Traversy"]

// You can even inline the callback function which is how most of the people write this.
names.map(function(name) { return name.firstName + ' ' + name.lastName })

// Let's write the same using arrow functions and template literals that we just learned earlier
names.map((name) => `${name.firstName} ${name.lastName}`)

// You can even omit the parenthesis around name
names.map(name => `${name.firstName} ${name.lastName}`)

// Let's use object destructuring
names.map(({firstName, lastName}) => `${firstName} ${lastName}`)

Syntax:

// callback takes a single array element as an argument.
// values is an array
values.map(callback)

Note:

• Calling this method will not change the original array

Array filter() Method

Now that we know the

Array map

The array filter method creates a new array with elements that satisfy some given criteria.

I always remember this as the

filter

// consider the following array of users
const users = [
    {id: 1, posts: 2},
    {id: 2, posts: 1},
    {id: 3, posts: 5},
    {id: 4, posts: 4},
    {id: 5, posts: 3},
]

// Let's say we want to have all users who have at least 3 posts.
users.filter((user) => user.posts >= 3) // [{id: 3, posts: 5}, {id: 4, posts: 4}, {id: 5, posts: 3}]

Syntax:

// callback takes a single array element as an argument.
// values is an array
values.filter(callback)

Note:

• Calling this method will not change the original array

Array reduce() method

The array reduce method reduces the array of values into a single value. It executes the callback function for each value of the array.

Lets see the syntax of the reduce method before seeing an example.

array.reduce(function(totalValue, currentValue, currentIndex, arr), initialValue)

const values = [2, 4, 6, 7, 8]

// Let's say that we want to have a sum of all these values.
// Let's see how we do it using a traditional for loop
let total = 0
for(let i = 0; i < values.length; i++) {
    const value = values[i]
    total = total + value
}
console.log(total)


// Let's use reduce method now
const initialValue = 0
values.reduce((total, currentValue) => total + currentValue, initialValue)

Notes:

• initialValue

  is an optional parameter.
• Calling this method will not change the original array

Array sort() method

The callback function takes two different values as arguments. Based on the return value of the callback function, the two elements' positions are decided.

• If the return value is negative, then the first value is considered to be before the second value.
• If the return value is zero, then there will be no change in the order of the values.
• If the return value is positive, then the first value is considered to be after the second value.

const values = [4, 10, 2, 1, 55]

// Let's say we want to sort this array in descending order
// so if a and b are given
// a should be before b if a > b
// a should be after b if a < b
// a and b can be at their own places if a === b

values.sort((a, b) => {
    if(a > b) {
        return -1
    }
    if(a < b) {
        return 1
    }
    return 0
}) // [55, 10, 4, 2, 1]


// You can also do this as follows
values.sort((a, b) => b - a)

Note:

• The return value of the function is the sorted array
• This changes the original array
• If you do not pass any callback function, this sorts the values as strings and in ascending order.

Array includes() Method

This returns

true

false

Syntax:

array.includes(element)

const values = [2, 3, 4]
values.includes(1) // false
values.includes(2) // true

Note:

• You can pass an optional parameter specifying the start index to start the search from.

  array.includes(element, startIndex)

Array slice() method

Syntax

array.slice(start, end)

Array slice will return the elements in the given range.

start

• starting index to select the elements from
• This is an optional parameter and by default, it takes the value of 0
• You can even pass a negative number.
• Negative number represents the position from the end.

• -1

  refers to the last element of the array,

  -2

  refers to last but one element, and so on.

end

• ending index up to where to select the elements
• This is an optional parameter. If this is not passed, then all elements up to the end of the array will be selected.
• The element at

  end

  will not be selected
• This also accepts a negative number as an argument and the meaning is the same as before.

const numbers = [0, 1, 2, 3, 4, 5, 6, 7]
console.log(numbers.slice())  // [0, 1, 2, 3, 4, 5, 6, 7]
console.log(numbers.slice(2)) // [2, 3, 4, 5, 6, 7]
console.log(numbers.slice(2, 6)) // [2, 3, 4, 5]
console.log(numbers.slice(-1)) // [7]
console.log(numbers.slice(-3)) // [5, 6, 7]
console.log(numbers.slice(-3, -1)) // [5, 6]

Note:

• This doesn't change the original array

Array splice() method

Syntax:

array.splice(index, count, item1, ....., itemX)

This method is used to add or remove elements in an array.

index

• The index at which the elements need to be added or removed. Can be a negative value too.

count

• The number of elements to remove.

item1, ....., itemX

• Items that will be added at

  index

const numbers = [0, 1, 2, 100, 6, 7]
// let's say we have to convert this array to [0, 1, 2, 3, 4, 5, 6, 7]
numbers.splice(3, 1, 3, 4, 5) 
console.log(numbers) // [0, 1, 2, 3, 4, 5, 6, 7]

Note:

• The return value of the splice method is the array of items removed.
• This changes the original array

To know more about different array methods, check out the amazing series Javascript Array Methods

Default Exports vs Named Exports

You often see yourself using ES Modules imports and exports while working with React. It's important to know how to import them when they are exported as default exports vs when they are exported as named exports.

Check out the following amazing articles to read about these.

• JavaScript Modules and how to effectively work with Export Import
• ES Modules

Promises

You also need to have a basic knowledge of what promises are and how to work them. They will be used quite often in React.

Check out this article to know more about them.

Basic DOM Document APIs

It is also good to be familiar with basic Document Apis like

createElement

getElementById

If you are working with javascript for a while now, it is most likely that you already know basic Document APIs.

MDN Docs are the best place to read up on these.

• Document - Web APIs | MDN

You might also like the following React articles that I wrote:

• My Review of Kent C. Dodds's EpicReact.Dev: Introduction
• Javascript You Need To Know For React
• Intro to React Raw APIs
• Understanding JSX
• Creating Custom Components
• Styling And Handling Forms
• Managing State With useState Hook
• How to Create a Reusable LocalStorage Hook
• Easily Detect Outside Click Using useRef Hook
• The Lifecycle of React Hooks Component

Previously published at https://blog.bhanuteja.dev/epic-react-javascript-you-need-to-know-for-react