Javascript You Need To Know For React

Hello World 👋

Welcome to the second article of the My Review of Kent C. Dodds’s EpicReact.Dev series which is based on the workshop material from EpicReact.Dev by Kent C. Dodds. In the previous article, you looked at the different topics that are going to be covered in EpicReact.Dev workshop. If you haven’t read the previous article of the series, please go and read it now and come back. In this article, I am going to explain the basic javascript concepts that you need to know before you start learning React.

My Review of Kent C. Dodds's EpicReact.Dev: Introduction

Hello World 👋 Welcome to the new series that I am starting on React called My Review of Kent C. Dodds's EpicReact.Dev. This is the first article in this series and I will start it off with the introduction to what this series will be about. This ser...

https://blog.bhanuteja.dev/epic-react-introduction

Let’s get to it.

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.

I will be covering the following topics. Feel free to skip over the topics that you already know.

Hello World 👋Logical AND (&&) and Logical OR (||) Operators Logical AND (&&) Operator Logical OR (||) Operator Template Literals Without using template literals Using template literals Without using template literals Using template literals Ternary Operator Syntax Shorthand Property Names Object Destructuring Array Destructuring Default Parameters Optional Chaining Nullish Coalescing Operator Spread Operator Array Spread Object Spread Rest Operator Function Arguments Usage with objects Arrow Functions Array Methods Array map() Method Array filter() Method Array reduce() method Array sort() method Array includes() Method Array slice() method Array splice() method Default Exports vs Named Exports Promises Basic DOM Document APIs What’s Next Until Next Time 👋

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

Logical AND (&&) Operator

Let's say we have the following expression - where `b` and `c` are [expressions](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Expressions_and_Operators)

b && c

This will be evaluated to the value of c only if b is truthy, otherwise, it will be evaluated to the value of b

Note:

If b is falsy, then the expression c is not even going to be evaluated.

This is called short-circuit evaluation.

This will be used quite a lot while using React.

Logical OR (||) Operator

Let's say we have the following expression - where b and c are expressions

b || c

This will be evaluated to the value of b if b is truthy, otherwise, it will be evaluated to the value of c.

Note:

Short-circuit 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 (number of blogs), name (name of the user), span (time span it took) as variables.

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(`) and you can write strings of text inside them and you have to wrap the javascript expressions around with ${ and }

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 and the name of the property of the object are same, then you can just write the variable name and omit the rest.

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 = sign after the parameter and add your default value there.

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 if a is not null and not undefined, otherwise, it evaluates to 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 if a is null or undefined, otherwise, it evaluates to 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 by @Skay

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.

Array Methods

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

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 method, it’s easy to understand other array methods. They all have a similar syntax.

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

I always remember this as the filter method filters out elements that do not satisfy the criteria.

// 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 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 if the element is included in the array, otherwise returns 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 - 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 made by @aman__tyagi

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 by @Tapas Adhikary

ES Modules by Kent C. Dodds

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 the following amazing articles to read about these.

What are JavaScript Promises? by @Skay

Promises and Async Await by Kent C. Dodds

Basic DOM Document APIs

It is also good to be familiar with basic Document Apis like createElement, getElementById etc. If you know these, you will appreciate the similarity and differences between React APIs and Document APIs.

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 already know some of the topics that I have explained in this article. Even if you didn’t before, now you know. These concepts are enough for you to follow along with EpicReact series. If you find something that can be improved or added to this article, feel free to comment on this article. I will try to incorporate your suggestions. Subscribe to my blog if you don’t want to miss the next article in this series.

What’s Next

In the next article, we are actually going to start the workshop - starting with React Fundamentals workshop.

Until Next Time 👋

If you want to learn more about the topics mentioned in this article, you can go through the following articles that I found being published on @Hashnode. Will be adding more articles to this list as they come along. Different people will find different explanations better.

Spread and Rest Operators

Arrow Functions

Optional Chaining and Nullish Coalescing

Destructuring

Array Methods

ES Modules

JavaScript Modules and how to effectively work with Export Import by @Tapas Adhikary

Promises