Codecademy Logo

JavaScript Introduction & React Overview


The console.log() method is used to log or print messages to the console. It can also be used to print objects and other info.

console.log('Hi there!');
// Prints: Hi there!


JavaScript is a programming language that powers the dynamic behavior on most websites. Alongside HTML and CSS, it is a core technology that makes the web run.


Methods return information about an object, and are called by appending an instance with a period ., the method name, and parentheses.

// Returns a number between 0 and 1

Built-in Objects

Built-in objects contain methods that can be called by appending the object name with a period ., the method name, and a set of parentheses.

// ☝️ Math is the built-in object


Numbers are a primitive data type. They include the set of all integers and floating point numbers.

let amount = 6;
let price = 4.99;

String .length

The .length property of a string returns the number of characters that make up the string.

let message = 'good nite';
// Prints: 9
// Prints: 5

Data Instances

When a new piece of data is introduced into a JavaScript program, the program keeps track of it in an instance of that data type. An instance is an individual case of a data type.


Booleans are a primitive data type. They can be either true or false.

let lateToWork = true;


The Math.random() method returns a floating-point, random number in the range from 0 (inclusive) up to but not including 1.

// Prints: 0 - 0.9999999999999999


The Math.floor() function returns the largest integer less than or equal to the given number.

// Prints: 5

Single Line Comments

In JavaScript, single-line comments are created with two consecutive forward slashes //.

// This line will denote a comment


Null is a primitive data type. It represents the intentional absence of value. In code, it is represented as null.

let x = null;


Strings are a primitive data type. They are any grouping of characters (letters, spaces, numbers, or symbols) surrounded by single quotes ' or double quotes ".

let single = 'Wheres my bandit hat?';
let double = "Wheres my bandit hat?";

Arithmetic Operators

JavaScript supports arithmetic operators for:

  • + addition
  • - subtraction
  • * multiplication
  • / division
  • % modulo
// Addition
5 + 5
// Subtraction
10 - 5
// Multiplication
5 * 10
// Division
10 / 5
// Modulo
10 % 5

Multi-line Comments

In JavaScript, multi-line comments are created by surrounding the lines with /* at the beginning and */ at the end. Comments are good ways for a variety of reasons like explaining a code block or indicating some hints, etc.

The below configuration must be
changed before deployment.
let baseUrl = 'localhost/taxwebapp/country';

Remainder / Modulo Operator

The remainder operator, sometimes called modulo, returns the number that remains after the right-hand number divides into the left-hand number as many times as it evenly can.

// calculates # of weeks in a year, rounds down to nearest integer
const weeksInYear = Math.floor(365/7);
// calcuates the number of days left over after 365 is divded by 7
const daysLeftOver = 365 % 7 ;
console.log("A year has " + weeksInYear + " weeks and " + daysLeftOver + " days");

Assignment Operators

An assignment operator assigns a value to its left operand based on the value of its right operand. Here are some of them:

  • += addition assignment
  • -= subtraction assignment
  • *= multiplication assignment
  • /= division assignment
let number = 100;
// Both statements will add 10
number = number + 10;
number += 10;
// Prints: 120

String Interpolation

String interpolation is the process of evaluating string literals containing one or more placeholders (expressions, variables, etc).

It can be performed using template literals: text ${expression} text.

let age = 7;
// String concatenation
'Tommy is ' + age + ' years old.';
// String interpolation
`Tommy is ${age} years old.`;


Variables are used whenever there’s a need to store a piece of data. A variable contains data that can be used in the program elsewhere. Using variables also ensures code re-usability since it can be used to replace the same value in multiple places.

const currency = '$';
let userIncome = 85000;
console.log(currency + userIncome + ' is more than the average income.');
// Prints: $85000 is more than the average income.


undefined is a primitive JavaScript value that represents lack of defined value. Variables that are declared but not initialized to a value will have the value undefined.

var a;
// Prints: undefined

Learn Javascript: Variables

A variable is a container for data that is stored in computer memory. It is referenced by a descriptive name that a programmer can call to assign a specific value and retrieve it.

// Examples of variables
let name = "Tammy";
const found = false;
var age = 3;
console.log(name, found, age);
// Prints: Tammy false 3

Declaring Variables

To declare a variable in JavaScript, any of these three keywords can be used along with a variable name:

  • var is used in pre-ES6 versions of JavaScript.
  • let is the preferred way to declare a variable when it can be reassigned.
  • const is the preferred way to declare a variable with a constant value.
var age;
let weight;
const numberOfFingers = 20;

Template Literals

Template literals are strings that allow embedded expressions, ${expression}. While regular strings use single ' or double " quotes, template literals use backticks instead.

let name = "Codecademy";
console.log(`Hello, ${name}`);
// Prints: Hello, Codecademy
console.log(`Billy is ${6+8} years old.`);
// Prints: Billy is 14 years old.

let Keyword

let creates a local variable in JavaScript & can be re-assigned. Initialization during the declaration of a let variable is optional. A let variable will contain undefined if nothing is assigned to it.

let count;
console.log(count); // Prints: undefined
count = 10;
console.log(count); // Prints: 10

const Keyword

A constant variable can be declared using the keyword const. It must have an assignment. Any attempt of re-assigning a const variable will result in JavaScript runtime error.

const numberOfColumns = 4;
numberOfColumns = 8;
// TypeError: Assignment to constant variable.

String Concatenation

In JavaScript, multiple strings can be concatenated together using the + operator. In the example, multiple strings and variables containing string values have been concatenated. After execution of the code block, the displayText variable will contain the concatenated string.

let service = 'credit card';
let month = 'May 30th';
let displayText = 'Your ' + service + ' bill is due on ' + month + '.';
// Prints: Your credit card bill is due on May 30th.

Learn More on Codecademy