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Introduction to JavaScript and Velo

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.


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");

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