Introduction to JavaScript

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.

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); // Tammy, false, 3

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; console.log(number); // Prints: 120

Booleans

Booleans are a primitive data type. They can be either true or false. let lateToWork = true;

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';

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 + '.'; console.log(displayText); // Prints: Your credit card bill is due on May 30th.

Single Line Comments

In JavaScript, single-line comments are created with two consecutive forward slashes //. // This line will denote a comment

JavaScript

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.

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

Libraries

Libraries contain methods that can be called by appending the library name with a period ., the method name, and a set of parentheses. Math.random(); // ☝️ Math is the library

Methods

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 Math.random();

Null

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

Numbers

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 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.`;

Strings

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?";

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.

String .length

The .length property of a string returns the number of characters that make up the string. let message = 'good nite'; console.log(message.length); // Prints: 9 console.log('howdy'.length); // Prints: 5

Math.floor()

The Math.floor() function returns the largest integer less than or equal to the given number. console.log(Math.floor(5.95)); // Prints: 5

Math.random()

The Math.random() function returns a floating-point, random number in the range from 0 (inclusive) up to but not including 1. console.log(Math.random()); // Prints: 0 - 0.9

console.log()

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!

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. const weeksInYear = Math.floor(365/7) // calculates # of weeks in a year, rounds down to nearest integer const daysLeftOver = 367 % 7 // calcuates the number of days left over after 365 is divded by 7 console.log("A year has " + weeksInYear + "weeks " and "daysLeftOver " + days)

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;

Variables

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.

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.

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

Undefined

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; console.log(a); // Prints: undefined