Cheatsheets JavaScript

Introduction

Cheatsheets ❇ Learn JavaScript

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!

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.

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();

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

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 .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

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

let lateToWork = true;

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

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

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

// This line will denote a comment

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

let x = null;

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 Kittie?';
let double = "Wheres my Kittie?";

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 . 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;

console.log(number);
// 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
'Hanii is ' + age + ' years old.';

// String interpolation
`Hanii is ${age} years old.`;

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.

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

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 = "Hanii";
const found = false;
var age = 3;
console.log(name, found, age);
// Hanii, false, 3

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

var age;
let weight;
const numberOfFingers = 20;

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.

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

let name = "Hanii";
console.log(`Hello, ${name}`);
// Prints: Hello, Hanii

console.log(`Hanii is ${16+8} years old.`);
// Prints: Hanii is 24 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 = 'Netflix';
let month = 'May 30th';
let displayText = 'Your ' + service  + ' bill is due on ' +  month + '.';

console.log(displayText);
// Prints: Your Netflix bill is due on May 30t

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