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Introduction

JavaScript​

JavaScript is a programming language that powers the dynamic behaviour of 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.

Math.random();

Libraries​

Libraries contain methods that can be called by appending the library name with a period ., method name and 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 = 7; 
let price = 3.99;

String .length​

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

let message = "I should be eating pizza instead"
console.log(message.length);
// prints: 32 (probably, if my counting is correct - spaces included)

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.

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​

created with foward slashes. 

// this line is a comment

Null​

Null is a primitive data type. in represents the intentional absense of a value. in code, it is represented as null

let x = null;

Strings​

Strings are a primitive data type. they are a grouping od characters(lletters, spaces, numbers or symbols) surrounded by 'single' or "double" quotes.

Arthithmetic Operators​

JavaScript supports arthimetic operators for:

*** + Addition - 5 + 5 = 10
*** - Subtraction - 10 - 5 = 5
*** * Multiplication - 5 * 10 = 50
*** / Division - 10 / 5 = 2
*** % Modulo / Remainder - 10 % 5 = 0.5

Multi-line Comments​

In JS, multi-line comments are created by using `/* and */ (start & finish)

/* <- this is the start, this is the end -> */

Remainder/Modulo Operator​

This operator returns the number that remains after the right hand number divides into the left hand number as many times as is can evenly.

// works out how many weeks in a year and then math.floor() rounds it down.
// prints 52 (rounds down 52.143?)

const weeksInYear = Math.floor(365/7):

`**const daysLeftOver = 365 % 7**`
// prints 25.55

`console.log(`A year has ${weeksInaYear} weeks and ${daysLeftOver} days`)`

Assignment Operator​

Assigns the value to the left operand based on the value to the right operand. As if operand is a word. here are some of them:

let number = 100;

number = number + 10; // prints 110
number += 10; // prints 110

String interpolation & Concatenation​

String interpolation: Is the process of evaluating string literals containing one or more placeholders (expressions,variables, etc). It can be performed using template literals: my ${name} is

String Concatenation: Strings can be cocatenated together by using the + operator.

let age = 13;

// string concatenation
'Jimmy is ' + age + ' years old';

//string interpolation
`Frankie is ${age} years old.`;

Variables​

These are used when theres a need to store data. this data can then be used elsewhere. Variables are stored in computer memory Using variables also allows code to be re-usable since it can be used to replace the same value in multiple places. Variables can store all types of data:

  • objects
  • strings
  • numbers
  • lists
  • functions
  • promises
  • booleans
  • undefined
const currency = '$';
let userIncome = 70000;

console.log(`${currency} ${userIncome} is the salary for this interview m'kay?`)
// prints `$70000 is the salary for this interview m'kay?

const gotTheJob = true;
const expectations = [];
const salaryExpectations = salaryChecker(employee, awesomenessLevel);

Varaible Declarations​

  • var is used pre-ES6 versions of JS.
  • let is preferred way to declare a varialbe which can be reassigned.
  • const is the preferred way to declare a variable with a constant value.
var age;
let weight;
const pie;

Undefined​

undefined is a primitive JS value that represents lack of defined value. Varialbles that re declared but not intialized to a value will have the value undefined.

var noValueAttached;

console.log(noValueAttached);
// prints undefined