JavaScript One

Introduction to JavaScript

What is JavaScript?

JavaScript was initially created "to make web pages alive"

JavaScript is often used alongside HTML and CSS, but can also be used for other purposes

JavaScript provides the dynamism and the action of a website. 

JavaScript Variables

Variables are used to store values in JavaScript. There are three ways to create, or declare, a variable:

var name = "Matias";

let name = "Matias";

const name = "Matias";

Declares a variable

The name of the variable

The value of the variable

JavaScript Data Types

JavaScript is built with several different data types.

Number

String

Boolean

Undefined

Null

Array

Object

let newString = "hello" 

let newNumber = 7

let newBoolean = true

let newUndefined = undefined

let newNull = null

let newArray = []

let newObject = {}

Numbers

Numbers refer to any integer or "floating point "number.

 

NaN, which stands for Not a Number, is, ironically, a number datatype.

 

Math operations can be performed on Number data types.

let num = 2;
let numTwo = 2;

let sum = 2 + 2; //=> 4

let summedNums = num + numTwo //=> 4

Above, both the 'sum' and 'summedNums' variables have a value of 4.

Strings

Strings are characters wrapped in quotes.

A string can be created using double, single, or back-tick quotes.

let name = "Matias";

let name = 'Matias';

let name = `Matias`;

Strings can also be added together, an operation called concatenation.

 

Using back-ticks, variables can also be placed directly into a string. This is known as a template string, or template literal.

let name = "Matias";
let greeting = "Hello, "
let nameGreet = greeting + name;
let tempString = `Hello, ${name}`;

Booleans

The boolean data type has only two values: true and false.


const isJeddyAwesome = true;

let isItPartyTime = false;

Null and Undefined

Null means the absence of a value, or that the value represents is nothing.

 

Undefined is a data type itself, and occurs when a variable is declared but not given a value;

let nothing = null;

let notDefined;

Arrays

Arrays in JavaScript are a data type that is used to create lists, or indexed data.

 

They are created using [square brackets], with each element separated by a comma.

 

These elements are organized by indexes, which start at 0.

 

Arrays can hold any data type in an index, including other arrays!

const myArray = [1, 'Hello', true, null];

0

1

2

3

Index:

Objects

Objects are a data type that is used to create representations of related data.

 

Objects are separated into key/value pairs, with the key being how we reference each value.

 

Object keys are also called properties interchangeably.

 

Objects are created using {curly brackets}.

const myObj = {
    name: 'Matias',
    age: 26
};

JavaScript Functions

Functions can be seen as 'actions' in our JavaScript, and they help bring our code to life.

 

Functions are what are used to "do stuff" or "make stuff happen" on a webpage

function sayHello(){
    return 'Hello!'
};

sayHello();

Function Syntax

There are two ways we can create functions, known as function declarations and function expressions.

//Function Declaration
function sayHello(){
    return 'Hello!'
};

//Function Expression
const sayHello = function(){
    return 'Hello!'
};

Function keyword tells JS that this is a function

Name of the function

What the function returns

The above code is just the blueprint of the function.

 

To actual use our function, we have to invoke, or call, the function, as seen below.

sayHello();

Parameters and Arguments

When building, or defining our function, we can specify parameters for our function.

Parameters are placeholders for arguments that will be passed in when the function is called. 

function toaster(slot1, slot2){
    return `${slot1} was toasted, ${slot2} was burned`;
};
toaster('rye bread', 'wheat bread');

Truthy and Falsy Values

In JavaScript, all values are either considered truthy or falsy values.

 

There are only six values in JavaScript that are falsy.

False

Null

Undefined

Empty String

0 (as a number)

NaN

 Operators

=  Assignment Operator

==  Equal to

===  Strictly Equal to

>  Greater than

>=  Greater than or equal to

<  Less than

<=  Less than or equal to

&&  Logical 'and' Operator

|| Logical 'or' Operator

!==  Not Equal to

If/Else Statements

If/Else Statements allow functionality dependent on conditions that are passed in

if(7 === 7){
   return'This is true!'
} else if(7 === '7'){
   return 'This is not true!'
} else {
   return 'Default Action'
}

Condition to be evaluated

Action block

Multiple comparisons can be evaluated in an if/else statement

 

The else block runs if none of the conditions pass.

Introduction to Scope

In JavaScript, everything has a scope; this refers to the 'level' the code is running in.

 

The top most scope is known as the global scope.

 

The scope in functions is known as the local, or lexical scope.

let global = "Global Scope";

function lexFunc(){
    return "Lexical Scope";
};

JavaScript 1

By Devmountain

Source: matias_perez

JavaScript 1

Introduction to JavaScript

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