Introduction to JavaScript

Outline

HTML/CSS Review

JavaScript Overview

JavaScript Variables

Advanced Array Functionality

Manipulating HTML Elements

HTML

  • HyperText Markup Language
  • Uses <tags> to describe rendering
  • Organize content into a tree structure
Style

Selectors

By ID

li { /* all <li> elements */
    property:value;
}

By class

.my-class { /* all elements with class "my-class" */
    property:value;
}

By element type

#my-id { /* element with id "my-id" */
    property:value;
}

The box model

JavaScript Overview

Javascript(JS)

  • The most popular programming language in the world
  • Most of the code you write will be JS
  • Can be used to create, manipulate, and remove HTML elements
  • Program behaviors of the webpage
  • Handle data
  • Written in a separate file, referenced by index.html
<script type="text/javascript" src="js/my-script.js"></script>

Javascript is a scripting language

  • Differs substantially from markup languages
  • Creates variables which are not represented in the DOM
  • Supports structured programming (ie, conditional if, else) statements
  • We will aim for a functional understanding of JS

JavaScript Variables

Variable types

Numeric: single quantitative value

var x = 13;

String: characters, in quotes (', or ")

var str = "hello there";

Array: Set of items in a vector (string or numeric)

var timesTwo = function(value) { return value * 2}

Object: Item that holds key-value pairs

var arr = [13, 'fourteen', 15];
var obj = {name:'steve', job:'lobbyist'};

Function: Executable block of code, returns a value

Properties and Methods

Properties describe a feature of an element

var characters = "This is a string"
characters.length // Returns the length of the variable
var arr = [1,2,3]
arr.length // Returns the number of items in the array

Methods are actions that variables can perform

var chars = "This is a string"
chars.replace("This", "Here") // Replaces "This" with "Here"
var arr = [1,2,3]
arr.push(4)// Pushes 4 into the array as the last element

Numbers and Strings

Numbers

Only one type of JavaScript number

var num = 1;
typeof(num) // returns 'number'
var num = 1.2;
typeof(num) // returns 'number'
var num = 1e8
typeof(num) // returns 'number'

Mathematical operations can be used

var num = 2*14 // num is 28
var num = 2/4 // num is .5

Advanced Math operations

var num = Math.sqrt(4) // num is 2

Strings

Create string variables with single or double quotes

var str = 'using single quotes'
var str = "using double quotes"

Lots and lots and lots of string functions

var name = "Mike"
var greeting = "Hello, my name is " + name // concatenation
var greeting2 = greeting.replace("Hello", "Sup") // replacing

Arrays

Comma separated items, denoted by square brackets []

var x = [13, 14, 15];
var x = ['one', 'two'];

Access values by index, starting at 0

var x = [13, 14]
x[0] // 13
x[1] // 14

Objects

Data items with key-value pairs

var person = {
    firstName:'Anna', 
    lastName:'Smith', 
    height:"5'4", 
}

Access values by referencing the key

person.firstName // Anna
person['firstName'] // Anna

Can be nested

var person = {
    firstName:'Anna', 
    favorites:{
        music:'bluegrass', 
        food:'pizza'
    } 
}
person.favorites.food // 'pizza

Functions

Perform blocks of code on parameters

var myFunction = function(a,b) {
    return a*b
}

Execute function using this syntax

var result = myFunction(2,4) // returns 8

Return a single item (object, array, etc.)

Conditional statements

Execute statements only if a condition is true (if)

if(a<b) {
    // execute some action
}

Provide a default alternative (else)

if(a<b) {
    // take some action
}
else if(a==b) {
    // another action
}
else {
    // default
}

Switch/case

Extension of if/else

Avoid having to retype conditions

var country = 'USA'
switch(country) {
	case 'USA':
		// take some action
		break;
	case 'FRA':
		// take some action
		break;
	default:
		// take default action
		break;
}

 

Functions can take functions as parameters

You have a function that compares values

var comparison = function(a,b) {
	if(a>b) {
            return 'a is greater that b'
	}
	else {
	    return 'b is less than or equal to a'
	}
}
var comparison = function(a,b, callback) {
	if(a>b) {
            callback()
       	    return 'a is greater that b'
	}
}

Pass that action in as a parameter

Advanced array manipulation

Execute a function on each element of an array by using the "map" method

  • The map method is a built in alternative to writing loops
  • Each array value is passed to a provided function
  • Values in the array are unchanged
var arr = [1,2,3]
var timesTwo = function(d) {return d*2}
var doubleArr = arr.map(timeTwo) // returns [2,4,6]

Map functions can also be written in-line

var arr = [1,2,3]
var doubleArr = arr.map(function(d) {
    return d*2
})

Filtering

var arr = [1,2,3]
var greaterThanTwo = arr.filter(function(d) {
    return d>2
})

Filtering only returns values that meet a given conditionality:

Same format as .map

Sorting

Will change your array (unlike map, or filter)

var arrToSort = [1,4,3,5,2]
sortFunction = function(a,b) {
    return a - b
}
arrToSort.sort(sortFunction)

Sorting takes 2 parameters

Parameters represent values to be compared

Connection to HTML

JavaScript DOM Manipulation

Get values using JavaScript

document.getElementById('my-input').value

Set properties using JavaScript

document.getElementById("demo").style.fontSize = "25px";

Libraries will make this less cumbersome (jQuery, D3, etc.)

Summary

  • Properties and methods of JavaScript variables
  • The use of functions
  • The ability to manipulate HTML elements

Assignment

JavaScript challenge (due Monday before class )

info343_js

By Michael Freeman

info343_js

  • 550
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