JS DOM / AJAX
Telerik Academy Alpha

 

Front-End

 Table of contents

Document Object Model

 The DOM Tree

  • The Document Object Model is an API for HTML and XML documents

    • Provides a structural representation of the document

    • Developers can modify the content and UI of a web page

 The DOM Tree

  • The DOM consists of objects to manipulate a web page

    • All the properties, methods and events are organized into objects

    • Those objects are accessible through programming languages and scripts

  • How to use the DOM of an HTML page?

    • Write JavaScript to interact with the DOM

      • JavaScript uses the DOM API (native implementation for each browser)

 The DOM API

  • The DOM API consists of objects and methods to interact with the HTML page

    • Can add or remove HTML elements

    • Can apply styles dynamically

    • Can add and remove HTML attributes

  • DOM introduces objects that represent HTML elements and their properties

    • document.documentElement is <html>

    • document.body is the body of the page, where the content of the page is

 DOM Objects

  • Each of the HTML elements has corresponding DOM object type

    • HTMLLIElement represents <li>

    • HTMLAudioElement represents <audio>

  • Each of these objects has the appropriate properties

    • HTMLAnchorElement has href property

    • HTMLImageElement has src property

  • The document object is a special object
    It represents the entry point for the DOM API

 Selecting DOM Elements

  • HTML elements can be found and cached into variables using the DOM API

var header = document.getElementById('header');
var nav = document.querySelector('#main-nav');

  • Select single element

  • Select a collection of elements

var inputs = document.getElementsByTagName('li');
var radiosGroup = document.getElementsByName('genders');
var header = document.querySelectorAll('#main-nav li');

  • Select a collection of elements

var links = document.links;
var forms = document.forms;

 NodeLists

  • A NodeList is a collection returned by the DOM selector methods:

    • getElementsByTagName(tagName)

    • getElementsByName(name)

    • getElementsByClassName(className)

    • querySelectorAll(selector)

var divs = document.getElementsByTagName('div'),
    queryDivs = document.querySelectorAll('div');
for(var i = 0, length = divs.length; i < length; i += 1){
   // do stuff with divs[i]…
}

 NodeLists

  • NodeList looks like an array, but is not

    • It's an object with properties similar to array

      • Has length and indexer

  • Traversing an array with for-in loop works unexpected:

console.log(Array.isArray(divs)); // false

for (var i in divs) {
   console.log('divs[' + i + '] = ' + divs[i]);
}

jQuery

 What is jQuery?

  • jQuery is a cross-browser JavaScript library

    • Designed to simplify the client-side scripting of HTML

    • The most popular JavaScript library in use today

    • Free, open source software

  • jQuery's syntax is designed to make it easier to

    • Navigate a document and select DOM elements

    • Create animations

    • Handle events

 Why use jQuery?

  • Easy to learn

    • Fluent programming style

  • Easy to extend

    • You create new jQuery plugins by creating newJavaScript functions

  • Powerful DOM Selection

    • Powered by CSS 3.0

  • Lightweight

  • Community Support

    • Large community of developers and geeks

 Installing jQuery

  • Download jQuery files from jquery.com

    • Self hosted

      • You can choose to self host the .js file

      • E.g. jquery-3.1.0.js or .min.js file

    • Use it from CDN (content delivery network)

      • Microsoft, jQuery, Google CDNs

      • e.g. code.jquery.com/jquery-3.1.0.min.js

      • ajax.microsoft.com/ajax/jquery/jquery-3.1.0.min.js

 Selectors

  • Selection of DOM elements in jQuery is much like as in pure JavaScript

    • Selection of elements using CSS selectors

    • Like querySelectorAll

//by tag
$("div") //document.querySelectorAll("div");

//by class
$(".menu-item"); //document.querySelectorAll(".menu-item");

//by id
$("#navigation");

//by combination of selectors
$("ul.menu li");

 Selectors

  • Selecting items with jQuery

    • Almost always returns a collection of the items

      • Even if there is only one item

    • Can be stored in a variable or used right away

    • The usage of the elements is always the same, nomatter whether a single or many elements

$(".widgets").fade(1);

let something = $("#something");
something.hide();

 DOM Traversal

  • $.next(), $.prev(), $.parent(), $.parents(selector)

    • Returns the next/prev sibling

    • Returns an HTML element

      • Not a [text] node

var $firstLi = $("li").first();
log($firstLi); //logs "Item 1"
log($firstLi.next()); //logs "Item 2"
<ul>
  <li>Item 1</li>
  <li>Item 2</li>
</ul>

 jQuery Objects

  • Selected with jQuery DOM elements are NOT pure DOM elements

    • They are extended

    • Have additional properties and methods

      • addClass(), removeClass(), toggleClass()

      • on(event, callback) for attaching events

      • animate(), fade(), etc…

//Parsing a regular DOM element to jQuery Element
var content = document.createElement("div");
var $content = $(content);

 jQuery Object Properties

  • jQuery elements extend regular DOM elements

  • Methods for altering the elements

    • $.css("color", "#f3f")

    • $.html() returns the innerHTML

      • $.html(content) sets the innerHTML

    • $.text(content) sets the innerHTML, by escaping the content

 jQuery Object Properties

  • $(document).ready(callback) calls the passed function once the DOM tree is fully loaded

// A $(document).ready() block.
$(document).ready(function() {
    console.log("ready!"); // Code to run when the document is ready.
});

  • Shorthands:

function readyFn( jQuery ) {    
    console.log("ready!");
}

$(document).ready(readyFn);
$(window).on("load", readyFn);
$(readyFn);

 jQuery Events

  • jQuery has a convenient way for attaching and detaching events

    • Works cross-browser

    • Using methods on() and off()

function onButtonClick(){
  $(".selected").removeClass("selected");
  $(this).addClass("selected");
}

$("a.button").on("click", onButtonClick);

AJAX

 What is AJAX?

  • AJAX is acronym of Asynchronous JavaScript and XML

    • Technique for background loading of dynamic content and data from the server side

    • Allows dynamic client-side changes

  • Two types of AJAX

    • Partial page rendering - loading of HTML fragment and showing it in a <div>

    • JSON service - loading JSON object and client-side processing it with JavaScript / jQuery

 What is AJAX?

  • Technically, AJAX is a group of technologies working together

    • HTML & CSS for presentation

    • The DOM for data display & interaction

    • XML (or JSON) for data interchange

    • XMLHttpRequest for async communication

    • JavaScript to use the above

 AJAX Requests

  • AJAX uses HTTP

    • Requests have headers - GET, POST, HEAD, etc.

    • Requests have bodies - XML, JSON or plain text

    • The request must target a resource with a URI

    • The resource must understand the request

      • Server-side logic

    • Requests get a HTTP Response

      • Header with a body

 AJAX jQuery

  • jQuery provides some methods for AJAX

    • $.ajax(options) – HTTP request with full control (headers, data, method, etc…)

    • $.get(url) – HTTP GET request

    • $.post(url) – HTTP POST request

    • $(selector).load(url) – loads the contents from the url inside the selected node

 jQuery AJAX

$.get('users.all')
  .then(function(usersString) {
    return JSON.parse(usersString);
  })
  .then(function(users) {
    myController.users = users;
  })
  .catch(fucntion(error) {
    console.log(error)
  }

 Promises

  • A promise is an object which represents an eventual (future) value

    • Methods "promise" they will return a value

    • Correct or representing an error

  • A Promises can be in one of three states:

    • Fulfilled (resolved, succeeded)

    • Rejected (an error happened)

    • Pending (unfulfilled yet, still being computed)

 Promises

  • Promise objects can be used in code as if their value is known

    • Actually we attach code which executes

    • When the promise is fulfilled

    • When the promise is rejected

    • When the promise reports progress (optionally)

  • Promises are a pattern

    • No defined implementation, but strict requirements

    • Initially described in CommonJS Promises/A

 Promises

  • More specifically:

    • Each promise has a .then() method accepting 3 params:

      • Success, Error and Progress functions

    • All parameters are optional

  • So we can write:

promiseMeSomething()
  .then(function (value) {
      //handle success here
  }, function (reason) {
      //handle error here
  });

// Note: Provided promiseMeSomething returns a promise

 Promises

  • Each .then() method returns a promise in turn

    • Meaning promises can be chained

  • Promises enable us to:

    • Remove the callback functions from the parameters and attach them to the "result"

    • Make a sequence of operations happen

    • Catch errors when we can process them

asyncComputeTheAnswerToEverything()
  .then(addTwo)
  .then(printResult, onError);

 Promises

let promise = new Promise((resolve, reject) => {
    setTimeout(() => {
        resolve(1000);
    }, 1500)
} )

promise
    .then((value) => {
        console.log("Success " + value);
    })
    .catch((error) => {
        console.log("Error " + error);
    })

 Promise Methods

  • Promise.all(iterable)

    • Wait until all settled

  • Promise.race(iterable)

    • Wait until 1 settles

  • Promise.reject(reason)

    • Create a Promise that is already rejected

    • Useful to not do async operation in some condition

  • Promise.resolve(value)

    • Create a promise that is already resolved

 Promise Methods - Example

let usersPromise = get('users.all');
let postsPromise = get('posts.everyone');

// Wait until BOTH are settled
Promise.all([usersPromise, postsPromise])
  .then((results) => {
  // results is an array of the responses
    myController.users = results[0];
    myController.posts = results[1];
  })
  .catch(() => {
    delete myController.users;
    delete myController.posts;
  })

Questions?

[FE] JS DOM / AJAX

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