JS DOM / AJAX
Telerik Academy Alpha
Front-End
Table of contents
Document Object Model
The DOM Tree
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The Document Object Model is an API for HTML and XML documents
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Provides a structural representation of the document
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Developers can modify the content and UI of a web page
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The DOM Tree
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The DOM consists of objects to manipulate a web page
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All the properties, methods and events are organized into objects
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Those objects are accessible through programming languages and scripts
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How to use the DOM of an HTML page?
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Write JavaScript to interact with the DOM
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JavaScript uses the DOM API (native implementation for each browser)
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The DOM API
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The DOM API consists of objects and methods to interact with the HTML page
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Can add or remove HTML elements
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Can apply styles dynamically
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Can add and remove HTML attributes
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DOM introduces objects that represent HTML elements and their properties
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document.documentElement is <html>
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document.body is the body of the page, where the content of the page is
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DOM Objects
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Each of the HTML elements has corresponding DOM object type
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HTMLLIElement represents <li>
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HTMLAudioElement represents <audio>
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Each of these objects has the appropriate properties
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HTMLAnchorElement has href property
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HTMLImageElement has src property
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The document object is a special object
It represents the entry point for the DOM API
Selecting DOM Elements
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HTML elements can be found and cached into variables using the DOM API
var header = document.getElementById('header');
var nav = document.querySelector('#main-nav');
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Select single element
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Select a collection of elements
var inputs = document.getElementsByTagName('li');
var radiosGroup = document.getElementsByName('genders');
var header = document.querySelectorAll('#main-nav li');
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Select a collection of elements
var links = document.links;
var forms = document.forms;
NodeLists
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A NodeList is a collection returned by the DOM selector methods:
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getElementsByTagName(tagName)
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getElementsByName(name)
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getElementsByClassName(className)
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querySelectorAll(selector)
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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
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NodeList looks like an array, but is not
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It's an object with properties similar to array
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Has length and indexer
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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?
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jQuery is a cross-browser JavaScript library
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Designed to simplify the client-side scripting of HTML
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The most popular JavaScript library in use today
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Free, open source software
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jQuery's syntax is designed to make it easier to
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Navigate a document and select DOM elements
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Create animations
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Handle events
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Why use jQuery?
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Easy to learn
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Fluent programming style
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Easy to extend
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You create new jQuery plugins by creating newJavaScript functions
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Powerful DOM Selection
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Powered by CSS 3.0
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Lightweight
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Community Support
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Large community of developers and geeks
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Installing jQuery
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Download jQuery files from jquery.com
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Self hosted
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You can choose to self host the .js file
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E.g. jquery-3.1.0.js or .min.js file
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Use it from CDN (content delivery network)
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Microsoft, jQuery, Google CDNs
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e.g. code.jquery.com/jquery-3.1.0.min.js
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ajax.microsoft.com/ajax/jquery/jquery-3.1.0.min.js
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Selectors
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Selection of DOM elements in jQuery is much like as in pure JavaScript
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Selection of elements using CSS selectors
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Like querySelectorAll
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//by tag
$("div") //document.querySelectorAll("div");
//by class
$(".menu-item"); //document.querySelectorAll(".menu-item");
//by id
$("#navigation");
//by combination of selectors
$("ul.menu li");
Selectors
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Selecting items with jQuery
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Almost always returns a collection of the items
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Even if there is only one item
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Can be stored in a variable or used right away
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The usage of the elements is always the same, nomatter whether a single or many elements
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$(".widgets").fade(1);
let something = $("#something");
something.hide();
DOM Traversal
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$.next(), $.prev(), $.parent(), $.parents(selector)
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Returns the next/prev sibling
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Returns an HTML element
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Not a [text] node
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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
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Selected with jQuery DOM elements are NOT pure DOM elements
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They are extended
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Have additional properties and methods
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addClass(), removeClass(), toggleClass()
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on(event, callback) for attaching events
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animate(), fade(), etc…
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//Parsing a regular DOM element to jQuery Element
var content = document.createElement("div");
var $content = $(content);
jQuery Object Properties
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jQuery elements extend regular DOM elements
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Methods for altering the elements
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$.css("color", "#f3f")
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$.html() returns the innerHTML
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$.html(content) sets the innerHTML
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$.text(content) sets the innerHTML, by escaping the content
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jQuery Object Properties
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$(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.
});
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Shorthands:
function readyFn( jQuery ) {
console.log("ready!");
}
$(document).ready(readyFn);
$(window).on("load", readyFn);
$(readyFn);
jQuery Events
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jQuery has a convenient way for attaching and detaching events
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Works cross-browser
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Using methods on() and off()
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function onButtonClick(){
$(".selected").removeClass("selected");
$(this).addClass("selected");
}
$("a.button").on("click", onButtonClick);
AJAX
What is AJAX?
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AJAX is acronym of Asynchronous JavaScript and XML
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Technique for background loading of dynamic content and data from the server side
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Allows dynamic client-side changes
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Two types of AJAX
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Partial page rendering - loading of HTML fragment and showing it in a <div>
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JSON service - loading JSON object and client-side processing it with JavaScript / jQuery
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What is AJAX?
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Technically, AJAX is a group of technologies working together
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HTML & CSS for presentation
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The DOM for data display & interaction
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XML (or JSON) for data interchange
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XMLHttpRequest for async communication
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JavaScript to use the above
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AJAX Requests
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AJAX uses HTTP
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Requests have headers - GET, POST, HEAD, etc.
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Requests have bodies - XML, JSON or plain text
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The request must target a resource with a URI
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The resource must understand the request
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Server-side logic
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Requests get a HTTP Response
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Header with a body
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AJAX jQuery
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jQuery provides some methods for AJAX
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$.ajax(options) – HTTP request with full control (headers, data, method, etc…)
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$.get(url) – HTTP GET request
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$.post(url) – HTTP POST request
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$(selector).load(url) – loads the contents from the url inside the selected node
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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
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A promise is an object which represents an eventual (future) value
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Methods "promise" they will return a value
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Correct or representing an error
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A Promises can be in one of three states:
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Fulfilled (resolved, succeeded)
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Rejected (an error happened)
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Pending (unfulfilled yet, still being computed)
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Promises
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Promise objects can be used in code as if their value is known
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Actually we attach code which executes
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When the promise is fulfilled
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When the promise is rejected
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When the promise reports progress (optionally)
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Promises are a pattern
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No defined implementation, but strict requirements
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Initially described in CommonJS Promises/A
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Promises
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More specifically:
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Each promise has a .then() method accepting 3 params:
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Success, Error and Progress functions
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All parameters are optional
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So we can write:
promiseMeSomething()
.then(function (value) {
//handle success here
}, function (reason) {
//handle error here
});
// Note: Provided promiseMeSomething returns a promise
Promises
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Each .then() method returns a promise in turn
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Meaning promises can be chained
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Promises enable us to:
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Remove the callback functions from the parameters and attach them to the "result"
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Make a sequence of operations happen
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Catch errors when we can process them
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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
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Promise.all(iterable)
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Wait until all settled
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Promise.race(iterable)
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Wait until 1 settles
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Promise.reject(reason)
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Create a Promise that is already rejected
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Useful to not do async operation in some condition
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Promise.resolve(value)
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Create a promise that is already resolved
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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
By telerikacademy
[FE] JS DOM / AJAX
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