Core concept: Selectors

• the parameter to the $ function is a Selector
• a Selector is a string that matches 0 or more HTML elements
• Selector syntax is CSS, with some extensions
  • $('a.nav[href=/home]') - selects all A elements whose class is 'nav' and whose href attribute is '/home'
• References:
  • Selectors Doc for more details
  • Interactive jQuery selector tester

Core concept: Collections 1

• when you call the $ function you always get back a jQuery Collection
  • even if only one element matched
  • or none
  • the docs call it a jQuery Object
• a Collection has a length property
  • but it's not an Array

Core concept: Collections 2

• every Collection has a gajillion methods on it
  • actually they're on its prototype
  • these are all documented at http://api.jquery.com/
• most of these methods return this, i.e. a pointer to the original collection
  • allows method chaining
  • some methods change the matched set
  • some methods return values, not collections, e.g. attr and html

Core concept: Collections 3

• gotcha: some of the methods only act on the first item in the collection
  • $('a').attr('href') -- returns the href attribute of the first matching element
  • you can use each to execute a function on all items
  • or eq to reduce the set to a single item
  • $('a').eq(2).attr('href') -- returns the href attribute of the third matching element
  • $('a').last().attr('href') -- returns the href attribute of the last matching element
  • see also Filtering doc
• gotcha: if you have an error in your selector, jQuery will not warn you
  • it will just return an empty collection

Loading jQuery

• http://docs.jquery.com/Downloading_jQuery

<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js">
</script>

• All Google jQuery links:

  • https://developers.google.com/speed/libraries/#jquery

• 3 reasons why you should let Google host jQuery for you

  1. Decreased Latency
  2. Increased parallelism
  3. Better caching

Animation

• jQuery has some fun methods to animate CSS attributes

$("#logo").animate({
    backgroundColor: "#aa0000"
}, 1000 );

• http://api.jquery.com/animate
  • You choose the attribute(s) and their final value, plus the duration of the entire effect
  • jQuery calculates and interpolates the details
• jQuery UI has lots more
  • http://jqueryui.com/, not http://jquery.com/

Event Listening

• jQuery's on method

$('#hi').on('click', function() {
    alert("hi");
});

• jQuery's convenience method for standard event types

$('#hi').click(function() {
    alert("hi");
});

• Warning: if you call click() with no parameters, it triggers a click

return false

• if the event listener returns false, all further processing stops
• usually used to stop a form from actually submitting after a handled submit event

More about jQuery Event Listening

var f = function(event) {....}

• convenience methods for standard event types

$('#hi').click(f);
$('#hi').change(f);

• using on directly

$('#hi').on('click', f);
$('#ho').on('change', f);

• on can hook many events at once

$('#hi').on('click change', f);

• or you can use chaining for the same effect

$('#hi').on('click', f)
        .on('change', f);

Ready, Fire, Aim

jQuery has a special event that is fired when the page is "ready", i.e. the DOM hierarchy has been fully constructed.

$(document).ready(function() {
    // set up your page
});

It's best to put setup code in a "ready" handler rather than directly inside a <script> tag since the document might not be ready yet.

This can be abbreviated, but this might be unclear:

$(function() {
    // set up your page
});

"In cases where code relies on loaded assets (for example, if the dimensions of an image are required), the code should be placed in a handler for the load event instead." - http://api.jquery.com/ready/

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