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the dom

(D)ocument (O)bject (M)odel:

the data structure of web pages.


one big tree

<html>
  <head><title>wow</title></head>
  <body>
    <h1 id="top">very tree</h1>
    <div class="msg">
      so <span>tall</span>
    </div>
  </body>
</html>

document

  • document.head -> <head>...</head>
  • document.body -> <body>...</body>

elements

Each html tag is called an element.

When you have an element reference, you get:

  • elem.childNodes - an array of children
  • elem.parentNode - the parent of the current element

document.head.childNodes[0] === <title>wow</title>


document.body.childNodes[0] === <h1 id="top">very tree</h1>


document.body.childNodes[1] === <div class="msg">so <span>tall</span></div>


document.body.childNodes[1].childNodes[0] === <span>tall</span>


query selector

Using document.body.[...] to fetch every element is not very flexible!

Instead, we can fetch elements with a css selector:

document.querySelector('h1') === <h1>very tree</h1>


css selectors

Select an element by a tag name:

  • span -> <span>tall</span>

Select an element by its class name:

  • .msg -> <div class="msg">...</div>

Select an element by its id:

  • #top -> <h1 id="top">very tree</h1>

Select an element by attribute:

  • [id="top"] -> <h1 id="top">very tree</h1>

css selector combinations

You can combine id, class, attribute, and element selectors.

All of the constraints must match:

document.querySelector('h1#cool.row[x="5"]')

will match:

<h1 id="cool" class="row" x="5">whatever</h1>

but not:

<h1 id="sweet" class="row" x="5">hey</h1>

nested css selectors

  • .msg span -> <span>tall</span>

  • #top span -> null (matches nothing)


querySelector, querySelectorAll

  • document.querySelector() -> first matching element
  • document.querySelectorAll() -> all matching elements

elem.querySelector, elem.querySelectorAll

If you have an element, you can also call .querySelector() on that element to query its descendents:

  • elem.querySelector() -> first matching element
  • elem.querySelectorAll() -> all matching elements

some other ways of querying

  • document.getElementById()
  • document.getElementsByClassName()

once you have an element reference, you can modify the contents in plenty of ways:

  • set the inner text or html
  • add and remove children
  • set and remove attributes

setting text

var elem = document.querySelector('.msg');
elem.textContent = 'purr cats <3 <3 <3'

With textContent you don't need to worry about elements being interpreted as html.


setting html

var elem = document.querySelector('.msg');
elem.innerHTML = '<h1>wow</h1>'

construct elements with code

var div = document.createElement('div');

construct elements with a string of html

var div = document.createElement('div');
div.innerHTML = '<b>wow such inner</b>';

add children with .appendChild()

var div = document.createElement('div');
var b = document.createElement('b');
b.textContent = 'wowsers';

div.appendChild(b);
document.body.appendChild(b);

remove children with .removeChild()

suppose we have some html:

<html>
  <body>
    <div>
      cool cool
      <span>get rid of me</span>
    </div>
  </body>
</html>

remove children with .removeChild()

we can get rid of the inner span by fetching references to the span and its parent, then on the parent we can call .removeChild():

var div = document.querySelector('div');
var span = div.querySelector('span');
div.removeChild(span);

remove children with .removeChild()

and now the html will be:

<html>
  <body>
    <div>
      cool cool
    </div>
  </body>
</html>

set an attribute with .setAttribute()

var input = document.createElement('input');
input.setAttribute('type', 'text');
input.setAttribute('value', 'wow');
document.body.appendChild(input);

set an attribute with .setAttribute()

and now the body will have an input element with type="text" and value="wow":

<html>
  <body>
    <input type="text" value="wow">
  </body>
</html>

.insertBefore()

We can also insert elements before another element. Given this html:

<html>
  <body>
    <ul>
      <li class="zero">zero</li>
      <li class="one">one</li>
      <li class="three">three</li>
    </ul>
  </body>
</html>

.insertBefore()

We can insert an element before <li>three</li>:

var ul = document.querySelector('ul');
var three = ul.querySelector('.three');

var two = document.createElement('li');
two.textContent = 'two';
two.setAttribute('class', 'two');

ul.insertBefore(two, three);

.insertBefore()

and now the html will be:

<html>
  <body>
    <ul>
      <li class="zero">zero</li>
      <li class="one">one</li>
      <li class="two">two</li>
      <li class="three">three</li>
    </ul>
  </body>
</html>

.style

We can adjust css on the fly with .style. Using the html from the previous example, we can do:

var zero = document.querySelector('.zero');
zero.style.color = 'purple';

and now the first element in the list will have purple text.


events

We can respond to page actions by using .addEventListener(). With this html:

<body>
  <button>wow</button>
</body>

events

we can insert an element when somebody clicks the button:

var button = document.querySelector('button');
button.addEventListener('click', function (ev) {
    var msg = document.createElement('div');
    msg.textContent = new Date().toISOString();
    document.body.appendChild(msg);
});

forms and preventDefault

Sometimes events have default actions, like forms will send a GET or POST request when the submit button is clicked. You can override these actions by calling preventDefault() on the event object.

Given this html:

<form>
  <input type="text" name="cool">
  <input type="submit">
</form>

forms and preventDefault

We can capture the submit event and prevent the default action:

var form = document.querySelector('form');
form.addEventListener('submit', function (ev) {
    ev.preventDefault();
    form.querySelector('[name=cool]')
});

registering events for multiple elements

Suppose we have some html:

<ul>
  <li>one</li>
  <li>two</li>
  <li>three</li>
</ul>

There are two ways we can register a "click" event for each li element.


register a listener on each element

We can register a "click" listener on each element:

var items = document.querySelectorAll('ul li');
for (var i = 0; i < items.length; i++) (function (elem) {
    elem.addEventListener('click', function (ev) {
        ev.target.style.fontWeight = 'bold';
    });
})(items[i]);

This is kind of messy. We need to use a closure because the for loop modifies the i index before the events fire so it is always the last index.


ev.target

A simpler way to register a listener for many elements is to register a listener on the parent ul and check the ev.target:

var ul = document.querySelector('ul');
ul.addEventListener('click', function (ev) {
    ev.target.style.fontWeight = 'bold';
});

xhr

We can make http requests with the DOM too!

It's really awkward to do this with raw javascript:

var xhr = new XMLHttpRequest;
xhr.addEventListener('readystatechange', function (ev) {
    if (xhr.readyState === 4) {
        console.log('body=', xhr.responseText);
    }
});
xhr.open('POST', '/', true);
xhr.send('foo=bar&x=5');

xhr

Luckily, there is a package on npm we can use. First do:

npm install xhr

then in your browser code you can do:

var xhr = require('xhr');
var opts = {
    method: 'POST',
    uri: '/',
    body: 'foo=bar&x=5'
};
xhr(opts, function (err, res, body) {
    console.log('body=', body);
});

xhr

or with the help of the built-in querystring module:

var xhr = require('xhr');
var qs = require('querystring');
var opts = {
    method: 'POST',
    uri: '/',
    body: qs.stringify({ foo: 'bar', x: 5 })
};
xhr(opts, function (err, res, body) {
    console.log('body=', body);
});

If you want to use require() to load modules from npm, you'll need to use a tool like browserify:

$ browserify browser.js > bundle.js

then in your html:

<script src="bundle.js"></script>