DOM

PreviousJavaScript Event Loop NextWeb APIs

Last updated 5 years ago

DOM

DOM = window.document
BOM = window.document, window.navigator, window.location, window.history, window.screen, etc
in DevTool, $0 returns the selected Element, so you can use normal DOM APIs to access: ex, $0.classList

DOM Ready

event DOMContentLoaded : the browser fully loaded HTML, and the DOM tree is built, but external resources like pictures <img> and stylesheets may be not yet loaded.
- document.addEventListener("DOMContentLoaded", ready);
- the defer scripts already loaded
- the async scripts might NOT be loaded
function load: the browser loaded all resources (images, styles etc)
- window.onload = function() {}
function beforeunload/unload : when the user is leaving the page
- window.unload = function() {}

// handle the case when we add the callback after the document is already loaded 
if (document.readyState === "complete" ||
    (document.readyState !== "loading" && !document.documentElement.doScroll)
) {
  callback();
} else {
  document.addEventListener("DOMContentLoaded", callback);
}

Script: async vs defer

Normally, if browser see <script> in <head>, it will stop parsing until downloaded
script in bottom of the body will not block. But its own problem is, the script won't be downloaded until the full page is parsed.
defer: async downloading scripts (download in order), execute after parsing HTML is done
async: async downloading scripts (downloading might be out of order), execute after downloading is done (will pause parsing HTML)
reference:

DOM Manipulation

Node (like root interface) -> Document -> Element

Selector

You can use the following selector on document or element.

NOTE you cannot use pure number in your selector

The selector attributes should be CSS identifiers or strings. So you cannot use number there!
var a = document.querySelector('a[data-a="1"]');

Select the first or null ($(selector) in DevTools):

const myElement = document.querySelector('#foo > div.bar input[name="login"]')
It's not live, compare to getElementsByTagName()

Select all matching ($$(selector) in DevTools):

const myElements = document.querySelectorAll('.bar')
iterate: for .. of or forEach or .item(i)

Compare/Match:

node.contains(element)
e.target.nodeName == "LI"
e.target.matches('input') (Element.matches)

Traverse:

traverse parents: element.closest('selector')

Others: using above is recommended.

  const items = document.getElementsByClassName('item');
  const item = document.getElementById('item');

Attributes

Generally, you can do:

element.getAttribute('id') or element.getAttribute('data-parent')
element.setAttribute('tabindex', 3);
element.id

js: it's accessible (read/write) from js by the dataset property. Note dashes are converted to camelCase)
css: attr()

// <article
//   id="electriccars"
//   data-columns="3"
//   data-index-number="12314"
//   data-parent="cars">
// ...
// </article>
var article = document.getElementById('electriccars');
article.dataset.columns // "3"
article.dataset.indexNumber // "12314"
article.dataset.parent = 'makes'; // "cars" -> 'makes'

// css
// article::before {
//   content: attr(data-parent);
// }

CSS

// CRUD class
myElement.classList.add('foo', 'bar')
myElement.classList.remove('bar')
myElement.classList.toggle('baz')
myElement.classList.replace('bar', 'baz')

// fetch class
myElement.className // -> 'foo bar'
myElement.classList.contains('foo') // -> 'foo bar'

// show or hide button
stopBtn.style.display = 'none'; // '';

// set style (this won't get all styles)
myElement.style.marginLeft = '2em'

// get all styles as text
var text = myElement.style.cssText; // "color: red;"

// get all styles
window.getComputedStyle(myElement).getPropertyValue('margin-left')

// Animation
const start = window.performance.now()
const duration = 2000

window.requestAnimationFrame(function fadeIn (now)) {
  const progress = now - start
  myElement.style.opacity = progress / duration

  if (progress < duration) {
    window.requestAnimationFrame(fadeIn)
  }
}

Element CRUD

// create
const myNewElement = document.createElement('a');
myNewElement.setAttribute('href', 'http://zurassic.com/');

// append as child
element1.appendChild(element2)
// this can also move existing element
element1.appendChild(document.querySelecotr('.existing'));

// insert into precise location, can be same level or as children
// [insertAdjacentHTML](https://developer.mozilla.org/en-US/docs/Web/API/Element/insertAdjacentHTML)
document.querySelector("body").insertAdjacentHTML("beforeend", '<div class="loader"></div>');
// this can also move existing element
document.querySelector("body").insertAdjacentHTML("beforeend", document.querySelecotr('.existing'));

// remove
myElement.parentNode.removeChild(myElement)

// clone
const myElementClone = myElement.cloneNode()

// preferment version using [DocumentFragment](https://developer.mozilla.org/en-US/docs/Web/API/DocumentFragment)
// It's minimal DOM, similar to StringBuilder
const fragment = document.createDocumentFragment()
fragment.appendChild(text)
fragment.appendChild(hr)

myElement.appendChild(fragment)

Update content

myElement.innerHTML = '<div>foo</div>'
myElement.textContent = 'abc'

Observe Changes

The MutationObserver interface provides the ability to watch for changes being made to the DOM tree.

You can observe changes (child, attributes, style, etc) and pause in dev tools:

function observe(el) {
  var MutationObserver = window.WebKitMutationObserver;

  var observer = new MutationObserver(function(mutations) {
    mutations.forEach(function(mutation) {
      console.log(
        "old",
        mutation.oldValue,
        "new",
        mutation.target.style.cssText,
        "mutation",
        mutation
      );
      if (mutation.attributeName == property) debugger;
    });
  });

  var config = {
    attributes: true,
    attributeFilter: ["class"], // only observe class changes
    attributeOldValue: true
  };

  observer.observe(el, config);
}

observe(document.querySelector(".my-test"));

Common Functions

// Rename a tag (change tag type). Ex, <div> -> <a>
function renameTag(target, newNode) {
  // copy attributes
  for (var i = 0; i < target.attributes.length; i++) {
    newNode.setAttribute(
      target.attributes[i].nodeName,
      target.attributes[i].nodeValue
    );
  }

  // move children. firstChild is a live API so we can 'while' on that
  while (target.firstChild) {
    newNode.appendChild(target.firstChild);
  }

  // copy in-line styles
  if (target.style.length > 0) {
    newNode.style.cssText = target.style.cssText;
  }

  return target.parentNode.replaceChild(newNode, target);
}

BOM

Reload window: location.reload();
Scroll to top: window.scrollTo(0, 0);

Don't use onclick: single property, essentially override it.

Event CRUD

// add
myElement.addEventListener('click', (event)=> {
  console.log(event.type + ' got fired')
})

// remove
myElement.addEventListener('change', function listener (event) {
  console.log(event.type + ' got triggered on ' + this)
  this.removeEventListener('change', listener)
})

// match all children (live too)
myForm.addEventListener('change', function (event) {
  const target = event.target
  if (target.matches('input')) {
    console.log(target.value)
  }
})

Override Default Event

preventDefault() stop default handling, it'll still propagate
stopPropagation() stop bubbling up
stopImmediatePropagation() stop handling in current layer, stop passing to other event handlers: If several listeners are attached to the same element for the same event type, they are called in order in which they have been added. If during one such call, event.stopImmediatePropagation() is called, no remaining listeners will be called.

e.target vs e.currentTarget:

e.target identifies the element on which the event occurred
e.currentTarget always refer to the the element to which the event handler has been attached

`this`

usually points to the DOM element where the handler is bound.
NOTE if you add event using arrow function then it's document instead of the element!

How to trigger event manually:

How to perform click? element.click()

Keyboard/Mouse events:

Custom events

interface for any custom event:

custom implementation of EventEmitter

Event Delegation

Use one event handler to handle all children events

  <div id="menu">
    <button data-action="save">Save</button>
    <button data-action="load">Load</button>
    <button data-action="search">Search</button>
  </div>

  <script>
    class Menu {
      constructor(elem) {
        this._elem = elem;
        elem.addEventListener('click', this.onClick.bind(this)); // (*)
      }

      save() {
        alert('saving');
      }

      load() {
        alert('loading');
      }

      search() {
        alert('searching');
      }

      onClick(event) {
        let action = event.target.dataset.action;
        if (action) {
          this[action]();
        }
      };
    }

    new Menu(menu);
  </script>

Event delegation can also be used to add behavior to any element (with data-attr):

  // use with <button data-counter>add</button>
  document.addEventListener('click', function(event) {
    if (event.target.dataset.counter != undefined) {
      // if the attribute exists...
      event.target.value++;
    }
  });

PreviousJavaScript Event Loop NextWeb APIs

Last updated 5 years ago