Examples of web and XML development using the DOM

This chapter provides some longer examples of web and XML development using the DOM. Wherever possible, the examples use common APIs, tricks, and patterns in JavaScript for manipulating the document object.

Example 1: height and width

The following example shows the use of the height and width properties alongside images of varying dimensions:

html

<p>
  Image 1: no height, width, or style
  <img id="image1" src="https://www.mozilla.org/images/mozilla-banner.gif" />
</p>

<p>
  Image 2: height="50", width="500", but no style
  <img
    id="image2"
    src="https://www.mozilla.org/images/mozilla-banner.gif"
    height="50"
    width="500" />
</p>

<p>
  Image 3: no height, width, but style="height: 50px; width: 500px;"
  <img
    id="image3"
    src="https://www.mozilla.org/images/mozilla-banner.gif"
    style="height: 50px; width: 500px;" />
</p>

<div id="output"></div>

const arrImages = [
  document.getElementById("image1"),
  document.getElementById("image2"),
  document.getElementById("image3"),
];

const objOutput = document.getElementById("output");
let strHtml = "<ul>";

for (const img of arrImages) {
  strHtml += `<li>image${i + 1}: height=${img.height}, width=${img.width}, style.height=${img.style.height}, style.width=${img.style.width}</li>`;
}

strHtml += "</ul>";

objOutput.innerHTML = strHtml;

Example 2: Image Attributes

html

<p>
  <img id="img1" src="image1.gif" width="100" height="100" alt="border test" />
</p>

<form name="FormName">
  <input type="button" id="btn1" value="Make border 20px-wide" />
  <input type="button" id="btn2" value="Make border 5px-wide" />
</form>

css

#img1 {
  border: 5px solid green;
}

function setBorderWidth(width) {
  document.getElementById("img1").style.borderWidth = `${width}px`;
}

document.getElementById("btn1").addEventListener("click", () => {
  setBorderWidth(20);
});
document.getElementById("btn2").addEventListener("click", () => {
  setBorderWidth(5);
});

Example 3: Manipulating Styles

In this simple example, some basic style properties of an HTML paragraph element are accessed using the style object on the element and that object's CSS style properties, which can be retrieved and set from the DOM. In this case, you are manipulating the individual styles directly. In the next example (see Example 4), you can use stylesheets and their rules to change styles for whole documents.

html

<p id="pid">Some text</p>
<form>
  <p><button type="button">Change text</button></p>
</form>

function changeText() {
  const p = document.getElementById("pid");

  p.style.color = "blue";
  p.style.fontSize = "18pt";
}

document.querySelector("button").addEventListener("click", () => {
  changeText();
});

Example 4: Using Stylesheets

The styleSheets property on the document object returns a list of the stylesheets that have been loaded on that document. You can access these stylesheets and their rules individually using the stylesheet, style, and CSSRule objects, as demonstrated in this example, which prints out all of the style rule selectors to the console.

for (const styleSheet of document.styleSheets) {
  for (const rule of styleSheet.cssRules) {
    console.log(`${rule.selectorText}\n`);
  }
}

For a document with a single stylesheet in which the following three rules are defined:

css

body {
  background-color: darkblue;
}
p {
  font-family: Arial;
  font-size: 10pt;
  margin-left: 0.125in;
}
#lumpy {
  display: none;
}

This script outputs the following:

BODY
P
#LUMPY

Example 5: Event Propagation

This example demonstrates how events fire and are handled in the DOM in a very simple way. When the BODY of this HTML document loads, an event listener is registered with the top row of the TABLE. The event listener handles the event by executing the function stopEvent, which changes the value in the bottom cell of the table.

However, stopEvent also calls an event object method, event.stopPropagation, which keeps the event from bubbling any further up into the DOM. Note that the table itself has an onclick event handler that ought to display a message when the table is clicked. But the stopEvent method has stopped propagation, and so after the data in the table is updated, the event phase is effectively ended, and an alert box is displayed to confirm this.

html

<table id="t-daddy">
  <tr id="tbl1">
    <td id="c1">one</td>
  </tr>
  <tr>
    <td id="c2">two</td>
  </tr>
</table>

css

#t-daddy {
  border: 1px solid red;
}

#c1 {
  background-color: pink;
}

function stopEvent(event) {
  const c2 = document.getElementById("c2");
  c2.textContent = "hello";

  // this ought to keep t-daddy from getting the click.
  event.stopPropagation();
  console.log("event propagation halted.");
}

const elem = document.getElementById("tbl1");
elem.addEventListener("click", stopEvent, false);

document.getElementById("t-daddy").addEventListener("click", () => {
  console.log("t-daddy clicked");
});

Example 6: getComputedStyle

This example demonstrates how the window.getComputedStyle method can be used to get the styles of an element that are not set using the style attribute or with JavaScript (e.g., elt.style.backgroundColor="rgb(173 216 230)"). These latter types of styles can be retrieved with the more direct elt.style property, whose properties are listed in the DOM CSS Properties List.

getComputedStyle() returns a CSSStyleDeclaration object, whose individual style properties can be referenced with this object's getPropertyValue() method, as the following example document shows.

html

<div id="d1">&nbsp;</div>

<form action="">
  <p>
    <button type="button">getComputedStyle</button>
    height<input id="t1" type="text" value="1" /> max-width<input
      id="t2"
      type="text"
      value="2" />
    bg-color<input id="t3" type="text" value="3" />
  </p>
</form>

css

#d1 {
  margin-left: 10px;
  background-color: rgb(173, 216, 230);
  height: 20px;
  max-width: 20px;
}

function cStyles() {
  const refDiv = document.getElementById("d1");
  const txtHeight = document.getElementById("t1");
  const hStyle = document.defaultView
    .getComputedStyle(refDiv, null)
    .getPropertyValue("height");

  txtHeight.value = hStyle;

  const txtWidth = document.getElementById("t2");
  const wStyle = document.defaultView
    .getComputedStyle(refDiv, null)
    .getPropertyValue("width");

  txtWidth.value = wStyle;

  const txtBackgroundColor = document.getElementById("t3");
  const bStyle = document.defaultView
    .getComputedStyle(refDiv, null)
    .getPropertyValue("background-color");

  txtBackgroundColor.value = bStyle;
}

document.querySelector("button").addEventListener("click", cStyles);

Example 7: Displaying Event Object Properties

This example uses DOM methods to display all the properties of the onload event object and their values in a table. It also shows a useful technique of using a for...in loop to iterate over the properties of an object to get their values.

The properties of event objects differs greatly between browsers, the WHATWG DOM Standard lists the standard properties, however many browsers have extended these greatly.

Put the following code into a blank text file and load it into a variety of browsers, you'll be surprised at the different number and names of properties. You might also like to add some elements in the page and call this function from different event handlers.

html

<h1>Properties of the DOM <span id="eventType"></span> Event Object</h1>

css

table {
  border-collapse: collapse;
}
thead {
  font-weight: bold;
}
td {
  padding: 2px 10px 2px 10px;
}

.odd {
  background-color: #efdfef;
}
.even {
  background-color: #ffffff;
}

function showEventProperties(e) {
  function addCell(row, text) {
    const cell = row.insertCell(-1);
    cell.appendChild(document.createTextNode(text));
  }

  const event = e || window.event;
  document.getElementById("eventType").textContent = event.type;

  const table = document.createElement("table");
  const thead = table.createTHead();
  let row = thead.insertRow(-1);
  const labelList = ["#", "Property", "Value"];
  const len = labelList.length;

  for (let i = 0; i < len; i++) {
    addCell(row, labelList[i]);
  }

  const tbody = document.createElement("tbody");
  table.appendChild(tbody);

  for (const p in event) {
    row = tbody.insertRow(-1);
    row.className = row.rowIndex % 2 ? "odd" : "even";
    addCell(row, row.rowIndex);
    addCell(row, p);
    addCell(row, event[p]);
  }

  document.body.appendChild(table);
}

window.onload = (event) => {
  showEventProperties(event);
};

Example 8: Using the DOM Table Interface

The DOM HTMLTableElement interface provides some convenience methods for creating and manipulating tables. Two frequently used methods are HTMLTableElement.insertRow and HTMLTableRowElement.insertCell.

To add a row and some cells to an existing table:

html

<table id="table0">
  <tr>
    <td>Row 0 Cell 0</td>
    <td>Row 0 Cell 1</td>
  </tr>
</table>

const table = document.getElementById("table0");
const row = table.insertRow(-1);
let cell;
let text;

for (let i = 0; i < 2; i++) {
  cell = row.insertCell(-1);
  text = `Row ${row.rowIndex} Cell ${i}`;
  cell.appendChild(document.createTextNode(text));
}

Notes

A table's innerHTML property should never be used to modify a table, although you can use it to write an entire table or the content of a cell.
If DOM Core methods document.createElement and Node.appendChild are used to create rows and cells, IE requires that they are appended to a <tbody> element, whereas other browsers will allow appending to a <table> element (the rows will be added to the last <tbody> element).
There are a number of other convenience methods belonging to the HTMLTableElement interface that can be used for creating and modifying tables.