The Element property innerHTML gets or sets the HTML or XML markup contained within the element.

To insert the HTML into the document rather than replace the contents of an element, use the method insertAdjacentHTML().

A string containing the HTML serialization of the element's descendants. Setting the value of innerHTML removes all of the element's descendants and replaces them with nodes constructed by parsing the HTML given in the string htmlString.

SyntaxError DOMException

Thrown if an attempt was made to set the value of innerHTML using a string which is not properly-formed HTML.

NoModificationAllowedError DOMException

Thrown if an attempt was made to insert the HTML into a node whose parent is a Document.

The innerHTML property can be used to examine the current HTML source of the page, including any changes that have been made since the page was initially loaded.

Reading innerHTML causes the user agent to serialize the HTML or XML fragment comprised of the element's descendants. The resulting string is returned.

let contents = myElement.innerHTML;

This lets you look at the HTML markup of the element's content nodes.

Setting the value of innerHTML lets you easily replace the existing contents of an element with new content.

For example, you can erase the entire contents of a document by clearing the contents of the document's body attribute:

document.body.innerHTML = "";

This example fetches the document's current HTML markup and replaces the "<" characters with the HTML entity "<", thereby essentially converting the HTML into raw text. This is then wrapped in a <pre> element. Then the value of innerHTML is changed to this new string. As a result, the document contents are replaced with a display of the page's entire source code.

document.documentElement.innerHTML = `<pre>${document.documentElement.innerHTML.replace(
  /</g,
  "&lt;",
)}</pre>`;

Operational details

What exactly happens when you set value of innerHTML? Doing so causes the user agent to follow these steps:

1. The specified value is parsed as HTML or XML (based on the document type), resulting in a DocumentFragment object representing the new set of DOM nodes for the new elements.
2. If the element whose contents are being replaced is a <template> element, then the <template> element's content attribute is replaced with the new DocumentFragment created in step 1.
3. For all other elements, the element's contents are replaced with the nodes in the new DocumentFragment.

Setting the value of innerHTML lets you append new contents to the existing one of an element.

For example, we can append a new list item (<li>) to the existing list (<ul>):

HTML

<ul id="list">
  <li><a href="#">Item 1</a></li>
  <li><a href="#">Item 2</a></li>
  <li><a href="#">Item 3</a></li>
</ul>

JavaScript

const list = document.getElementById("list");

list.innerHTML += `<li><a href="#">Item ${list.children.length + 1}</a></li>`;

Please note that using innerHTML to append HTML elements (e.g. el.innerHTML += "<a href='…'>link</a>") will result in the removal of any previously set event listeners. That is, after you append any HTML element that way you won't be able to listen to the previously set event listeners.

It is not uncommon to see innerHTML used to insert text into a web page. There is potential for this to become an attack vector on a site, creating a potential security risk.

let name = "John";
// assuming 'el' is an HTML DOM element
el.innerHTML = name; // harmless in this case

// …

name = "<script>alert('I am John in an annoying alert!')</script>";
el.innerHTML = name; // harmless in this case

Although this may look like a cross-site scripting attack, the result is harmless. HTML specifies that a <script> tag inserted with innerHTML should not execute.

However, there are ways to execute JavaScript without using <script> elements, so there is still a security risk whenever you use innerHTML to set strings over which you have no control. For example:

const name = "<img src='x' onerror='alert(1)'>";
el.innerHTML = name; // shows the alert

For that reason, it is recommended that instead of innerHTML you use:

• Element.setHTML() to sanitize the text before it is inserted into the DOM.
• Node.textContent when inserting plain text, as this inserts it as raw text rather than parsing it as HTML.

This example uses innerHTML to create a mechanism for logging messages into a box on a web page.

function log(msg) {
  const logElem = document.querySelector(".log");

  const time = new Date();
  const timeStr = time.toLocaleTimeString();
  logElem.innerHTML += `${timeStr}: ${msg}<br/>`;
}

log("Logging mouse events inside this container…");

The log() function creates the log output by getting the current time from a Date object using toLocaleTimeString(), and building a string with the timestamp and the message text. Then the message is appended to the box with the class "log".

We add a second method that logs information about MouseEvent based events (such as mousedown, click, and mouseenter):

function logEvent(event) {
  const msg = `Event <strong>${event.type}</strong> at <em>${event.clientX}, ${event.clientY}</em>`;
  log(msg);
}

Then we use this as the event handler for a number of mouse events on the box that contains our log:

const boxElem = document.querySelector(".box");

boxElem.addEventListener("mousedown", logEvent);
boxElem.addEventListener("mouseup", logEvent);
boxElem.addEventListener("click", logEvent);
boxElem.addEventListener("mouseenter", logEvent);
boxElem.addEventListener("mouseleave", logEvent);

The HTML is quite simple for our example.

<div class="box">
  <div><strong>Log:</strong></div>
  <div class="log"></div>
</div>

The <div> with the class "box" is just a container for layout purposes, presenting the contents with a box around it. The <div> whose class is "log" is the container for the log text itself.

The following CSS styles our example content.

.box {
  width: 600px;
  height: 300px;
  border: 1px solid black;
  padding: 2px 4px;
  overflow-y: scroll;
  overflow-x: auto;
}

.log {
  margin-top: 8px;
  font-family: monospace;
}

The resulting content looks like this. You can see output into the log by moving the mouse in and out of the box, clicking in it, and so forth.

• Node.textContent and HTMLElement.innerText
• Element.insertAdjacentHTML()
• Element.outerHTML
• Element.setHTML
• Parsing HTML or XML into a DOM tree: DOMParser
• Serializing a DOM tree into an XML string: XMLSerializer