Element: innerHTML property

Getting the property returns a string containing the HTML serialization of the element's descendants.

Setting the property accepts either a TrustedHTML object or a string. It parses this value as HTML and replaces all the element's descendants with the result. When set to the null value, that null value is converted to the empty string (""), so elt.innerHTML = null is equivalent to elt.innerHTML = "".

SyntaxError DOMException

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

TypeError

Thrown if the property is set to a string when Trusted Types are enforced by a CSP and no default policy is defined.

NoModificationAllowedError DOMException

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

innerHTML gets a serialization of the nested child DOM elements within the element, or sets HTML or XML that should be parsed to replace the DOM tree within the element.

Note that some browsers serialize the < and > characters as < and > when they appear in attribute values (see Browser compatibility). This is to prevent a potential security vulnerability (mutation XSS) in which an attacker can craft input that bypasses a sanitization function, enabling a cross-site scripting (XSS) attack.

The serialization of the DOM tree read from the property does not include shadow roots — if you want to get a HTML string that includes shadow roots, you must instead use the Element.getHTML() or ShadowRoot.getHTML() methods.

Similarly, when setting element content using innerHTML, the HTML string is parsed into DOM elements that do not contain shadow roots. So for example <template> is parsed into as HTMLTemplateElement, whether or not the shadowrootmode attribute is specified. In order to set an element's contents from an HTML string that includes declarative shadow roots, you must instead use Element.setHTMLUnsafe() or ShadowRoot.setHTMLUnsafe().

The innerHTML property is probably the most common vector for Cross-site-scripting (XSS) attacks, where potentially unsafe strings provided by a user are injected into the DOM without first being sanitized. While the property does prevent <script> elements from executing when they are injected, it is susceptible to many other ways that attackers can craft HTML to run malicious JavaScript. For example, the following example would execute the code in the error event handler, because the <img> src value is not a valid image URL:

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

You can mitigate these issues by always assigning TrustedHTML objects instead of strings, and enforcing trusted type using the require-trusted-types-for CSP directive. This ensures that the input is passed through a transformation function, which has the chance to sanitize the input to remove potentially dangerous markup before it is injected.

Reading innerHTML causes the user agent to serialize the element's descendants.

Given the following HTML:

<div id="example">
  <p>My name is Joe</p>
</div>

You can get and log the markup for the contents of the outer <div> as shown:

const myElement = document.querySelector("#example");
const contents = myElement.innerHTML;
console.log(contents); // "\n  <p>My name is Joe</p>\n"

In this example we'll replace an element's DOM by assigning HTML to the element's innerHTML property. To mitigate the risk of XSS, we'll first create a TrustedHTML object from the string containing the HTML, and then assign that object to innerHTML.

Trusted types are not yet supported on all browsers, so first we define the trusted types tinyfill. This acts as a transparent replacement for the trusted types JavaScript API:

if (typeof trustedTypes === "undefined")
  trustedTypes = { createPolicy: (n, rules) => rules };

Next we create a TrustedTypePolicy that defines a createHTML() for transforming an input string into TrustedHTML instances. Commonly implementations of createHTML() use a library such as DOMPurify to sanitize the input as shown below:

const policy = trustedTypes.createPolicy("my-policy", {
  createHTML: (input) => DOMPurify.sanitize(input),
});

Then we use this policy object to create a TrustedHTML object from the potentially unsafe input string, and assign the result to the element:

// The potentially malicious string
const untrustedString = "<p>I might be XSS</p><img src='x' onerror='alert(1)'>";

// Create a TrustedHTML instance using the policy
const trustedHTML = policy.createHTML(untrustedString);

// Inject the TrustedHTML (which contains a trusted string)
const element = document.querySelector("#container");
element.innerHTML = trustedHTML;

• Node.textContent and HTMLElement.innerText
• Element.insertAdjacentHTML()
• Element.outerHTML
• Parsing HTML or XML into a DOM tree: DOMParser
• Serializing a DOM tree into an XML string: XMLSerializer
• Element.getHTML()
• ShadowRoot.getHTML()
• Element.setHTMLUnsafe()
• ShadowRoot.setHTMLUnsafe()
• Trusted Types API