files/en-us/web/api/htmliframeelement/contentwindow/index.md
{{APIRef("HTML DOM")}}
The contentWindow property returns the Window object of an HTMLIFrameElement.
This attribute is read-only.
A Window object.
Access to the {{domxref("Window")}} returned by contentWindow is subject to the rules defined by the same-origin policy, meaning that if the iframe is same-origin with the parent, then the parent can access the iframe's document and its internal DOM, and if they are cross-origin, it gets very limited access to the window's attributes. See "Cross-origin script API access" for details.
Pages can also use this property to find out which iframe sent a message using {{domxref("Window.postMessage()")}}, by comparing it with the message event's {{domxref("MessageEvent.source", "source")}} property.
This example modifies the style attribute of the document body.
Note that this only works if the iframe's window is same-origin with its parent: otherwise attempting to access contentWindow.document will throw an exception.
const iframe = document.querySelector("iframe").contentWindow;
iframe.document.querySelector("body").style.backgroundColor = "blue";
// this would turn the 1st iframe in document blue.
This example could run in a page that hosts several iframes, any of which can send it messages using {{domxref("Window.postMessage()")}}. When the page receives a message, it wants to know which iframe contains the window that sent the message.
To do this, when it receives a message, the page first checks that the message was from the expected origin, and then finds the iframe that was the source of the message by comparing the message event's {{domxref("MessageEvent.source", "source")}} property with the iframe's contentWindow property.
const expectedOrigin = "https://example.org";
const iframes = Array.from(document.querySelectorAll("iframe"));
window.addEventListener("message", (e) => {
if (e.origin !== expectedOrigin) return;
const sourceIframe = iframes.find(
(iframe) => iframe.contentWindow === e.source,
);
console.log(sourceIframe);
});
{{Specifications}}
{{Compat}}