Evaluates an expression in the context of the inspected page, and displays the result in the extension sidebar pane.

The expression's execution context is the same as that for inspectedWindow.eval().

 JSON objects and DOM nodes are displayed as an expandable tree, as in the JSON viewer in Firefox. You can optionally specify a rootTitle string: this will be displayed as the title of the tree's root.

This is an asynchronous function that returns a Promise.


var evaluating = browser.devtools.panels.setExpression(
  expression,       // string
  rootTitle         // string


string. The expression to evaluate.
rootTitle Optional
string. The title of the root of the tree in which results are displayed.

Return value

A Promise that will be fulfilled with no arguments, once the expression has been evaluated.

Browser compatibility

ChromeEdgeFirefoxFirefox for AndroidOpera
Basic support Yes1 No572 No Yes

1. The expression must evaluate to a JavaScript object or a DOM node, or nothing is shown in the sidebar.

2. The expression must evaluate to an object that can be serialized to JSON, or nothing is shown in the sidebar. In particular, JavaScript cyclic objects and DOM nodes are not supported. See bug 1403130.


This code creates a sidebar pane that displays the tagName of the currently selected element:

function onCreated(sidebarPane) {

  browser.devtools.panels.elements.onSelectionChanged.addListener(() => {
    const exp = "$0 && $0.tagName";
    const title = "Selected Element tagName";
    sidebarPane.setExpression(exp, title);


browser.devtools.panels.elements.createSidebarPane("My pane").then(onCreated);


This API is based on Chromium's chrome.devtools.panels API.

Document Tags and Contributors

 Contributors to this page: wbamberg
 Last updated by: wbamberg,