This value is used to report an error message from an asynchronous API, when the asynchronous API is given a callback. This is useful for extensions that are using the callback-based version of the WebExtension APIs.

You don't need to check this property if you are using the promise-based version of the APIs: instead, pass an error handler to the promise:

const gettingCookies = browser.cookies.getAll();
gettingCookies.then(onGot, onError);

The runtime.lastError property is set when an asynchronous function has an error condition that it needs to report to its caller.

If you call an asynchronous function that may set lastError, you are expected to check for the error when you handle the result of the function. If lastError has been set and you don't check it within the callback function, then an error will be raised.


let myError = browser.runtime.lastError;  // null or Error object


An Error object representing the error. The message property is a string with a human-readable description of the error. If lastError has not been set, the value is null.


Set a cookie, using a callback to log the new cookie or report an error:

function logCookie(c) {
  if (browser.runtime.lastError) {
  } else {

browser.cookies.set({ url: "" }, logCookie);

The same, but using a promise to handle the result of setCookie():

function logCookie(c) {

function logError(e) {

const setCookie = browser.cookies.set({
  url: "",

setCookie.then(logCookie, logError);

Note: runtime.lastError is an alias for extension.lastError. They are set together, and checking either one will work.

Browser compatibility

BCD tables only load in the browser

Example extensions

Note: This API is based on Chromium's chrome.runtime API. This documentation is derived from runtime.json in the Chromium code.