webRequest.onBeforeRequest

This event is triggered when a request is about to be made, and before headers are available. This is a good place to listen if you want to cancel or redirect the request.

To cancel or redirect the request, first include "blocking" in the extraInfoSpec array argument to addListener(). Then, in the listener function, return a BlockingResponse object, setting the appropriate property:

• to cancel the request, include a property cancel with the value true.
• to redirect the request, include a property redirectUrl with the value set to the URL to which you want to redirect.

From Firefox 52 onwards, instead of returning BlockingResponse, the listener can return a Promise which is resolved with a BlockingResponse. This enables the listener to process the request asynchronously.

If you use "blocking", you must have the "webRequestBlocking" API permission in your manifest.json.

Syntax

browser.webRequest.onBeforeRequest.addListener(
  listener,             // function
  filter,               //  object
  extraInfoSpec         //  optional array of strings
)
browser.webRequest.onBeforeRequest.removeListener(listener)
browser.webRequest.onBeforeRequest.hasListener(listener)

Events have three functions:

addListener(callback, filter, extraInfoSpec)

Adds a listener to this event.

removeListener(listener)

Stop listening to this event. The listener argument is the listener to remove.

hasListener(listener)

Check whether listener is registered for this event. Returns true if it is listening, false otherwise.

addListener syntax

Parameters

callback

Function that will be called when this event occurs. The function will be passed the following arguments:

details

object. Details about the request. See details below.

Returns: webRequest.BlockingResponse. If "blocking" is specified in the extraInfoSpec parameter, the event listener should return a BlockingResponse object, and can set either its cancel or its redirectUrl properties. From Firefox 52 onwards, instead of returning BlockingResponse, the listener can return a Promise which is resolved with a BlockingResponse. This enables the listener to process the request asynchronously.

filter

webRequest.RequestFilter. A filter that restricts the events that will be sent to this listener.

extraInfoSpecOptional

array of string. Extra options for the event. You can pass any of the following values:

• "blocking": make the request synchronous, so you can cancel or redirect the request
• "requestBody": include requestBody in the details object passed to the listener

Additional objects

details

requestId

string. The ID of the request. Request IDs are unique within a browser session, so you can use them to relate different events associated with the same request.

url

string. Target of the request.

method

string. Standard HTTP method: for example, "GET" or "POST".

frameId

integer. Zero if the request happens in the main frame; a positive value is the ID of a subframe in which the request happens. If the document of a (sub-)frame is loaded (type is main_frame or sub_frame), frameId indicates the ID of this frame, not the ID of the outer frame. Frame IDs are unique within a tab.

parentFrameId

integer. ID of the frame that contains the frame which sent the request. Set to -1 if no parent frame exists.

requestBodyOptional

object. Contains the HTTP request body data. Only provided if extraInfoSpec contains "requestBody".

errorOptional

string. This is set if any errors were encountered when obtaining request body data.

formDataOptional

object. This object is present if the request method is POST and the body is a sequence of key-value pairs encoded in UTF-8 as either "multipart/form-data" or "application/x-www-form-urlencoded".

It is a dictionary in which each key contains the list of all values for that key. For example: {'key': ['value1', 'value2']}. If the data is of another media type, or if it is malformed, the object is not present.

rawOptional

array of webRequest.UploadData. If the request method is PUT or POST, and the body is not already parsed in formData, then this array contains the unparsed request body elements.

tabId

integer. ID of the tab in which the request takes place. Set to -1 if the request isn't related to a tab.

type

webRequest.ResourceType. The type of resource being requested: for example, "image", "script", "stylesheet".

timeStamp

number. The time when this event fired, in milliseconds since the epoch.

originUrl

string. URL of the resource that triggered this request. Note that this may not be the same as the URL of the page into which the requested resource will be loaded. For example, if a document triggers a load in a different window through the target attribute of a link, or a CSS document includes an image using the url() functional notation, then this will be the URL of the original document or of the CSS document, respectively.

Browser compatibility

The compatibility table in this page is generated from structured data. If you'd like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request.

Examples

This code logs the URL for every resource requested which matches the <all_urls> pattern:

function logURL(requestDetails) {
  console.log("Loading: " + requestDetails.url);
}
browser.webRequest.onBeforeRequest.addListener(
  logURL,
  {urls: ["<all_urls>"]}
);

This code cancels requests for images that are made to URLs under "https://mdn.mozillademos.org/" (to see the effect, visit any page on MDN that contains images, such as Firefox Developer Edition):

// match pattern for the URLs to redirect
var pattern = "https://mdn.mozillademos.org/*";
// cancel function returns an object
// which contains a property `cancel` set to `true`
function cancel(requestDetails) {
  console.log("Canceling: " + requestDetails.url);
  return {cancel: true};
}
// add the listener,
// passing the filter argument and "blocking"
browser.webRequest.onBeforeRequest.addListener(
  cancel,
  {urls: [pattern], types: ["image"]},
  ["blocking"]
);

This code replaces, by redirection, all network requests for images that are made to URLs under "https://mdn.mozillademos.org/" (to see the effect, visit any page on MDN that contains images, such as Firefox Developer Edition):

// match pattern for the URLs to redirect
var pattern = "https://mdn.mozillademos.org/*";
// redirect function
// returns an object with a property `redirectURL`
// set to the new URL
function redirect(requestDetails) {
  console.log("Redirecting: " + requestDetails.url);
  return {
    redirectUrl: "https://38.media.tumblr.com/tumblr_ldbj01lZiP1qe0eclo1_500.gif"
  };
}
// add the listener,
// passing the filter argument and "blocking"
browser.webRequest.onBeforeRequest.addListener(
  redirect,
  {urls:[pattern], types:["image"]},
  ["blocking"]
);

This code is exactly like the previous example, except that the listener handles the request asynchronously. It returns a Promise that sets a timer, and resolves with the redirect URL when the timer expires:

// match pattern for the URLs to redirect
var pattern = "https://mdn.mozillademos.org/*";
// URL we will redirect to
var redirectUrl = "https://38.media.tumblr.com/tumblr_ldbj01lZiP1qe0eclo1_500.gif";
// redirect function returns a Promise
// which is resolved with the redirect URL when a timer expires
function redirectAsync(requestDetails) {
  console.log("Redirecting async: " + requestDetails.url);
  var asyncCancel = new Promise((resolve, reject) => {
    window.setTimeout(() => {
      resolve({redirectUrl});
    }, 2000);
  });
  return asyncCancel;
}
// add the listener,
// passing the filter argument and "blocking"
browser.webRequest.onBeforeRequest.addListener(
  redirectAsync,
  {urls: [pattern], types: ["image"]},
  ["blocking"]
);

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