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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 valuetrue
. - to redirect the request, include property
redirectUrl
with the value set to the URL to which you want to redirect.
If you use "blocking"
, you must have the "webRequestBlocking" API permission in your manifest.json.
Syntax
chrome.webRequest.onBeforeRequest.addListener( function( // function details // object ) {...}, filter, // object extraInfoSpec // optional array of strings ) chrome.webRequest.onBeforeRequest.removeListener(listener) chrome.webRequest.onBeforeRequest.hasListener(listener)
This API is also available as browser.webRequest.onBeforeRequest.*
.
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. Returnstrue
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:
Returns:
webRequest.BlockingResponse
. If"blocking"
is specified in theextraInfoSpec
parameter, the event listener should return aBlockingResponse
object, and can set either itscancel
or itsredirectUrl
properties. filter
webRequest.RequestFilter
. A filter that restricts the events that will be sent to this listener.extraInfoSpec
Optionalarray
ofstring
. 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"
: includerequestBody
in thedetails
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
ismain_frame
orsub_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.requestBody
Optionalobject
. Contains the HTTP request body data. Only provided ifextraInfoSpec
contains"requestBody"
.-
error
Optionalstring
. This is set if any errors were encountered when obtaining request body data.formData
Optionalobject
. 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. raw
Optionalarray
of
. If the request method is PUT or POST, and the body is not already parsed inwebRequest.UploadData
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 theurl()
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.
Chrome | Edge | Firefox | Firefox for Android | Opera | |
---|---|---|---|---|---|
Basic support | Yes | Yes | 46.0 | 48.0 | 33 |
Compatibility notes
Chrome
- 'originUrl' is not supported.
Firefox
- 'requestBody' is not supported.
- 'originUrl' is supported from version 48.
Firefox for Android
- 'requestBody' is not supported.
Opera
- 'originUrl' is not supported.
Examples
This code logs the URL for every resource requested which matches the <all_urls> pattern:
function logURL(requestDetails) { console.log("Loading: " + requestDetails.url); } chrome.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" chrome.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" chrome.webRequest.onBeforeRequest.addListener( redirect, {urls:[pattern], types:["image"]}, ["blocking"] );
This API is based on Chromium's chrome.webRequest
API. This documentation is derived from web_request.json
in the Chromium code.
Microsoft Edge compatibility data is supplied by Microsoft Corporation and is included here under the Creative Commons Attribution 3.0 United States License.
// Copyright 2015 The Chromium Authors. All rights reserved. // // Redistribution and use in source and binary forms, with or without // modification, are permitted provided that the following conditions are // met: // // * Redistributions of source code must retain the above copyright // notice, this list of conditions and the following disclaimer. // * Redistributions in binary form must reproduce the above // copyright notice, this list of conditions and the following disclaimer // in the documentation and/or other materials provided with the // distribution. // * Neither the name of Google Inc. nor the names of its // contributors may be used to endorse or promote products derived from // this software without specific prior written permission. // // THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS // "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT // LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR // A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT // OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, // SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT // LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, // DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY // THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.