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
browser.webRequest.onBeforeRequest.addListener(function( details // object ) {...}) 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. 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.
Browser compatibility
Desktop | Mobile | ||||
---|---|---|---|---|---|
Edge | Firefox | Chrome | Opera | Firefox | |
Basic Support | ? | 46.01 | Yes | 33 | 48.02 |
1. Firefox does not support the "requestBody" option.
2. Firefox does not support the "requestBody" option.
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.
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