webRequest.StreamFilter.onstart

An event handler that will be called when the stream is opened and is about to begin delivering data. From this point on, the extension may use filter functions like write(), disconnect(), or close().

Browser compatibility

	Desktop						Mobile
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	WebView Android	Chrome Android	Firefox for Android	Opera Android	Safari on IOS	Samsung Internet
`onstart`	No	No	57	?	No	No	?	?	57	?	?	?

Examples

This example will replace the page content with "replacement text":

function listener(details) {
  let filter = browser.webRequest.filterResponseData(details.requestId);

  filter.onstart = event => {
    console.log("started");
    let encoder = new TextEncoder();
    filter.write(encoder.encode("replacement content"));
    filter.close();
  }
}

browser.webRequest.onBeforeRequest.addListener(
  listener,
  {urls: ["https://example.org/"], types: ["main_frame"]},
  ["blocking"]
);

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https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/webRequest/StreamFilter/onstart