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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 ? No ?

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