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

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webextensions-desktopwebextensions-mobile
ChromeEdgeFirefoxOperaSafariFirefox for Android
onstart

Legend

Full support
Full support
No support
No support

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"]
);