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

BCD tables only load in the browser

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"],
);