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webRequest.StreamFilter.ondata

An event handler that will be called repeatedly when response data is available. The handler is passed an event object which contains a data property, which contains a chunk of the response data as an ArrayBuffer.

To decode the data use either TextDecoder or Blob.

WebExtension Examples

This example adds an ondata listener which replaces "WebExtension Example" in the response with "WebExtension WebExtension Example" using the replace() method.

Note: This example only works for occurrences of "WebExtension Example" that are entirely contained within a data chunk, and not ones that straddle two chunks (which might happen ~0.1% of the time for large documents). Additionally it only deals with UTF-8-coded documents. A real implementation of this would have to be more complex.

function listener(details) {
  let filter = browser.webRequest.filterResponseData(details.requestId);
  let decoder = new TextDecoder("utf-8");
  let encoder = new TextEncoder();

  filter.ondata = event => {
    let str = decoder.decode(event.data, {stream: true});
    // Just change any instance of WebExtension Example in the HTTP response
    // to WebExtension WebExtension Example.
    str = str.replace(/WebExtension Example/g, 'WebExtension WebExtension Example');
    filter.write(encoder.encode(str));
    // Doing filter.disconnect(); here would make us process only
    // the first chunk, and let the rest through unchanged. Note
    // that this would break multi-byte characters that occur on
    // the chunk boundary!
  }

  filter.onstop = event => {
    filter.close();
  }

  //return {}; // not needed
}

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

Another example for handling large documents:

function listener(details) {
  let filter = browser.webRequest.filterResponseData(details.requestId);
  let decoder = new TextDecoder("utf-8");
  let encoder = new TextEncoder();

  let data = [];
  filter.ondata = event => {
    data.push(event.data);
  };

  filter.onstop = event => {
    let str = "";
    if (data.length == 1) {
      str = decoder.decode(data[0]);
    }
    else {
      for (let i = 0; i < data.length; i++) {
        let stream = (i == data.length - 1) ? false : true;
        str += decoder.decode(data[i], {stream});
      }
    }
    // Just change any instance of WebExtension Example in the HTTP response
    // to WebExtension WebExtension Example.
    str = str.replace(/WebExtension Example/g, 'WebExtension $&');
    filter.write(encoder.encode(str));
    filter.close();
  };
}

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

Here's another version:

function listener(details) {
  let filter = browser.webRequest.filterResponseData(details.requestId);
  let decoder = new TextDecoder("utf-8");
  let encoder = new TextEncoder();

  let data = [];
  filter.ondata = event => {
    data.push(event.data);
  };

  filter.onstop = event => {
    let str = "";
    for (let buffer of data) {
      str += decoder.decode(buffer, {stream: true});
    }
    str += decoder.decode(); // end-of-stream

    // Just change any instance of WebExtension Example in the HTTP response
    // to WebExtension WebExtension Example.
    str = str.replace(/WebExtension Example/g, 'WebExtension $&');
    filter.write(encoder.encode(str));
    filter.close();
  };
}

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

The above example can also be written like so:

function listener(details) {
  let filter = browser.webRequest.filterResponseData(details.requestId);
  let decoder = new TextDecoder("utf-8");
  let encoder = new TextEncoder();

  let data = [];
  filter.ondata = event => {
    data.push(decoder.decode(event.data, {stream: true}));
  };

  filter.onstop = event => {
    data.push(decoder.decode());

    let str = data.join("");
    // Just change any instance of WebExtension Example in the HTTP response
    // to WebExtension WebExtension Example.
    str = str.replace(/WebExtension Example/g, 'WebExtension $&');
    filter.write(encoder.encode(str));
    filter.close();
  };
}

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

This example uses a Blob:

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

  let data = [];
  filter.ondata = event => {
    data.push(event.data);
  };

  filter.onstop = async event => {
    let blob = new Blob(data, {type: 'text/html'});
    let str = await blob.text();

    // Just change any instance of WebExtension Example in the HTTP response
    // to WebExtension WebExtension Example.
    str = str.replace(/WebExtension Example/g, 'WebExtension $&');
    filter.write(encoder.encode(str));
    filter.close();
  };
}

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

This example makes use of the DOMParser interface:

function listener(details) {
  let filter = browser.webRequest.filterResponseData(details.requestId);
  let encoder = new TextEncoder();
  let parser = new DOMParser();

  let data = [];
  filter.ondata = event => {
    data.push(event.data);
  };

  filter.onstop = async event => {
    let blob = new Blob(data, {type: 'text/html'});
    let str = await blob.text();
    let doc = parser.parseFromString(str, blob.type);
    let nodes = doc.querySelectorAll("title, h1");
    for (let node of nodes) {
      node.innerText = node.innerText.replace('WebExtension Example', 'WebExtension $&');
    }
    filter.write(encoder.encode(doc.documentElement.outerHTML));
    filter.close();
  };
}

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

This example combines all buffers into a single buffer:

function listener(details) {
  let filter = browser.webRequest.filterResponseData(details.requestId);
  let decoder = new TextDecoder("utf-8");
  let encoder = new TextEncoder();

  let data = [];
  filter.ondata = event => {
    data.push(new Uint8Array(event.data));
  };

  filter.onstop = event => {
    let combinedLength = 0;
    for (let buffer of data) {
      combinedLength += buffer.length;
    }
    let combinedArray = new Uint8Array(combinedLength);
    let writeOffset = 0;
    for (let buffer of data) {
      combinedArray.set(buffer, writeOffset);
      writeOffset += buffer.length;
    }
    let str = decoder.decode(combinedArray);
    // Just change any instance of WebExtension Example in the HTTP response
    // to WebExtension WebExtension Example.
    str = str.replace(/WebExtension Example/g, 'WebExtension $&');
    filter.write(encoder.encode(str));
    filter.close();
  };
}

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

The above example can also be written like so:

function listener(details) {
  let filter = browser.webRequest.filterResponseData(details.requestId);
  let decoder = new TextDecoder('utf-8');
  let encoder = new TextEncoder();

  let data = [];
  filter.ondata = event => {
    data.push(event.data);
  };

  filter.onstop = async event => {
    let blob = new Blob(data, {type: 'text/html'});
    let buffer = await blob.arrayBuffer();
    let str = decoder.decode(buffer);
    str = str.replace(/WebExtension Example/g, 'WebExtension $&');
    filter.write(encoder.encode(str));
    filter.close();
  };
}

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

This example demonstrates, how one can detect, if it's the final chunk in the response:

function listener(details) {
  let filter = browser.webRequest.filterResponseData(details.requestId);
  let encoder = new TextEncoder();
  let decoder = new TextDecoder("utf-8");

  let str = "";
  filter.ondata = event => {
    let stream = true;
    let data = new Uint8Array(event.data.slice(-8, -1));
    if (String.fromCharCode(...data) == "</html>") {
      stream = false; // end-of-stream
    }
    str += decoder.decode(event.data, {stream});
  };

  filter.onstop = event => {
    // Just change any instance of WebExtension Example in the HTTP response
    // to WebExtension WebExtension Example.
    str = str.replace(/WebExtension Example/g, 'WebExtension $&');
    filter.write(encoder.encode(str));
    filter.close();
  };
}

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

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
`ondata`	No	No	57	?	No	No	?	?	57	?	?	?

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Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/webRequest/StreamFilter/ondata