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Version: 18.0.3

Request interception

Once request interception is enabled, every request will stall unless it's continued, responded or aborted.

An example of a naïve request interceptor that aborts all image requests:

const puppeteer = require('puppeteer');

(async () => {
  const browser = await puppeteer.launch();
  const page = await browser.newPage();
  await page.setRequestInterception(true);
  page.on('request', interceptedRequest => {
    if (interceptedRequest.isInterceptResolutionHandled()) return;
    if (
      interceptedRequest.url().endsWith('.png') ||
      interceptedRequest.url().endsWith('.jpg')
    )
      interceptedRequest.abort();
    else interceptedRequest.continue();
  });
  await page.goto('https://example.com');
  await browser.close();
})();

Multiple Intercept Handlers and Asynchronous Resolutions​

By default Puppeteer will raise a Request is already handled! exception if request.abort, request.continue, or request.respond are called after any of them have already been called.

Always assume that an unknown handler may have already called abort/continue/respond. Even if your handler is the only one you registered, 3rd party packages may register their own handlers. It is therefore important to always check the resolution status using request.isInterceptResolutionHandled before calling abort/continue/respond.

Importantly, the intercept resolution may get handled by another listener while your handler is awaiting an asynchronous operation. Therefore, the return value of request.isInterceptResolutionHandled is only safe in a synchronous code block. Always execute request.isInterceptResolutionHandled and abort/continue/respond synchronously together.

This example demonstrates two synchronous handlers working together:

/*
This first handler will succeed in calling request.continue because the request interception has never been resolved.
*/
page.on('request', interceptedRequest => {
  if (interceptedRequest.isInterceptResolutionHandled()) return;
  interceptedRequest.continue();
});

/*
This second handler will return before calling request.abort because request.continue was already
called by the first handler.
*/
page.on('request', interceptedRequest => {
  if (interceptedRequest.isInterceptResolutionHandled()) return;
  interceptedRequest.abort();
});

This example demonstrates asynchronous handlers working together:

/*
This first handler will succeed in calling request.continue because the request interception has never been resolved.
*/
page.on('request', interceptedRequest => {
  // The interception has not been handled yet. Control will pass through this guard.
  if (interceptedRequest.isInterceptResolutionHandled()) return;

  // It is not strictly necessary to return a promise, but doing so will allow Puppeteer to await this handler.
  return new Promise(resolve => {
    // Continue after 500ms
    setTimeout(() => {
      // Inside, check synchronously to verify that the intercept wasn't handled already.
      // It might have been handled during the 500ms while the other handler awaited an async op of its own.
      if (interceptedRequest.isInterceptResolutionHandled()) {
        resolve();
        return;
      }
      interceptedRequest.continue();
      resolve();
    }, 500);
  });
});
page.on('request', async interceptedRequest => {
  // The interception has not been handled yet. Control will pass through this guard.
  if (interceptedRequest.isInterceptResolutionHandled()) return;

  await someLongAsyncOperation();
  // The interception *MIGHT* have been handled by the first handler, we can't be sure.
  // Therefore, we must check again before calling continue() or we risk Puppeteer raising an exception.
  if (interceptedRequest.isInterceptResolutionHandled()) return;
  interceptedRequest.continue();
});

For finer-grained introspection (see Cooperative Intercept Mode below), you may also call request.interceptResolutionState synchronously before using abort/continue/respond.

Here is the example above rewritten using request.interceptResolutionState

/*
This first handler will succeed in calling request.continue because the request interception has never been resolved.
*/
page.on('request', interceptedRequest => {
  // The interception has not been handled yet. Control will pass through this guard.
  const {action} = interceptedRequest.interceptResolutionState();
  if (action === InterceptResolutionAction.AlreadyHandled) return;

  // It is not strictly necessary to return a promise, but doing so will allow Puppeteer to await this handler.
  return new Promise(resolve => {
    // Continue after 500ms
    setTimeout(() => {
      // Inside, check synchronously to verify that the intercept wasn't handled already.
      // It might have been handled during the 500ms while the other handler awaited an async op of its own.
      const {action} = interceptedRequest.interceptResolutionState();
      if (action === InterceptResolutionAction.AlreadyHandled) {
        resolve();
        return;
      }
      interceptedRequest.continue();
      resolve();
    }, 500);
  });
});
page.on('request', async interceptedRequest => {
  // The interception has not been handled yet. Control will pass through this guard.
  if (
    interceptedRequest.interceptResolutionState().action ===
    InterceptResolutionAction.AlreadyHandled
  )
    return;

  await someLongAsyncOperation();
  // The interception *MIGHT* have been handled by the first handler, we can't be sure.
  // Therefore, we must check again before calling continue() or we risk Puppeteer raising an exception.
  if (
    interceptedRequest.interceptResolutionState().action ===
    InterceptResolutionAction.AlreadyHandled
  )
    return;
  interceptedRequest.continue();
});

Cooperative Intercept Mode​

request.abort, request.continue, and request.respond can accept an optional priority to work in Cooperative Intercept Mode. When all handlers are using Cooperative Intercept Mode, Puppeteer guarantees that all intercept handlers will run and be awaited in order of registration. The interception is resolved to the highest-priority resolution. Here are the rules of Cooperative Intercept Mode:

  • All resolutions must supply a numeric priority argument to abort/continue/respond.
  • If any resolution does not supply a numeric priority, Legacy Mode is active and Cooperative Intercept Mode is inactive.
  • Async handlers finish before intercept resolution is finalized.
  • The highest priority interception resolution "wins", i.e. the interception is ultimately aborted/responded/continued according to which resolution was given the highest priority.
  • In the event of a tie, abort > respond > continue.

For standardization, when specifying a Cooperative Intercept Mode priority use 0 or DEFAULT_INTERCEPT_RESOLUTION_PRIORITY (exported from HTTPRequest) unless you have a clear reason to use a higher priority. This gracefully prefers respond over continue and abort over respond and allows other handlers to work cooperatively. If you do intentionally want to use a different priority, higher priorities win over lower priorities. Negative priorities are allowed. For example, continue({}, 4) would win over continue({}, -2).

To preserve backward compatibility, any handler resolving the intercept without specifying priority (Legacy Mode) causes immediate resolution. For Cooperative Intercept Mode to work, all resolutions must use a priority. In practice, this means you must still test for request.isInterceptResolutionHandled because a handler beyond your control may have called abort/continue/respond without a priority (Legacy Mode).

In this example, Legacy Mode prevails and the request is aborted immediately because at least one handler omits priority when resolving the intercept:

// Final outcome: immediate abort()
page.setRequestInterception(true);
page.on('request', request => {
  if (request.isInterceptResolutionHandled()) return;

  // Legacy Mode: interception is aborted immediately.
  request.abort('failed');
});
page.on('request', request => {
  if (request.isInterceptResolutionHandled()) return;
  // Control will never reach this point because the request was already aborted in Legacy Mode

  // Cooperative Intercept Mode: votes for continue at priority 0.
  request.continue({}, 0);
});

In this example, Legacy Mode prevails and the request is continued because at least one handler does not specify a priority:

// Final outcome: immediate continue()
page.setRequestInterception(true);
page.on('request', request => {
  if (request.isInterceptResolutionHandled()) return;

  // Cooperative Intercept Mode: votes to abort at priority 0.
  request.abort('failed', 0);
});
page.on('request', request => {
  if (request.isInterceptResolutionHandled()) return;

  // Control reaches this point because the request was cooperatively aborted which postpones resolution.

  // { action: InterceptResolutionAction.Abort, priority: 0 }, because abort @ 0 is the current winning resolution
  console.log(request.interceptResolutionState());

  // Legacy Mode: intercept continues immediately.
  request.continue({});
});
page.on('request', request => {
  // { action: InterceptResolutionAction.AlreadyHandled }, because continue in Legacy Mode was called
  console.log(request.interceptResolutionState());
});

In this example, Cooperative Intercept Mode is active because all handlers specify a priority. continue() wins because it has a higher priority than abort().

// Final outcome: cooperative continue() @ 5
page.setRequestInterception(true);
page.on('request', request => {
  if (request.isInterceptResolutionHandled()) return;

  // Cooperative Intercept Mode: votes to abort at priority 10
  request.abort('failed', 0);
});
page.on('request', request => {
  if (request.isInterceptResolutionHandled()) return;

  // Cooperative Intercept Mode: votes to continue at priority 5
  request.continue(request.continueRequestOverrides(), 5);
});
page.on('request', request => {
  // { action: InterceptResolutionAction.Continue, priority: 5 }, because continue @ 5 > abort @ 0
  console.log(request.interceptResolutionState());
});

In this example, Cooperative Intercept Mode is active because all handlers specify priority. respond() wins because its priority ties with continue(), but respond() beats continue().

// Final outcome: cooperative respond() @ 15
page.setRequestInterception(true);
page.on('request', request => {
  if (request.isInterceptResolutionHandled()) return;

  // Cooperative Intercept Mode: votes to abort at priority 10
  request.abort('failed', 10);
});
page.on('request', request => {
  if (request.isInterceptResolutionHandled()) return;

  // Cooperative Intercept Mode: votes to continue at priority 15
  request.continue(request.continueRequestOverrides(), 15);
});
page.on('request', request => {
  if (request.isInterceptResolutionHandled()) return;

  // Cooperative Intercept Mode: votes to respond at priority 15
  request.respond(request.responseForRequest(), 15);
});
page.on('request', request => {
  if (request.isInterceptResolutionHandled()) return;

  // Cooperative Intercept Mode: votes to respond at priority 12
  request.respond(request.responseForRequest(), 12);
});
page.on('request', request => {
  // { action: InterceptResolutionAction.Respond, priority: 15 }, because respond @ 15 > continue @ 15 > respond @ 12 > abort @ 10
  console.log(request.interceptResolutionState());
});

Cooperative Request Continuation​

Puppeteer requires request.continue() to be called explicitly or the request will hang. Even if your handler means to take no special action, or 'opt out', request.continue() must still be called.

With the introduction of Cooperative Intercept Mode, two use cases arise for cooperative request continuations: Unopinionated and Opinionated.

The first case (common) is that your handler means to opt out of doing anything special the request. It has no opinion on further action and simply intends to continue by default and/or defer to other handlers that might have an opinion. But in case there are no other handlers, we must call request.continue() to ensure that the request doesn't hang.

We call this an Unopinionated continuation because the intent is to continue the request if nobody else has a better idea. Use request.continue({...}, DEFAULT_INTERCEPT_RESOLUTION_PRIORITY) (or 0) for this type of continuation.

The second case (uncommon) is that your handler actually does have an opinion and means to force continuation by overriding a lower-priority abort() or respond() issued elsewhere. We call this an Opinionated continuation. In these rare cases where you mean to specify an overriding continuation priority, use a custom priority.

To summarize, reason through whether your use of request.continue is just meant to be default/bypass behavior vs falling within the intended use case of your handler. Consider using a custom priority for in-scope use cases, and a default priority otherwise. Be aware that your handler may have both Opinionated and Unopinionated cases.

Upgrading to Cooperative Intercept Mode for package maintainers​

If you are package maintainer and your package uses intercept handlers, you can update your intercept handlers to use Cooperative Intercept Mode. Suppose you have the following existing handler:

page.on('request', interceptedRequest => {
  if (request.isInterceptResolutionHandled()) return;
  if (
    interceptedRequest.url().endsWith('.png') ||
    interceptedRequest.url().endsWith('.jpg')
  )
    interceptedRequest.abort();
  else interceptedRequest.continue();
});

To use Cooperative Intercept Mode, upgrade continue() and abort():

page.on('request', interceptedRequest => {
  if (request.isInterceptResolutionHandled()) return;
  if (
    interceptedRequest.url().endsWith('.png') ||
    interceptedRequest.url().endsWith('.jpg')
  )
    interceptedRequest.abort('failed', 0);
  else
    interceptedRequest.continue(
      interceptedRequest.continueRequestOverrides(),
      0
    );
});

With those simple upgrades, your handler now uses Cooperative Intercept Mode instead.

However, we recommend a slightly more robust solution because the above introduces several subtle issues:

  1. Backward compatibility. If any handler still uses a Legacy Mode resolution (ie, does not specify a priority), that handler will resolve the interception immediately even if your handler runs first. This could cause disconcerting behavior for your users because suddenly your handler is not resolving the interception and a different handler is taking priority when all the user did was upgrade your package.
  2. Hard-coded priority. Your package user has no ability to specify the default resolution priority for your handlers. This can become important when the user wishes to manipulate the priorities based on use case. For example, one user might want your package to take a high priority while another user might want it to take a low priority.

To resolve both of these issues, our recommended approach is to export a setInterceptResolutionConfig() from your package. The user can then call setInterceptResolutionConfig() to explicitly activate Cooperative Intercept Mode in your package so they aren't surprised by changes in how the interception is resolved. They can also optionally specify a custom priority using setInterceptResolutionConfig(priority) that works for their use case:

// Defaults to undefined which preserves Legacy Mode behavior
let _priority = undefined;

// Export a module configuration function
export const setInterceptResolutionConfig = (priority = 0) =>
  (_priority = priority);

/**
 * Note that this handler uses `DEFAULT_INTERCEPT_RESOLUTION_PRIORITY` to "pass" on this request. It is important to use
 * the default priority when your handler has no opinion on the request and the intent is to continue() by default.
 */
page.on('request', interceptedRequest => {
  if (request.isInterceptResolutionHandled()) return;
  if (
    interceptedRequest.url().endsWith('.png') ||
    interceptedRequest.url().endsWith('.jpg')
  )
    interceptedRequest.abort('failed', _priority);
  else
    interceptedRequest.continue(
      interceptedRequest.continueRequestOverrides(),
      DEFAULT_INTERCEPT_RESOLUTION_PRIORITY // Unopinionated continuation
    );
});

If your package calls for more fine-grained control over resolution priorities, use a config pattern like this:

interface InterceptResolutionConfig {
  abortPriority?: number;
  continuePriority?: number;
}

// This approach supports multiple priorities based on situational
// differences. You could, for example, create a config that
// allowed separate priorities for PNG vs JPG.
const DEFAULT_CONFIG: InterceptResolutionConfig = {
  abortPriority: undefined, // Default to Legacy Mode
  continuePriority: undefined, // Default to Legacy Mode
};

// Defaults to undefined which preserves Legacy Mode behavior
let _config: Partial<InterceptResolutionConfig> = {};

export const setInterceptResolutionConfig = (
  config: InterceptResolutionConfig
) => (_config = {...DEFAULT_CONFIG, ...config});

page.on('request', interceptedRequest => {
  if (request.isInterceptResolutionHandled()) return;
  if (
    interceptedRequest.url().endsWith('.png') ||
    interceptedRequest.url().endsWith('.jpg')
  ) {
    interceptedRequest.abort('failed', _config.abortPriority);
  } else {
    // Here we use a custom-configured priority to allow for Opinionated
    // continuation.
    // We would only want to allow this if we had a very clear reason why
    // some use cases required Opinionated continuation.
    interceptedRequest.continue(
      interceptedRequest.continueRequestOverrides(),
      _config.continuePriority // Why would we ever want priority!==0 here?
    );
  }
});

The above solutions ensure backward compatibility while also allowing the user to adjust the importance of your package in the resolution chain when Cooperative Intercept Mode is being used. Your package continues to work as expected until the user has fully upgraded their code and all third party packages to use Cooperative Intercept Mode. If any handler or package still uses Legacy Mode, your package can still operate in Legacy Mode too.