Prebid.js: Uncaught (in promise) TypeError: Cannot read properties of undefined (reading 'makeBidRequests')

Discussed this today; the plan is to ship a change in Prebid 7 that would prevent the second copy of prebid from running if it’s using the same globalVarName as an instance of Prebid that’s already running.

If you’re using an earlier version of Prebid, and you’re affected by this issue, you should prevent the second loading of Prebid yourself. Note that even in 6.12 and earlier, the Prebid codebase assumes one instance per globalVarName - just because it doesn’t show obvious exceptions doesn’t mean it works if you load it multiple times; we never tested that scenario.

I did some more digging, and I think the proper fix is to prevent Prebid from loading twice - I would not trust the switch to import to solve this. (Paging @coreyb-cbs as well).

The issue with the proper fix is that it might be a breaking change if we do it within Prebid. I’ll plan to get it in for 7 at least.

What happens is:

• webpack maintains a registry of all modules (where modules are roughly javascript files). It starts out empty.
• the first time Prebid is loaded, webpack fills out the registry recursively - e.g. when it’s time to run auction.js, it creates an object for it, then executes the code so that it can be filled with the exported symbols. When auction.js does require('./adapterManager.js'), the same happens - and so on along the dependency graph, as you’d expect.
• the second time Prebid is loaded, because the registry already contains entries for adapterManager etc, code runs in a different order - it does not follow dependencies. In particular, because of a circular import, it runs auction.js before the bulk of adapterManager.js. The object returned by require('./adapterManager.js') is incomplete.
• the reason import seems to fix it is that it gets compiled in a slightly different way. Instead of:

  // with require
  var adapterManager = (__webpack_require__("./src/adapterManager.js")["default"]);
  // ....
  function callBids() {
      var bidRequests = adapterManager.makeBidRequests(_adUnits, _auctionStart, _auctionId, _timeout, _labels);  
  

  it does:

  // with import
  var _adapterManager_js__WEBPACK_IMPORTED_MODULE_4__ = __webpack_require__("./src/adapterManager.js");
  function callBids() {
     var bidRequests = _adapterManager_js__WEBPACK_IMPORTED_MODULE_4__["default"].makeBidRequests(_adUnits, 
  _auctionStart, _auctionId, _timeout, _labels);
  }
  

That is, import happens to generate a lazy dereference in this case. But not in general; these idioms would have the same problem:

import * as am from './src/adapterManager.js';
const adapterManager = am['default'];
function callBids() {
   adapterManager.makeBidRequests(...)
}

// or when there's no option to be lazy

import {someSymbol} from './someModule';
someSymbol();

The bottom line is that even if everything happens to work right now after switching to import, it could easily break (for the second load of Prebid) just by shuffling around their order, introduction of new circular imports, or refactoring of imports to forms that will appear equivalent.

Just used your file https://s3.amazonaws.com/files.prebid.org/test/dgirardi/prebid-undef-test.js on https://7buoei.vercel.app/test.html

No more errors in the console ✅

If you’re using Webpack, can you post its version and the configuration you’re using for it?

I encountered the exact same issue yesterday while attempting to upgrade from Prebid 6.8 to 6.22 and, like you, I also noticed that the issue is resolved if the require is changed to an import in auction.js.

• @dgirardi this is what we discussed via Slack yesterday