Summary

Select2 4.1.0 throws when loading AJAX results:

TypeError: Cannot read properties of undefined (reading 'container')
    at SelectAdapter._normalizeItem (…)

The results dropdown stays on “Searching…” and never renders options.

This does not occur on 4.0.13 (that release does not normalize AJAX results in ajax.js).

Cause

#6241 added AJAX result normalization in src/js/select2/data/ajax.js:

results.results = results.results.map(
  AjaxAdapter.prototype._normalizeItem
);

SelectAdapter.prototype._normalizeItem uses this.container (and this.generateResultId). Passing the method unbound to Array#map leaves this as undefined when _normalizeItem is a strict-mode function (typical after ESM bundling with Vite/Webpack/Rollup).

#6239 added the same unbound pattern for optgroup children in select.js.

Reproduction

Minimal Vite repro (recommended): https://gist.github.com/madejejej/8c73bc3159e097027447a10616a52284

# clone gist files into vite-repro/ or download from gist
cd vite-repro
npm install
npm run build && npm run preview
# open http://127.0.0.1:4173

Expected output in <pre id="log">:

direct normalize(): Cannot read properties of undefined (reading 'container')
Array#map(unbound): Cannot read properties of undefined (reading 'container')

Upstream diff: 4.0.13 → 4.1.0 only adds the results.results.map(…_normalizeItem) block in ajax.js (plus Array.isArray / abort guard tweaks).

Note: Loading select2.full.min.js via a classic non-module <script> may mask the bug (sloppy this / window). Modern apps import select2 as an npm package through bundlers.

Suggested fix

Bind the adapter instance in AjaxAdapter.prototype.query:

results.results = results.results.map(function (item) {
  return self._normalizeItem(item);
});

Mirror for optgroup children in _normalizeItem:

item.children = item.children.map(function (child) {
  return this._normalizeItem(child);
}, this);

Context

• Introduced by Fixup: unselection of items from AJAX data sources with non-string identifiers #6241 (resolves Cannot unselect by clicking selected when using dataAdapter #6128) — milestone 4.1.0
• Related: Fixup: convert optgroup child data identifiers to string datatype #6239 (optgroup children)
• Not documented in the 4.1.0 changelog as an AJAX behavior change

Proposed test

See proposed-ajax-tests.js in the gist for a QUnit case that exercises AjaxAdapter#query with a synchronous transport.