Description
The legacy compatibility layer (assets/src/js/_acf-compatibility.js) only works because the production webpack bundle keeps it in non-strict mode:
maybe_get() assigns to an undeclared keys variable — a global leak; under strict mode this throws ReferenceError.add_action()'s multi-action split and Field→$el argument conversion rely on arguments aliasing, which strict mode disables — under strict mode the legacy callback wrapping silently stops working.
Any future tooling change that forces strict mode (e.g. converting the module to an ES module, or a bundler default change) breaks the acf.* legacy API used by third-party add-ons — one loudly, one silently.
Reproduction
Characterized in #450: tests/js/compatibility.test.js loads the raw source via indirect eval specifically to match production sloppy-mode semantics (see file comments).
Suggested fix
Declare var keys, capture arguments explicitly — two tiny changes that make the module strict-mode-safe without behavior change.
Found during the 2026-06 test campaign (see PR #450).
Description
The legacy compatibility layer (
assets/src/js/_acf-compatibility.js) only works because the production webpack bundle keeps it in non-strict mode:maybe_get()assigns to an undeclaredkeysvariable — a global leak; under strict mode this throwsReferenceError.add_action()'s multi-action split and Field→$elargument conversion rely onargumentsaliasing, which strict mode disables — under strict mode the legacy callback wrapping silently stops working.Any future tooling change that forces strict mode (e.g. converting the module to an ES module, or a bundler default change) breaks the
acf.*legacy API used by third-party add-ons — one loudly, one silently.Reproduction
Characterized in #450:
tests/js/compatibility.test.jsloads the raw source via indirectevalspecifically to match production sloppy-mode semantics (see file comments).Suggested fix
Declare
var keys, capture arguments explicitly — two tiny changes that make the module strict-mode-safe without behavior change.Found during the 2026-06 test campaign (see PR #450).