From 9d5aa363cf3031b586e9945cf990e178f5b370db Mon Sep 17 00:00:00 2001
From: Nils Knappmeier <github@knappi.org>
Date: Mon, 13 Jan 2020 21:39:01 +0100
Subject: [PATCH] fix: don't wrap helpers that are not functions

- helpers should always be a function, but in #1639 one seems to
  be undefined. This was not a problem before 4.6 because helpers
  weren't wrapped then.
  Now, we must take care only to wrap helpers (when adding
  the "lookupProperty" function to the options), if they
  are really functions.
---
 lib/handlebars/internal/wrapHelper.js | 5 +++++
 spec/regressions.js                   | 9 +++++++++
 2 files changed, 14 insertions(+)

diff --git a/lib/handlebars/internal/wrapHelper.js b/lib/handlebars/internal/wrapHelper.js
index 0010eb8c2..29d65b033 100644
--- a/lib/handlebars/internal/wrapHelper.js
+++ b/lib/handlebars/internal/wrapHelper.js
@@ -1,4 +1,9 @@
 export function wrapHelper(helper, transformOptionsFn) {
+  if (typeof helper !== 'function') {
+    // This should not happen, but apparently it does in https://github.com/wycats/handlebars.js/issues/1639
+    // We try to make the wrapper least-invasive by not wrapping it, if the helper is not a function.
+    return helper;
+  }
   let wrapper = function(/* dynamic arguments */) {
     const options = arguments[arguments.length - 1];
     arguments[arguments.length - 1] = transformOptionsFn(options);
diff --git a/spec/regressions.js b/spec/regressions.js
index 3a84dd7da..f6147ad6e 100644
--- a/spec/regressions.js
+++ b/spec/regressions.js
@@ -518,4 +518,13 @@ describe('Regressions', function() {
       sinon.restore();
     });
   });
+
+  describe("GH-1639: TypeError: Cannot read property 'apply' of undefined\" when handlebars version > 4.6.0 (undocumented, deprecated usage)", function() {
+    it('should treat undefined helpers like non-existing helpers', function() {
+      expectTemplate('{{foo}}')
+        .withHelper('foo', undefined)
+        .withInput({ foo: 'bar' })
+        .toCompileTo('bar');
+    });
+  });
 });