From 1035818108cacdfec2b813a5a770f483ff49253b Mon Sep 17 00:00:00 2001
From: Alec Gibson <12036746+alecgibson@users.noreply.github.com>
Date: Tue, 5 Oct 2021 09:48:59 +0100
Subject: [PATCH] =?UTF-8?q?=F0=9F=A5=85=20Throw=20a=20more=20useful=20mess?=
 =?UTF-8?q?age=20when=20subtypes=20are=20not=20invertible?=
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The `invert()` method assumes that all subtypes are invertible. However,
the OT type `invert()` method is [optional][1], so not all types will
have it implemented (`rich-text` is a notable example).

This change checks for presence of the `invert()` function, and throws
a more descriptive error if it's not present.

[1]: https://github.com/ottypes/docs#optional-properties
---
 lib/json0.js | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/lib/json0.js b/lib/json0.js
index 9f538ee..abea8ff 100644
--- a/lib/json0.js
+++ b/lib/json0.js
@@ -70,6 +70,9 @@ json.invertComponent = function(c) {
 
   // handle subtype ops
   if (c.t && subtypes[c.t]) {
+    if (typeof subtypes[c.t].invert !== 'function') {
+      throw new Error("Subtype '" + c.t + "' is not invertible");
+    }
     c_.t = c.t;
     c_.o = subtypes[c.t].invert(c.o);
   }