Bug 1879018 - Use replacer function to avoid calling JSON.stringify o…

…n XPCOM objects in Assert.sys.mjs. r=jmaher

This is a better way to fix the problem than my previous attempt.

Differential Revision: https://phabricator.services.mozilla.com/D200908

testing/modules/Assert.sys.mjs

@@ -57,6 +57,13 @@ function replacer(key, value) {
   if (typeof value === "function" || instanceOf(value, "RegExp")) {
     return value.toString();
   }
+  if (
+    typeof value === "object" &&
+    value !== null &&
+    "QueryInterface" in value
+  ) {
+    return value.toString();
+  }
   return value;
 }
 
@@ -74,25 +81,11 @@ function getMessage(error, prefix = "") {
   // Wrap calls to JSON.stringify in try...catch blocks, as they may throw. If
   // so, fall back to toString().
   try {
-    if (
-      typeof error.actual == "object" &&
-      error.actual !== null &&
-      "QueryInterface" in error.actual
-    ) {
-      throw new Error("don't attempt to stringify this XPCOM object");
-    }
     actual = JSON.stringify(error.actual, replacer);
   } catch (ex) {
     actual = Object.prototype.toString.call(error.actual);
   }
   try {
-    if (
-      typeof error.expected == "object" &&
-      error.expected !== null &&
-      "QueryInterface" in error.expected
-    ) {
-      throw new Error("don't attempt to stringify this XPCOM object");
-    }
     expected = JSON.stringify(error.expected, replacer);
   } catch (ex) {
     expected = Object.prototype.toString.call(error.expected);