Fuzzing in JS VMs: Print types when we have nothing better (#3773)

This matches #3747 which makes us not log out reference values, instead
we print just their types.

This also prints a type for non-reference things, replacing a previous
exception, which affects things like SIMD and BigInts, but those trap
anyhow at the JS boundary I believe (or did that change for SIMD?).
Anyhow, printing the type won't give a false "ok" when comparing wasm2js
output to the interpreter, assuming the interpreter prints out a value and
not just a type (which is the case). We could try to do better, but this
code is on the JS side, where we don't have the type - just a string
representation of it, which we'd need to parse etc.

2 files changed, 4 insertions, 2 deletions

diff --git a/src/tools/js-wrapper.h b/src/tools/js-wrapper.h
index 3a1a1aa4a..9568bccc0 100644
--- a/src/tools/js-wrapper.h
+++ b/src/tools/js-wrapper.h
@@ -60,7 +60,8 @@ static std::string generateJSWrapper(Module& wasm) {
          "      ret += Number(x).toString();\n"
          "      break;\n"
          "    }\n"
-         "    default: throw 'what?';\n"
+         "    // For anything else, just print the type.\n"
+         "    default: ret += type; break;\n"
          "  }\n"
          "  return ret;\n"
          "}\n"
diff --git a/test/passes/emit-js-wrapper=a.js.wast.js b/test/passes/emit-js-wrapper=a.js.wast.js
index 60f2449b0..a3c0a22ee 100644
--- a/test/passes/emit-js-wrapper=a.js.wast.js
+++ b/test/passes/emit-js-wrapper=a.js.wast.js
@@ -30,7 +30,8 @@ function literal(x, type) {
       ret += Number(x).toString();
       break;
     }
-    default: throw 'what?';
+    // For anything else, just print the type.
+    default: ret += type; break;
   }
   return ret;
 }