Handle unprintable objects better in print() (#834)

Before this commit, such objects were printed as `<exception>` because
print() and console.log() had no good way to turn them into strings.

Instead perform the C equivalent of Object.prototype.toString.call(o)
and print the result (which can still error but at least we tried.)

Fixes: #832

Author: bnoordhuis

quickjs-libc.c

@@ -3984,12 +3984,20 @@ static JSValue js_print(JSContext *ctx, JSValue this_val,
     DWORD mode;
 #endif
     const char *s;
+    JSValue v;
     DynBuf b;
     int i;
 
     dbuf_init(&b);
     for(i = 0; i < argc; i++) {
-        s = JS_ToCString(ctx, argv[i]);
+        v = argv[i];
+        s = JS_ToCString(ctx, v);
+        if (!s && JS_IsObject(v)) {
+            JS_FreeValue(ctx, JS_GetException(ctx));
+            v = JS_ToObjectString(ctx, v);
+            s = JS_ToCString(ctx, v);
+            JS_FreeValue(ctx, v);
+        }
         if (s) {
             dbuf_printf(&b, "%s%s", &" "[!i], s);
             JS_FreeCString(ctx, s);

quickjs.c

@@ -37079,6 +37079,11 @@ static JSValue js_object_toString(JSContext *ctx, JSValue this_val,
     return JS_ConcatString3(ctx, "[object ", tag, "]");
 }
 
+JSValue JS_ToObjectString(JSContext *ctx, JSValue val)
+{
+    return js_object_toString(ctx, val, 0, NULL);
+}
+
 static JSValue js_object_toLocaleString(JSContext *ctx, JSValue this_val,
                                         int argc, JSValue *argv)
 {

quickjs.h

@@ -694,6 +694,7 @@ JS_EXTERN JSValue JS_NewObjectClass(JSContext *ctx, int class_id);
 JS_EXTERN JSValue JS_NewObjectProto(JSContext *ctx, JSValue proto);
 JS_EXTERN JSValue JS_NewObject(JSContext *ctx);
 JS_EXTERN JSValue JS_ToObject(JSContext *ctx, JSValue val);
+JS_EXTERN JSValue JS_ToObjectString(JSContext *ctx, JSValue val);
 
 JS_EXTERN bool JS_IsFunction(JSContext* ctx, JSValue val);
 JS_EXTERN bool JS_IsConstructor(JSContext* ctx, JSValue val);

run-test262.c

@@ -463,26 +463,35 @@ static JSValue js_print_262(JSContext *ctx, JSValue this_val,
                         int argc, JSValue *argv)
 {
     ThreadLocalStorage *tls = JS_GetRuntimeOpaque(JS_GetRuntime(ctx));
+    const char *s;
+    JSValue v;
     int i;
-    const char *str;
 
     for (i = 0; i < argc; i++) {
-        str = JS_ToCString(ctx, argv[i]);
-        if (!str)
+        v = argv[i];
+        s = JS_ToCString(ctx, v);
+        // same logic as js_print in quickjs-libc.c
+        if (local && !s && JS_IsObject(v)) {
+            JS_FreeValue(ctx, JS_GetException(ctx));
+            v = JS_ToObjectString(ctx, v);
+            s = JS_ToCString(ctx, v);
+            JS_FreeValue(ctx, v);
+        }
+        if (!s)
             return JS_EXCEPTION;
-        if (!strcmp(str, "Test262:AsyncTestComplete")) {
+        if (!strcmp(s, "Test262:AsyncTestComplete")) {
             tls->async_done++;
-        } else if (js__strstart(str, "Test262:AsyncTestFailure", NULL)) {
+        } else if (js__strstart(s, "Test262:AsyncTestFailure", NULL)) {
             tls->async_done = 2; /* force an error */
         }
         if (outfile) {
             if (i != 0)
                 fputc(' ', outfile);
-            fputs(str, outfile);
+            fputs(s, outfile);
         }
         if (verbose > 1)
-            printf("%s%s", &" "[i < 1], str);
-        JS_FreeCString(ctx, str);
+            printf("%s%s", &" "[i < 1], s);
+        JS_FreeCString(ctx, s);
     }
     if (outfile)
         fputc('\n', outfile);

tests.conf

@@ -4,6 +4,7 @@ verbose=yes
 testdir=tests
 
 [exclude]
+tests/empty.js
 tests/fixture_cyclic_import.js
 tests/microbench.js
 tests/test_worker_module.js

tests/bug832.js

@@ -0,0 +1,2 @@
+const m = await import("./empty.js")
+print(m) // should not throw