[New] respect Symbol.toStringTag on objects

Author: ljharb

index.js

@@ -19,8 +19,17 @@ var gOPS = Object.getOwnPropertySymbols;
 var symToString = typeof Symbol === 'function' ? Symbol.prototype.toString : null;
 var isEnumerable = Object.prototype.propertyIsEnumerable;
 
+var gPO = (typeof Reflect === 'function' ? Reflect.getPrototypeOf : Object.getPrototypeOf) || (
+    [].__proto__ === Array.prototype // eslint-disable-line no-proto
+        ? function (O) {
+            return O.__proto__; // eslint-disable-line no-proto
+        }
+        : null
+);
+
 var inspectCustom = require('./util.inspect').custom;
 var inspectSymbol = inspectCustom && isSymbol(inspectCustom) ? inspectCustom : null;
+var toStringTag = typeof Symbol === 'function' && typeof Symbol.toStringTag === 'symbol' ? Symbol.toStringTag : null;
 
 module.exports = function inspect_(obj, options, depth, seen) {
     var opts = options || {};
@@ -178,11 +187,16 @@ module.exports = function inspect_(obj, options, depth, seen) {
     }
     if (!isDate(obj) && !isRegExp(obj)) {
         var ys = arrObjKeys(obj, inspect);
-        if (ys.length === 0) { return '{}'; }
+        var isPlainObject = gPO ? gPO(obj) === Object.prototype : obj instanceof Object || obj.constructor === Object;
+        var protoTag = obj instanceof Object ? '' : 'null prototype';
+        var stringTag = !isPlainObject && toStringTag && toStringTag in obj ? toStr(obj).slice(8, -1) : protoTag ? 'Object' : '';
+        var constructorTag = isPlainObject || typeof obj.constructor !== 'function' ? '' : obj.constructor.name ? obj.constructor.name + ' ' : '';
+        var tag = constructorTag + (stringTag || protoTag ? '[' + [].concat(stringTag || [], protoTag || []).join(': ') + '] ' : '');
+        if (ys.length === 0) { return tag + '{}'; }
         if (indent) {
-            return '{' + indentedJoin(ys, indent) + '}';
+            return tag + '{' + indentedJoin(ys, indent) + '}';
         }
-        return '{ ' + ys.join(', ') + ' }';
+        return tag + '{ ' + ys.join(', ') + ' }';
     }
     return String(obj);
 };

test/toStringTag.js

@@ -0,0 +1,40 @@
+'use strict';
+
+var test = require('tape');
+var hasSymbols = require('has-symbols')();
+
+var inspect = require('..');
+
+test('Symbol.toStringTag', { skip: !hasSymbols || typeof Symbol.toStringTag !== 'symbol' }, function (t) {
+    t.plan(4);
+
+    var obj = { a: 1 };
+    t.equal(inspect(obj), '{ a: 1 }', 'object, no Symbol.toStringTag');
+
+    obj[Symbol.toStringTag] = 'foo';
+    t.equal(inspect(obj), '{ a: 1, [Symbol(Symbol.toStringTag)]: \'foo\' }', 'object with Symbol.toStringTag');
+
+    t.test('null objects', { skip: 'toString' in { __proto__: null } }, function (st) {
+        st.plan(2);
+
+        var dict = { __proto__: null, a: 1 };
+        st.equal(inspect(dict), '[Object: null prototype] { a: 1 }', 'null object with Symbol.toStringTag');
+
+        dict[Symbol.toStringTag] = 'Dict';
+        st.equal(inspect(dict), '[Dict: null prototype] { a: 1, [Symbol(Symbol.toStringTag)]: \'Dict\' }', 'null object with Symbol.toStringTag');
+    });
+
+    t.test('instances', function (st) {
+        st.plan(4);
+
+        function C() {
+            this.a = 1;
+        }
+        st.equal(Object.prototype.toString.call(new C()), '[object Object]', 'instance, no toStringTag, Object.prototype.toString');
+        st.equal(inspect(new C()), 'C { a: 1 }', 'instance, no toStringTag');
+
+        C.prototype[Symbol.toStringTag] = 'Class!';
+        st.equal(Object.prototype.toString.call(new C()), '[object Class!]', 'instance, with toStringTag, Object.prototype.toString');
+        st.equal(inspect(new C()), 'C [Class!] { a: 1 }', 'instance, with toStringTag');
+    });
+});