lib: make the global console [[Prototype]] an empty object

From the WHATWG console spec: > For historical web-compatibility reasons, the namespace object for > console must have as its [[Prototype]] an empty object, created as > if by ObjectCreate(%ObjectPrototype%), instead of %ObjectPrototype%. Since in Node.js, the Console constructor has been exposed through require('console'), we need to keep the Console constructor but we cannot actually use `new Console` to construct the global console. This patch changes the prototype chain of the global console object, so the console.Console.prototype is not in the global console prototype chain anymore. ``` const proto = Object.getPrototypeOf(global.console); // Before this patch proto.constructor === global.console.Console // After this patch proto.constructor === Object ``` But, we still maintain that ``` global.console instanceof global.console.Console ``` through a custom Symbol.hasInstance function of Console that tests for a special symbol kIsConsole for backwards compatibility. This fixes a case in the console Web Platform Test that we commented out. Refs: https://console.spec.whatwg.org/#console-namespace Refs: whatwg/console#3
nodejs · joyeecheung · Oct 12, 2018 · Oct 25, 2018 · 5556e47aaf1c5b07e0d791e17e3afbcdf926c5ce
commit 5556e47aaf1c5b07e0d791e17e3afbcdf926c5ce
diff --git a/lib/console.js b/lib/console.js
@@ -60,17 +60,21 @@ let cliTable;
 
 // Track amount of indentation required via `console.group()`.
 const kGroupIndent = Symbol('kGroupIndent');
-
 const kFormatForStderr = Symbol('kFormatForStderr');
 const kFormatForStdout = Symbol('kFormatForStdout');
 const kGetInspectOptions = Symbol('kGetInspectOptions');
 const kColorMode = Symbol('kColorMode');
+const kIsConsole = Symbol('kIsConsole');
 
 function Console(options /* or: stdout, stderr, ignoreErrors = true */) {
-  if (!(this instanceof Console)) {
+  // We have to test new.target here to see if this function is called
+  // with new, because we need to define a custom instanceof to accommodate
+  // the global console.
+  if (!new.target) {
     return new Console(...arguments);
   }
 
+  this[kIsConsole] = true;
   if (!options || typeof options.write === 'function') {
     options = {
       stdout: options,
@@ -125,7 +129,7 @@ function Console(options /* or: stdout, stderr, ignoreErrors = true */) {
   var keys = Object.keys(Console.prototype);
   for (var v = 0; v < keys.length; v++) {
     var k = keys[v];
-    this[k] = this[k].bind(this);
+    this[k] = Console.prototype[k].bind(this);
   }
 }
 
@@ -465,10 +469,50 @@ Console.prototype.table = function(tabularData, properties) {
   return final(keys, values);
 };
 
-module.exports = new Console({
+function noop() {}
+
+// See https://console.spec.whatwg.org/#console-namespace
+// > For historical web-compatibility reasons, the namespace object
+// > for console must have as its [[Prototype]] an empty object,
+// > created as if by ObjectCreate(%ObjectPrototype%),
+// > instead of %ObjectPrototype%.
+
+// Since in Node.js, the Console constructor has been exposed through
+// require('console'), we need to keep the Console constructor but
+// we cannot actually use `new Console` to construct the global console.
+// Therefore, the console.Console.prototype is not
+// in the global console prototype chain anymore.
+const globalConsole = Object.create({});
+const tempConsole = new Console({
   stdout: process.stdout,
   stderr: process.stderr
 });
-module.exports.Console = Console;
 
-function noop() {}
+// Since Console is not on the prototype chain of the global console,
+// the symbol properties on Console.prototype have to be looked up from
+// the global console itself.
+for (const prop of Object.getOwnPropertySymbols(Console.prototype)) {
+  globalConsole[prop] = Console.prototype[prop];
+}
+
+// Reflect.ownKeys() is used here for retrieving Symbols
+for (const prop of Reflect.ownKeys(tempConsole)) {
+  const desc = { ...(Reflect.getOwnPropertyDescriptor(tempConsole, prop)) };
+  // Since Console would bind method calls onto the instance,
+  // make sure the methods are called on globalConsole instead of
+  // tempConsole.
+  if (typeof Console.prototype[prop] === 'function') {
+    desc.value = Console.prototype[prop].bind(globalConsole);
+  }
+  Reflect.defineProperty(globalConsole, prop, desc);
+}
+
+globalConsole.Console = Console;
+
+Object.defineProperty(Console, Symbol.hasInstance, {
+  value(instance) {
+    return instance[kIsConsole];
+  }
+});
+
+module.exports = globalConsole;
diff --git a/test/parallel/test-console-instance.js b/test/parallel/test-console-instance.js
@@ -23,7 +23,8 @@
 const common = require('../common');
 const assert = require('assert');
 const Stream = require('stream');
-const Console = require('console').Console;
+const requiredConsole = require('console');
+const Console = requiredConsole.Console;
 
 const out = new Stream();
 const err = new Stream();
@@ -35,6 +36,11 @@ process.stdout.write = process.stderr.write = common.mustNotCall();
 // Make sure that the "Console" function exists.
 assert.strictEqual('function', typeof Console);
 
+assert.strictEqual(requiredConsole, global.console);
+// Make sure the custom instanceof of Console works
+assert.ok(global.console instanceof Console);
+assert.ok(!({} instanceof Console));
+
 // Make sure that the Console constructor throws
 // when not given a writable stream instance.
 common.expectsError(
@@ -62,46 +68,57 @@ common.expectsError(
 
 out.write = err.write = (d) => {};
 
-const c = new Console(out, err);
+{
+  const c = new Console(out, err);
+  assert.ok(c instanceof Console);
 
-out.write = err.write = common.mustCall((d) => {
-  assert.strictEqual(d, 'test\n');
-}, 2);
+  out.write = err.write = common.mustCall((d) => {
+    assert.strictEqual(d, 'test\n');
+  }, 2);
 
-c.log('test');
-c.error('test');
+  c.log('test');
+  c.error('test');
 
-out.write = common.mustCall((d) => {
-  assert.strictEqual(d, '{ foo: 1 }\n');
-});
+  out.write = common.mustCall((d) => {
+    assert.strictEqual(d, '{ foo: 1 }\n');
+  });
 
-c.dir({ foo: 1 });
+  c.dir({ foo: 1 });
 
-// Ensure that the console functions are bound to the console instance.
-let called = 0;
-out.write = common.mustCall((d) => {
-  called++;
-  assert.strictEqual(d, `${called} ${called - 1} [ 1, 2, 3 ]\n`);
-}, 3);
+  // Ensure that the console functions are bound to the console instance.
+  let called = 0;
+  out.write = common.mustCall((d) => {
+    called++;
+    assert.strictEqual(d, `${called} ${called - 1} [ 1, 2, 3 ]\n`);
+  }, 3);
 
-[1, 2, 3].forEach(c.log);
+  [1, 2, 3].forEach(c.log);
+}
 
-// Console() detects if it is called without `new` keyword.
-Console(out, err);
+// Test calling Console without the `new` keyword.
+{
+  const withoutNew = Console(out, err);
+  assert.ok(withoutNew instanceof Console);
+}
 
-// Extending Console works.
-class MyConsole extends Console {
-  hello() {}
+// Test extending Console
+{
+  class MyConsole extends Console {
+    hello() {}
+  }
+  const myConsole = new MyConsole(process.stdout);
+  assert.strictEqual(typeof myConsole.hello, 'function');
+  assert.ok(myConsole instanceof Console);
 }
-const myConsole = new MyConsole(process.stdout);
-assert.strictEqual(typeof myConsole.hello, 'function');
 
 // Instance that does not ignore the stream errors.
-const c2 = new Console(out, err, false);
+{
+  const c2 = new Console(out, err, false);
 
-out.write = () => { throw new Error('out'); };
-err.write = () => { throw new Error('err'); };
+  out.write = () => { throw new Error('out'); };
+  err.write = () => { throw new Error('err'); };
 
-assert.throws(() => c2.log('foo'), /^Error: out$/);
-assert.throws(() => c2.warn('foo'), /^Error: err$/);
-assert.throws(() => c2.dir('foo'), /^Error: out$/);
+  assert.throws(() => c2.log('foo'), /^Error: out$/);
+  assert.throws(() => c2.warn('foo'), /^Error: err$/);
+  assert.throws(() => c2.dir('foo'), /^Error: out$/);
+}
diff --git a/test/parallel/test-whatwg-console-is-a-namespace.js b/test/parallel/test-whatwg-console-is-a-namespace.js
@@ -38,9 +38,7 @@ test(() => {
   const prototype1 = Object.getPrototypeOf(console);
   const prototype2 = Object.getPrototypeOf(prototype1);
 
-  // This got commented out from the original test because in Node.js all
-  // functions are declared on the prototype.
-  // assert_equals(Object.getOwnPropertyNames(prototype1).length, 0, "The [[Prototype]] must have no properties");
+  assert_equals(Object.getOwnPropertyNames(prototype1).length, 0, "The [[Prototype]] must have no properties");
   assert_equals(prototype2, Object.prototype, "The [[Prototype]]'s [[Prototype]] must be %ObjectPrototype%");
 }, "The prototype chain must be correct");