util: use a shared symbol for util.inspect.custom

Define `util.inspect.custom` as
`Symbol.for("nodejs.util.inspect.custom")` rather than
`Symbol("util.inspect.custom")`. This allows `inspect` hooks to
easily/safely be defined in non-Node.js environments.

Fixes: https://github.com/nodejs/node/issues/20821
Refs: https://github.com/nodejs/node/pull/22684

PR-URL: https://github.com/nodejs/node/pull/20857
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: John-David Dalton <john.david.dalton@gmail.com>

doc/api/util.md

@@ -574,9 +574,10 @@ terminals.
 
 <!-- type=misc -->
 
-Objects may also define their own `[util.inspect.custom](depth, opts)` function
-that `util.inspect()` will invoke and use the result of when inspecting the
-object:
+Objects may also define their own
+[`[util.inspect.custom](depth, opts)`][util.inspect.custom] function,
+which `util.inspect()` will invoke and use the result of when inspecting
+the object:
 
 ```js
 const util = require('util');
@@ -628,10 +629,41 @@ util.inspect(obj);
 ### util.inspect.custom
 <!-- YAML
 added: v6.6.0
+changes:
+  - version: REPLACEME
+    pr-url: https://github.com/nodejs/node/pull/20857
+    description: This is now defined as a shared symbol.
 -->
 
-* {symbol} that can be used to declare custom inspect functions, see
-[Custom inspection functions on Objects][].
+* {symbol} that can be used to declare custom inspect functions.
+
+In addition to being accessible through `util.inspect.custom`, this
+symbol is [registered globally][global symbol registry] and can be
+accessed in any environment as `Symbol.for('nodejs.util.inspect.custom')`.
+
+```js
+const inspect = Symbol.for('nodejs.util.inspect.custom');
+
+class Password {
+  constructor(value) {
+    this.value = value;
+  }
+
+  toString() {
+    return 'xxxxxxxx';
+  }
+
+  [inspect]() {
+    return `Password <${this.toString()}>`;
+  }
+}
+
+const password = new Password('r0sebud');
+console.log(password);
+// Prints Password <xxxxxxxx>
+```
+
+See [Custom inspection functions on Objects][] for more details.
 
 ### util.inspect.defaultOptions
 <!-- YAML
@@ -2076,7 +2108,6 @@ Deprecated predecessor of `console.log`.
 [`Array.isArray()`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/isArray
 [`ArrayBuffer`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/ArrayBuffer
 [`ArrayBuffer.isView()`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/ArrayBuffer/isView
-[async function]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/async_function
 [`assert.deepStrictEqual()`]: assert.html#assert_assert_deepstrictequal_actual_expected_message
 [`Buffer.isBuffer()`]: buffer.html#buffer_class_method_buffer_isbuffer_obj
 [`console.error()`]: console.html#console_console_error_data_args
@@ -2118,6 +2149,9 @@ Deprecated predecessor of `console.log`.
 [Module Namespace Object]: https://tc39.github.io/ecma262/#sec-module-namespace-exotic-objects
 [WHATWG Encoding Standard]: https://encoding.spec.whatwg.org/
 [Common System Errors]: errors.html#errors_common_system_errors
+[async function]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/async_function
 [constructor]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/constructor
+[global symbol registry]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol/for
 [list of deprecated APIS]: deprecations.html#deprecations_list_of_deprecated_apis
 [semantically incompatible]: https://github.com/nodejs/node/issues/4179
+[util.inspect.custom]: #util_util_inspect_custom

lib/internal/util.js

@@ -400,7 +400,7 @@ module.exports = {
 
   // Symbol used to provide a custom inspect function for an object as an
   // alternative to using 'inspect'
-  customInspectSymbol: Symbol('util.inspect.custom'),
+  customInspectSymbol: Symbol.for('nodejs.util.inspect.custom'),
 
   // Used by the buffer module to capture an internal reference to the
   // default isEncoding implementation, just in case userland overrides it.

test/parallel/test-assert.js

@@ -603,7 +603,7 @@ assert.throws(
     '+ {}\n' +
     '- {\n' +
     "-   loop: 'forever',\n" +
-    '-   [Symbol(util.inspect.custom)]: [Function]\n' +
+    '-   [Symbol(nodejs.util.inspect.custom)]: [Function]\n' +
     '- }'
   });
 

test/parallel/test-console.js

@@ -214,11 +214,11 @@ for (const expected of expectedStrings) {
 }
 
 assert.strictEqual(strings.shift(),
-                   "{ foo: 'bar',\n  [Symbol(util.inspect.custom)]: " +
-                    '[Function: [util.inspect.custom]] }\n');
+                   "{ foo: 'bar',\n  [Symbol(nodejs.util.inspect.custom)]: " +
+                    '[Function: [nodejs.util.inspect.custom]] }\n');
 assert.strictEqual(strings.shift(),
-                   "{ foo: 'bar',\n  [Symbol(util.inspect.custom)]: " +
-                    '[Function: [util.inspect.custom]] }\n');
+                   "{ foo: 'bar',\n  [Symbol(nodejs.util.inspect.custom)]: " +
+                    '[Function: [nodejs.util.inspect.custom]] }\n');
 assert.ok(strings.shift().includes('foo: [Object]'));
 assert.strictEqual(strings.shift().includes('baz'), false);
 assert.strictEqual(strings.shift(), 'inspect inspect\n');

test/parallel/test-util-inspect.js

@@ -627,7 +627,7 @@ assert.strictEqual(util.inspect(Object.create(Date.prototype)), 'Date {}');
 {
   const x = { [util.inspect.custom]: util.inspect };
   assert(util.inspect(x).includes(
-    '[Symbol(util.inspect.custom)]:\n   { [Function: inspect]'));
+    '[Symbol(nodejs.util.inspect.custom)]:\n   { [Function: inspect]'));
 }
 
 // `util.inspect` should display the escaped value of a key.
@@ -758,12 +758,26 @@ util.inspect({ hasOwnProperty: null });
   };
 
   util.inspect(subject, { customInspectOptions: true });
+
+  // util.inspect.custom is a shared symbol which can be accessed as
+  // Symbol.for("nodejs.util.inspect.custom").
+  const inspect = Symbol.for('nodejs.util.inspect.custom');
+
+  subject[inspect] = () => ({ baz: 'quux' });
+
+  assert.strictEqual(util.inspect(subject), '{ baz: \'quux\' }');
+
+  subject[inspect] = (depth, opts) => {
+    assert.strictEqual(opts.customInspectOptions, true);
+  };
+
+  util.inspect(subject, { customInspectOptions: true });
 }
 
 {
   // Returning `this` from a custom inspection function works.
   const subject = { a: 123, [util.inspect.custom]() { return this; } };
-  const UIC = 'util.inspect.custom';
+  const UIC = 'nodejs.util.inspect.custom';
   assert.strictEqual(util.inspect(subject),
                      `{ a: 123,\n  [Symbol(${UIC})]: [Function: [${UIC}]] }`);
 }
@@ -1188,7 +1202,7 @@ util.inspect(process);
   const obj = { [util.inspect.custom]: 'fhqwhgads' };
   assert.strictEqual(
     util.inspect(obj),
-    "{ [Symbol(util.inspect.custom)]: 'fhqwhgads' }"
+    "{ [Symbol(nodejs.util.inspect.custom)]: 'fhqwhgads' }"
   );
 }