util: allow symbol-based custom inspection methods

Add a `util.inspect.custom` Symbol which can be used to customize
`util.inspect()` output. Providing `obj[util.inspect.custom]`
works like providing `obj.inspect`, except that the former allows
avoiding name clashes with other `inspect()` methods.

Fixes: #8071
PR-URL: #8174
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>

doc/api/util.md

@@ -275,32 +275,48 @@ The predefined color codes are: `white`, `grey`, `black`, `blue`, `cyan`,
 Color styling uses ANSI control codes that may not be supported on all
 terminals.
 
-### Custom `inspect()` function on Objects
+### Custom inspection functions on Objects
 
 <!-- type=misc -->
 
-Objects may also define their own `inspect(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)`
+(or, equivalently `inspect(depth, opts)`) function that `util.inspect()` will
+invoke and use the result of when inspecting the object:
 
 ```js
 const util = require('util');
 
 const obj = { name: 'nate' };
-obj.inspect = function(depth) {
+obj[util.inspect.custom] = function(depth) {
   return `{${this.name}}`;
 };
 
 util.inspect(obj);
   // "{nate}"
 ```
 
-Custom `inspect(depth, opts)` functions typically return a string but may
-return a value of any type that will be formatted accordingly by
+Custom `[util.inspect.custom](depth, opts)` functions typically return a string
+but may return a value of any type that will be formatted accordingly by
 `util.inspect()`.
 
 ```js
 const util = require('util');
 
+const obj = { foo: 'this will not show up in the inspect() output' };
+obj[util.inspect.custom] = function(depth) {
+  return { bar: 'baz' };
+};
+
+util.inspect(obj);
+  // "{ bar: 'baz' }"
+```
+
+A custom inspection method can alternatively be provided by exposing
+an `inspect(depth, opts)` method on the object:
+
+```js
+const util = require('util');
+
 const obj = { foo: 'this will not show up in the inspect() output' };
 obj.inspect = function(depth) {
   return { bar: 'baz' };
@@ -330,6 +346,14 @@ util.inspect.defaultOptions.maxArrayLength = null;
 console.log(arr); // logs the full array
 ```
 
+### util.inspect.custom
+<!-- YAML
+added: REPLACEME
+-->
+
+A Symbol that can be used to declare custom inspect functions, see
+[Custom inspection functions on Objects][].
+
 ## Deprecated APIs
 
 The following APIs have been deprecated and should no longer be used. Existing
@@ -807,6 +831,7 @@ similar built-in functionality through [`Object.assign()`].
 [semantically incompatible]: https://github.com/nodejs/node/issues/4179
 [`util.inspect()`]: #util_util_inspect_object_options
 [Customizing `util.inspect` colors]: #util_customizing_util_inspect_colors
+[Custom inspection functions on Objects]: #util_custom_inspection_functions_on_objects
 [`Error`]: errors.html#errors_class_error
 [`console.log()`]: console.html#console_console_log_data
 [`console.error()`]: console.html#console_console_error_data

lib/buffer.js

@@ -500,8 +500,8 @@ Buffer.prototype.equals = function equals(b) {
 };
 
 
-// Inspect
-Buffer.prototype.inspect = function inspect() {
+// Override how buffers are presented by util.inspect().
+Buffer.prototype[internalUtil.inspectSymbol] = function inspect() {
   var str = '';
   var max = exports.INSPECT_MAX_BYTES;
   if (this.length > 0) {
@@ -511,6 +511,7 @@ Buffer.prototype.inspect = function inspect() {
   }
   return '<' + this.constructor.name + ' ' + str + '>';
 };
+Buffer.prototype.inspect = Buffer.prototype[internalUtil.inspectSymbol];
 
 Buffer.prototype.compare = function compare(target,
                                             start,

lib/internal/util.js

@@ -9,6 +9,10 @@ const kDecoratedPrivateSymbolIndex = binding['decorated_private_symbol'];
 exports.getHiddenValue = binding.getHiddenValue;
 exports.setHiddenValue = binding.setHiddenValue;
 
+// The `buffer` module uses this. Defining it here instead of in the public
+// `util` module makes it accessible without having to `require('util')` there.
+exports.customInspectSymbol = Symbol('util.inspect.custom');
+
 // All the internal deprecations have to use this function only, as this will
 // prepend the prefix to the actual message.
 exports.deprecate = function(fn, msg) {

lib/util.js

@@ -242,7 +242,10 @@ inspect.styles = {
   'regexp': 'red'
 };
 
+const customInspectSymbol = internalUtil.customInspectSymbol;
+
 exports.inspect = inspect;
+exports.inspect.custom = customInspectSymbol;
 
 function stylizeWithColor(str, styleType) {
   var style = inspect.styles[styleType];
@@ -350,18 +353,20 @@ function formatValue(ctx, value, recurseTimes) {
 
   // Provide a hook for user-specified inspect functions.
   // Check that value is an object with an inspect function on it
-  if (ctx.customInspect &&
-      value &&
-      typeof value.inspect === 'function' &&
-      // Filter out the util module, it's inspect function is special
-      value.inspect !== exports.inspect &&
-      // Also filter out any prototype objects using the circular check.
-      !(value.constructor && value.constructor.prototype === value)) {
-    var ret = value.inspect(recurseTimes, ctx);
-    if (typeof ret !== 'string') {
-      ret = formatValue(ctx, ret, recurseTimes);
+  if (ctx.customInspect && value) {
+    const maybeCustomInspect = value[customInspectSymbol] || value.inspect;
+
+    if (typeof maybeCustomInspect === 'function' &&
+        // Filter out the util module, its inspect function is special
+        maybeCustomInspect !== exports.inspect &&
+        // Also filter out any prototype objects using the circular check.
+        !(value.constructor && value.constructor.prototype === value)) {
+      let ret = maybeCustomInspect.call(value, recurseTimes, ctx);
+      if (typeof ret !== 'string') {
+        ret = formatValue(ctx, ret, recurseTimes);
+      }
+      return ret;
     }
-    return ret;
   }
 
   // Primitive types cannot have properties

test/parallel/test-util-inspect.js

@@ -451,7 +451,7 @@ assert.doesNotThrow(function() {
 
 // new API, accepts an "options" object
 {
-  let subject = { foo: 'bar', hello: 31, a: { b: { c: { d: 0 } } } };
+  const subject = { foo: 'bar', hello: 31, a: { b: { c: { d: 0 } } } };
   Object.defineProperty(subject, 'hidden', { enumerable: false, value: null });
 
   assert.strictEqual(
@@ -482,9 +482,11 @@ assert.doesNotThrow(function() {
     util.inspect(subject, { depth: null }).includes('{ d: 0 }'),
     true
   );
+}
 
+{
   // "customInspect" option can enable/disable calling inspect() on objects
-  subject = { inspect: function() { return 123; } };
+  const subject = { inspect: function() { return 123; } };
 
   assert.strictEqual(
     util.inspect(subject, { customInspect: true }).includes('123'),
@@ -515,6 +517,56 @@ assert.doesNotThrow(function() {
   util.inspect(subject, { customInspectOptions: true });
 }
 
+{
+  // "customInspect" option can enable/disable calling [util.inspect.custom]()
+  const subject = { [util.inspect.custom]: function() { return 123; } };
+
+  assert.strictEqual(
+    util.inspect(subject, { customInspect: true }).includes('123'),
+    true
+  );
+  assert.strictEqual(
+    util.inspect(subject, { customInspect: false }).includes('123'),
+    false
+  );
+
+  // a custom [util.inspect.custom]() should be able to return other Objects
+  subject[util.inspect.custom] = function() { return { foo: 'bar' }; };
+
+  assert.strictEqual(util.inspect(subject), '{ foo: \'bar\' }');
+
+  subject[util.inspect.custom] = function(depth, opts) {
+    assert.strictEqual(opts.customInspectOptions, true);
+  };
+
+  util.inspect(subject, { customInspectOptions: true });
+}
+
+{
+  // [util.inspect.custom] takes precedence over inspect
+  const subject = {
+    [util.inspect.custom]() { return 123; },
+    inspect() { return 456; }
+  };
+
+  assert.strictEqual(
+    util.inspect(subject, { customInspect: true }).includes('123'),
+    true
+  );
+  assert.strictEqual(
+    util.inspect(subject, { customInspect: false }).includes('123'),
+    false
+  );
+  assert.strictEqual(
+    util.inspect(subject, { customInspect: true }).includes('456'),
+    false
+  );
+  assert.strictEqual(
+    util.inspect(subject, { customInspect: false }).includes('456'),
+    false
+  );
+}
+
 // util.inspect with "colors" option should produce as many lines as without it
 function test_lines(input) {
   var count_lines = function(str) {