util: pass through the inspect function to custom inspect functions

This allows to use more portable custom inspect functions.

Fixes: #35956

Signed-off-by: Ruben Bridgewater <ruben@bridgewater.de>

PR-URL: #41019
Reviewed-By: Michaël Zasso <targos@protonmail.com>

doc/api/util.md

@@ -577,7 +577,7 @@ changes:
     codes. Colors are customizable. See [Customizing `util.inspect` colors][].
     **Default:** `false`.
   * `customInspect` {boolean} If `false`,
-    `[util.inspect.custom](depth, opts)` functions are not invoked.
+    `[util.inspect.custom](depth, opts, inspect)` functions are not invoked.
     **Default:** `true`.
   * `showProxy` {boolean} If `true`, `Proxy` inspection includes
     the [`target` and `handler`][] objects. **Default:** `false`.
@@ -872,10 +872,18 @@ ignored, if not supported.
 
 <!-- type=misc -->
 
+<!-- YAML
+added: v0.1.97
+changes:
+  - version: REPLACEME
+    pr-url: https://github.com/nodejs/node/pull/41019
+    description: The inspect argument is added for more interoperability.
+-->
+
 Objects may also define their own
-[`[util.inspect.custom](depth, opts)`][util.inspect.custom] function,
+[`[util.inspect.custom](depth, opts, inspect)`][util.inspect.custom] function,
 which `util.inspect()` will invoke and use the result of when inspecting
-the object:
+the object.
 
 ```js
 const util = require('util');
@@ -885,7 +893,7 @@ class Box {
     this.value = value;
   }
 
-  [util.inspect.custom](depth, options) {
+  [util.inspect.custom](depth, options, inspect) {
     if (depth < 0) {
       return options.stylize('[Box]', 'special');
     }
@@ -896,8 +904,8 @@ class Box {
 
     // Five space padding because that's the size of "Box< ".
     const padding = ' '.repeat(5);
-    const inner = util.inspect(this.value, newOptions)
-                      .replace(/\n/g, `\n${padding}`);
+    const inner = inspect(this.value, newOptions)
+                  .replace(/\n/g, `\n${padding}`);
     return `${options.stylize('Box', 'special')}< ${inner} >`;
   }
 }
@@ -908,9 +916,9 @@ util.inspect(box);
 // Returns: "Box< true >"
 ```
 
-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()`.
+Custom `[util.inspect.custom](depth, opts, inspect)` 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');
@@ -940,8 +948,13 @@ 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')`.
 
+Using this allows code to be written in a portable fashion, so that the custom
+inspect function is used in an Node.js environment and ignored in the browser.
+The `util.inspect()` function itself is passed as third argument to the custom
+inspect function to allow further portability.
+
 ```js
-const inspect = Symbol.for('nodejs.util.inspect.custom');
+const customInspectSymbol = Symbol.for('nodejs.util.inspect.custom');
 
 class Password {
   constructor(value) {
@@ -952,7 +965,7 @@ class Password {
     return 'xxxxxxxx';
   }
 
-  [inspect]() {
+  [customInspectSymbol](depth, inspectOptions, inspect) {
     return `Password <${this.toString()}>`;
   }
 }

lib/internal/util/inspect.js

@@ -778,7 +778,12 @@ function formatValue(ctx, value, recurseTimes, typedArray) {
       const isCrossContext =
         proxy !== undefined || !(context instanceof Object);
       const ret = FunctionPrototypeCall(
-        maybeCustom, context, depth, getUserOptions(ctx, isCrossContext));
+        maybeCustom,
+        context,
+        depth,
+        getUserOptions(ctx, isCrossContext),
+        inspect
+      );
       // If the custom inspection method returned `this`, don't go into
       // infinite recursion.
       if (ret !== context) {
@@ -1143,7 +1148,7 @@ function getClassBase(value, constructor, tag) {
 
 function getFunctionBase(value, constructor, tag) {
   const stringified = FunctionPrototypeToString(value);
-  if (stringified.slice(0, 5) === 'class' && stringified.endsWith('}')) {
+  if (stringified.startsWith('class') && stringified.endsWith('}')) {
     const slice = stringified.slice(5, -1);
     const bracketIndex = slice.indexOf('{');
     if (bracketIndex !== -1 &&

test/parallel/test-util-inspect.js

@@ -974,7 +974,7 @@ util.inspect({ hasOwnProperty: null });
 
   assert.strictEqual(util.inspect(subject), "{ foo: 'bar' }");
 
-  subject[util.inspect.custom] = common.mustCall((depth, opts) => {
+  subject[util.inspect.custom] = common.mustCall((depth, opts, inspect) => {
     const clone = { ...opts };
     // This might change at some point but for now we keep the stylize function.
     // The function should either be documented or an alternative should be
@@ -984,12 +984,13 @@ util.inspect({ hasOwnProperty: null });
     assert.strictEqual(opts.budget, undefined);
     assert.strictEqual(opts.indentationLvl, undefined);
     assert.strictEqual(opts.showHidden, false);
+    assert.strictEqual(inspect, util.inspect);
     assert.deepStrictEqual(
-      new Set(Object.keys(util.inspect.defaultOptions).concat(['stylize'])),
+      new Set(Object.keys(inspect.defaultOptions).concat(['stylize'])),
       new Set(Object.keys(opts))
     );
     opts.showHidden = true;
-    return { [util.inspect.custom]: common.mustCall((depth, opts2) => {
+    return { [inspect.custom]: common.mustCall((depth, opts2) => {
       assert.deepStrictEqual(clone, opts2);
     }) };
   });