util: show Weak(Set|Map) entries in inspect

This adds support for WeakMap and WeakSet entries in `util.inspect`.
The output is limited to a maximum entry length of `maxArrayLength`.

Fixes: #19001

Author: BridgeAR

doc/api/util.md

@@ -345,6 +345,9 @@ stream.write('With ES6');
 <!-- YAML
 added: v0.3.0
 changes:
+  - version: REPLACEME
+    pr-url: https://github.com/nodejs/node/pull/REPLACEME
+    description: WeakMap and WeakSet entries can now be inspected as well.
   - version: REPLACEME
     pr-url: https://github.com/nodejs/node/pull/17907
     description: The `depth` default changed to Infinity.
@@ -369,7 +372,8 @@ changes:
 * `object` {any} Any JavaScript primitive or Object.
 * `options` {Object}
   * `showHidden` {boolean} If `true`, the `object`'s non-enumerable symbols and
-    properties will be included in the formatted result. Defaults to `false`.
+    properties will be included in the formatted result as well as [`WeakMap`][]
+    and [`WeakSet`][] entries. Defaults to `false`.
   * `colors` {boolean} If `true`, the output will be styled with ANSI color
     codes. Defaults to `false`. Colors are customizable, see
     [Customizing `util.inspect` colors][].
@@ -378,10 +382,14 @@ changes:
   * `showProxy` {boolean} If `true`, then objects and functions that are
     `Proxy` objects will be introspected to show their `target` and `handler`
     objects. Defaults to `false`.
-  * `maxArrayLength` {number} Specifies the maximum number of array and
-    `TypedArray` elements to include when formatting. Defaults to `100`. Set to
-    `null` or `Infinity` to show all array elements. Set to `0` or negative to
-    show no array elements.
+    <!--
+    TODO(BridgeAR): Deprecate `maxArrayLength` and replace it with
+                    `maxEntries`.
+    -->
+  * `maxArrayLength` {number} Specifies the maximum number of `Array`,
+    [`TypedArray`][], [`WeakMap`][] and [`WeakSet`][] elements to include when
+    formatting. Defaults to `100`. Set to `null` or `Infinity` to show all
+    elements. Set to `0` or negative to show no elements.
   * `breakLength` {number} The length at which an object's keys are split
     across multiple lines. Set to `Infinity` to format an object as a single
     line. Defaults to 60 for legacy compatibility.
@@ -501,6 +509,25 @@ console.log(util.inspect(o, { compact: false, breakLength: 80 }));
 // chunks.
 ```
 
+Using the `showHidden` option allows to inspect [`WeakMap`][] and [`WeakSet`][]
+entries. If there are more entries than `maxArrayLength`, there is no guarantee
+which entries are displayed. That means retrieving the same ['WeakSet'][]
+entries twice might actually result in a different output. Besides this any item
+might be collected at any point of time by the garbage collector if there is no
+strong reference left to that object. Therefore there is no guarantee to get a
+reliable output.
+
+```js
+const { inspect } = require('util');
+
+const obj = { a: 1 };
+const obj2 = { b: 2 };
+const weakSet = new WeakSet([obj, obj2]);
+
+console.log(inspect(weakSet, { showHidden: true }));
+// WeakSet { { a: 1 }, { b: 2 } }
+```
+
 Please note that `util.inspect()` is a synchronous method that is mainly
 intended as a debugging tool. Some input values can have a significant
 performance overhead that can block the event loop. Use this function

lib/internal/bootstrap/node.js

@@ -474,6 +474,8 @@
     const v8 = NativeModule.require('internal/v8');
     v8.previewMapIterator(new Map().entries());
     v8.previewSetIterator(new Set().entries());
+    v8.previewWeakMap(new WeakMap(), 1);
+    v8.previewWeakSet(new WeakSet(), 1);
     // Disable --allow_natives_syntax again unless it was explicitly
     // specified on the command line.
     const re = /^--allow[-_]natives[-_]syntax$/;

lib/internal/v8.js

@@ -17,7 +17,23 @@ function previewSetIterator(it) {
   return %SetIteratorClone(it);
 }
 
+// Retrieve all WeakMap instance key / value pairs up to `max`. `max` limits the
+// number of key / value pairs returned. Make sure it is a positive number,
+// otherwise V8 aborts. Passing through `0` returns all elements.
+function previewWeakMap(weakMap, max) {
+  return %GetWeakMapEntries(weakMap, max);
+}
+
+// Retrieve all WeakSet instance values up to `max`. `max` limits the
+// number of key / value pairs returned. Make sure it is a positive number,
+// otherwise V8 aborts. Passing through `0` returns all elements.
+function previewWeakSet(weakSet, max) {
+  return %GetWeakSetValues(weakSet, max);
+}
+
 module.exports = {
   previewMapIterator,
-  previewSetIterator
+  previewSetIterator,
+  previewWeakMap,
+  previewWeakSet
 };

lib/util.js

@@ -32,7 +32,9 @@ const { isBuffer } = require('buffer').Buffer;
 
 const {
   previewMapIterator,
-  previewSetIterator
+  previewSetIterator,
+  previewWeakMap,
+  previewWeakSet
 } = require('internal/v8');
 
 const {
@@ -54,6 +56,8 @@ const {
   isPromise,
   isSet,
   isSetIterator,
+  isWeakMap,
+  isWeakSet,
   isRegExp,
   isDate,
   isTypedArray
@@ -291,6 +295,8 @@ function inspect(value, opts) {
     colors: inspectDefaultOptions.colors,
     customInspect: inspectDefaultOptions.customInspect,
     showProxy: inspectDefaultOptions.showProxy,
+    // TODO(BridgeAR): Deprecate `maxArrayLength` and replace it with
+    // `maxEntries`.
     maxArrayLength: inspectDefaultOptions.maxArrayLength,
     breakLength: inspectDefaultOptions.breakLength,
     indentationLvl: 0,
@@ -328,6 +334,8 @@ Object.defineProperty(inspect, 'defaultOptions', {
     if (options === null || typeof options !== 'object') {
       throw new ERR_INVALID_ARG_TYPE('options', 'Object');
     }
+    // TODO(BridgeAR): Add input validation and make sure `defaultOptions` are
+    // not configurable.
     return _extend(inspectDefaultOptions, options);
   }
 });
@@ -465,6 +473,7 @@ function formatValue(ctx, value, recurseTimes, ln) {
   let braces;
   let noIterator = true;
   let raw;
+  let extra;
 
   // Iterators and the rest are split to reduce checks
   if (value[Symbol.iterator]) {
@@ -562,6 +571,20 @@ function formatValue(ctx, value, recurseTimes, ln) {
     } else if (isPromise(value)) {
       braces[0] = `${prefix}{`;
       formatter = formatPromise;
+    } else if (isWeakSet(value)) {
+      braces[0] = `${prefix}{`;
+      if (ctx.showHidden) {
+        formatter = formatWeakSet;
+      } else {
+        extra = '[items unknown]';
+      }
+    } else if (isWeakMap(value)) {
+      braces[0] = `${prefix}{`;
+      if (ctx.showHidden) {
+        formatter = formatWeakMap;
+      } else {
+        extra = '[items unknown]';
+      }
     } else {
       // Check boxed primitives other than string with valueOf()
       // NOTE: `Date` has to be checked first!
@@ -616,6 +639,9 @@ function formatValue(ctx, value, recurseTimes, ln) {
   ctx.seen.push(value);
   const output = formatter(ctx, value, recurseTimes, keys);
 
+  if (extra !== undefined)
+    output.unshift(extra);
+
   for (var i = 0; i < symbols.length; i++) {
     output.push(formatProperty(ctx, value, recurseTimes, symbols[i], 0));
   }
@@ -839,6 +865,46 @@ function formatMap(ctx, value, recurseTimes, keys) {
   return output;
 }
 
+function formatWeakSet(ctx, value, recurseTimes, keys) {
+  const maxArrayLength = Math.max(ctx.maxArrayLength, 0);
+  const entries = previewWeakSet(value, maxArrayLength + 1);
+  const maxLength = Math.min(maxArrayLength, entries.length);
+  let output = new Array(maxLength);
+  for (var i = 0; i < maxLength; ++i)
+    output[i] = formatValue(ctx, entries[i], recurseTimes);
+  // Sort all entries to have a halfway reliable output (if more entries than
+  // retrieved ones exist, we can not reliably return the same output).
+  output = output.sort();
+  if (entries.length > maxArrayLength)
+    output.push('... more items');
+  for (i = 0; i < keys.length; i++)
+    output.push(formatProperty(ctx, value, recurseTimes, keys[i], 0));
+  return output;
+}
+
+function formatWeakMap(ctx, value, recurseTimes, keys) {
+  const maxArrayLength = Math.max(ctx.maxArrayLength, 0);
+  const entries = previewWeakMap(value, maxArrayLength + 1);
+  // Entries exist as [key1, val1, key2, val2, ...]
+  const remainder = entries.length / 2 > maxArrayLength;
+  const len = entries.length / 2 - (remainder ? 1 : 0);
+  const maxLength = Math.min(maxArrayLength, len);
+  let output = new Array(maxLength);
+  for (var i = 0; i < len; i++) {
+    const pos = i * 2;
+    output[i] = `${formatValue(ctx, entries[pos], recurseTimes)} => ` +
+      formatValue(ctx, entries[pos + 1], recurseTimes);
+  }
+  // Sort all entries to have a halfway reliable output (if more entries than
+  // retrieved ones exist, we can not reliably return the same output).
+  output = output.sort();
+  if (remainder > 0)
+    output.push('... more items');
+  for (i = 0; i < keys.length; i++)
+    output.push(formatProperty(ctx, value, recurseTimes, keys[i], 0));
+  return output;
+}
+
 function formatCollectionIterator(preview, ctx, value, recurseTimes, keys) {
   const output = [];
   for (const entry of preview(value)) {

test/parallel/test-util-inspect.js

@@ -1349,3 +1349,51 @@ util.inspect(process);
   expect = '{\n  a: \'12 45 78 01 34 \' +\n    \'67 90 23\'\n}';
   assert.strictEqual(out, expect);
 }
+
+{ // Test WeakMap
+  const obj = {};
+  const arr = [];
+  const weakMap = new WeakMap([[obj, arr], [arr, obj]]);
+  let out = util.inspect(weakMap, { showHidden: true });
+  let expect = 'WeakMap { [ [length]: 0 ] => {}, {} => [ [length]: 0 ] }';
+  assert.strictEqual(out, expect);
+
+  out = util.inspect(weakMap);
+  expect = 'WeakMap { [items unknown] }';
+  assert.strictEqual(out, expect);
+
+  out = util.inspect(weakMap, { maxArrayLength: 0, showHidden: true });
+  expect = 'WeakMap { ... more items }';
+  assert.strictEqual(out, expect);
+
+  weakMap.extra = true;
+  out = util.inspect(weakMap, { maxArrayLength: 1, showHidden: true });
+  // It is not possible to determine the output reliable.
+  expect = 'WeakMap { [ [length]: 0 ] => {}, ... more items, extra: true }';
+  const expectAlt = 'WeakMap { {} => [ [length]: 0 ], ... more items, ' +
+                    'extra: true }';
+  assert(out === expect || out === expectAlt);
+}
+
+{ // Test WeakSet
+  const weakSet = new WeakSet([{}, [1]]);
+  let out = util.inspect(weakSet, { showHidden: true });
+  let expect = 'WeakSet { [ 1, [length]: 1 ], {} }';
+  assert.strictEqual(out, expect);
+
+  out = util.inspect(weakSet);
+  expect = 'WeakSet { [items unknown] }';
+  assert.strictEqual(out, expect);
+
+  out = util.inspect(weakSet, { maxArrayLength: -2, showHidden: true });
+  expect = 'WeakSet { ... more items }';
+  assert.strictEqual(out, expect);
+
+  weakSet.extra = true;
+  out = util.inspect(weakSet, { maxArrayLength: 1, showHidden: true });
+  // It is not possible to determine the output reliable.
+  expect = 'WeakSet { {}, ... more items, extra: true }';
+  const expectAlt = 'WeakSet { [ 1, [length]: 1 ], ... more items, ' +
+                    'extra: true }';
+  assert(out === expect || out === expectAlt);
+}