util: add compact depth mode

This overloads the `compact` option from `util.inspect()`. If it's
set to a number, it is going to align all most inner entries on the
same lign if they adhere to the following:

* The entries do not exceed the `breakLength` options value.
* The entry is one of the local most inner levels up the the one
  provided in `compact`.

PR-URL: https://github.com/nodejs/node/pull/26269
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

doc/api/util.md

@@ -388,6 +388,9 @@ stream.write('With ES6');
 <!-- YAML
 added: v0.3.0
 changes:
+  - version: REPLACEME
+    pr-url: https://github.com/nodejs/node/pull/26269
+    description: The `compact` option accepts numbers for a new output mode.
   - version: REPLACEME
     pr-url: https://github.com/nodejs/node/pull/24971
     description: Internal properties no longer appear in the context argument
@@ -461,11 +464,13 @@ changes:
   * `breakLength` {integer} The length at which an object's keys are split
     across multiple lines. Set to `Infinity` to format an object as a single
     line. **Default:** `60` for legacy compatibility.
-  * `compact` {boolean} Setting this to `false` causes each object key to
+  * `compact` {boolean|integer} Setting this to `false` causes each object key
-    be displayed on a new line. It will also add new lines to text that is
+    to be displayed on a new line. It will also add new lines to text that is
-    longer than `breakLength`. Note that no text will be reduced below 16
+    longer than `breakLength`. If set to a number, the most `n` inner elements
-    characters, no matter the `breakLength` size. For more information, see the
+    are united on a single line as long as all properties fit into
-    example below. **Default:** `true`.
+    `breakLength`. Note that no text will be reduced below 16 characters, no
+    matter the `breakLength` size. For more information, see the example below.
+    **Default:** `true`.
   * `sorted` {boolean|Function} If set to `true` or a function, all properties
     of an object, and `Set` and `Map` entries are sorted in the resulting
     string. If set to `true` the [default sort][] is used. If set to a function,

lib/internal/util/inspect.js

@@ -164,6 +164,7 @@ function inspect(value, opts) {
     budget: {},
     indentationLvl: 0,
     seen: [],
+    currentDepth: 0,
     stylize: stylizeNoColor,
     showHidden: inspectDefaultOptions.showHidden,
     depth: inspectDefaultOptions.depth,
@@ -769,6 +770,7 @@ function formatRaw(ctx, value, recurseTimes, typedArray) {
   recurseTimes += 1;
 
   ctx.seen.push(value);
+  ctx.currentDepth = recurseTimes;
   let output;
   const indentationLvl = ctx.indentationLvl;
   try {
@@ -792,7 +794,10 @@ function formatRaw(ctx, value, recurseTimes, typedArray) {
     }
   }
 
-  const res = reduceToSingleString(ctx, output, base, braces);
+  const combine = typeof ctx.compact === 'number' &&
+                  ctx.currentDepth - recurseTimes < ctx.compact;
+
+  const res = reduceToSingleString(ctx, output, base, braces, combine);
   const budget = ctx.budget[ctx.indentationLvl] || 0;
   const newLength = budget + res.length;
   ctx.budget[ctx.indentationLvl] = newLength;
@@ -833,7 +838,7 @@ function formatBigInt(fn, value) {
 
 function formatPrimitive(fn, value, ctx) {
   if (typeof value === 'string') {
-    if (ctx.compact === false &&
+    if (ctx.compact !== true &&
       ctx.indentationLvl + value.length > ctx.breakLength &&
       value.length > kMinLineLength) {
       const rawMaxLineLength = ctx.breakLength - ctx.indentationLvl;
@@ -1143,7 +1148,7 @@ function formatProperty(ctx, value, recurseTimes, key, type) {
   const desc = Object.getOwnPropertyDescriptor(value, key) ||
     { value: value[key], enumerable: true };
   if (desc.value !== undefined) {
-    const diff = (type !== kObjectType || ctx.compact === false) ? 2 : 3;
+    const diff = (type !== kObjectType || ctx.compact !== true) ? 2 : 3;
     ctx.indentationLvl += diff;
     str = formatValue(ctx, desc.value, recurseTimes);
     if (diff === 3) {
@@ -1200,16 +1205,27 @@ function formatProperty(ctx, value, recurseTimes, key, type) {
   return `${name}:${extra}${str}`;
 }
 
-function reduceToSingleString(ctx, output, base, braces) {
+function reduceToSingleString(ctx, output, base, braces, combine = false) {
   const breakLength = ctx.breakLength;
   let i = 0;
-  if (ctx.compact === false) {
+  if (ctx.compact !== true) {
-    const indentation = ' '.repeat(ctx.indentationLvl);
+    if (combine) {
-    let res = `${base ? `${base} ` : ''}${braces[0]}\n${indentation}  `;
+      const totalLength = output.reduce((sum, cur) => sum + cur.length, 0);
+      if (totalLength + output.length * 2 < breakLength) {
+        let res = `${base ? `${base} ` : ''}${braces[0]} `;
         for (; i < output.length - 1; i++) {
-      res += `${output[i]},\n${indentation}  `;
+          res += `${output[i]}, `;
         }
-    res += `${output[i]}\n${indentation}${braces[1]}`;
+        res += `${output[i]} ${braces[1]}`;
+        return res;
+      }
+    }
+    const indentation = `\n${' '.repeat(ctx.indentationLvl)}`;
+    let res = `${base ? `${base} ` : ''}${braces[0]}${indentation}  `;
+    for (; i < output.length - 1; i++) {
+      res += `${output[i]},${indentation}  `;
+    }
+    res += `${output[i]}${indentation}${braces[1]}`;
     return res;
   }
   if (output.length * 2 <= breakLength) {

test/parallel/test-util-inspect.js

@@ -1498,6 +1498,42 @@ util.inspect(process);
 
   assert.strict.equal(out, expected);
 
+  out = util.inspect(map, { compact: 2, showHidden: true, depth: 9 });
+
+  expected = [
+    'Map {',
+    '  Promise {',
+    '    [',
+    '      [',
+    '        1,',
+    '        Set { [ 1, 2, [length]: 2 ], [size]: 1 },',
+    '        [length]: 2',
+    '      ],',
+    '      [length]: 1',
+    '    ]',
+    '  } => Uint8Array [',
+    '    [BYTES_PER_ELEMENT]: 1,',
+    '    [length]: 0,',
+    '    [byteLength]: 0,',
+    '    [byteOffset]: 0,',
+    '    [buffer]: ArrayBuffer { byteLength: 0, foo: true }',
+    '  ],',
+    '  [Set Iterator] { [ 1, 2, [length]: 2 ] } => [Map Iterator] {',
+    '    Uint8Array [',
+    '      [BYTES_PER_ELEMENT]: 1,',
+    '      [length]: 0,',
+    '      [byteLength]: 0,',
+    '      [byteOffset]: 0,',
+    '      [buffer]: ArrayBuffer { byteLength: 0, foo: true }',
+    '    ],',
+    '    [Circular]',
+    '  },',
+    '  [size]: 2',
+    '}'
+  ].join('\n');
+
+  assert.strict.equal(out, expected);
+
   out = util.inspect(map, { showHidden: true, depth: 9, breakLength: 4 });
   expected = [
     'Map {',