feat: LRUCache/LRUMap family -- inspect and toString improvements

* LRUCache and family .inspect limits its output -- showing the youngest items, and ellipsis, and the oldest item. Options allow dumping the raw object or controlling the size of output (and the number of items a console.log will mindlessly iterate over).
* LRUCache and family all have inspect wired up to the magic 'nodejs.util.inspect.custom' symbol property that drives console.log output
* LRUCache and family all have a summaryString method returning eg 'LRUCache[8/200]' for a cache with size 8 and capacity 200, wired to the magic Symbol.toStringTag property that drives string interpolation (partially addresses #129).

lru-cache-with-delete.js

@@ -35,8 +35,21 @@ function LRUCacheWithDelete(Keys, Values, capacity) {
 
 for (var k in LRUCache.prototype)
   LRUCacheWithDelete.prototype[k] = LRUCache.prototype[k];
-if (typeof Symbol !== 'undefined')
+
+/**
+ * If possible, attaching the
+ * * #.entries method to Symbol.iterator (allowing `for (const foo of cache) { ... }`)
+ * * the summaryString method to Symbol.toStringTag (allowing `\`${cache}\`` to work)
+ * * the inspect method to Symbol.for('nodejs.util.inspect.custom') (making `console.log(cache)` liveable)
+ */
+if (typeof Symbol !== 'undefined') {
   LRUCacheWithDelete.prototype[Symbol.iterator] = LRUCache.prototype[Symbol.iterator];
+  Object.defineProperty(LRUCacheWithDelete.prototype, Symbol.toStringTag, {
+    get: function () { return `${this.constructor.name}:${this.size}/${this.capacity}`; },
+  });
+  LRUCacheWithDelete.prototype[Symbol.for('nodejs.util.inspect.custom')] = LRUCache.prototype.inspect;
+}
+Object.defineProperty(LRUCacheWithDelete.prototype, 'summary', Object.getOwnPropertyDescriptor(LRUCache.prototype, 'summary'));
 
 /**
  * Method used to clear the structure.

lru-cache.js

@@ -18,7 +18,8 @@
 var Iterator = require('obliterator/iterator'),
     forEach = require('obliterator/foreach'),
     typed = require('./utils/typed-arrays.js'),
-    iterables = require('./utils/iterables.js');
+    iterables = require('./utils/iterables.js'),
+    {snipToLast} = require('./utils/snip.js');
 
 /**
  * LRUCache.
@@ -368,29 +369,59 @@ LRUCache.prototype.entries = function() {
   });
 };
 
+/**
+ * Return a short string for interpolation: `LRUCache:size/capacity`
+ */
+Object.defineProperty(LRUCache.prototype, 'summary', {
+  get: function() { return `${this.constructor.name}:${this.size}/${this.capacity}`; },
+});
+
 /**
  * Attaching the #.entries method to Symbol.iterator if possible.
  */
-if (typeof Symbol !== 'undefined')
+if (typeof Symbol !== 'undefined') {
   LRUCache.prototype[Symbol.iterator] = LRUCache.prototype.entries;
+  Object.defineProperty(LRUCache.prototype, Symbol.toStringTag, {
+    get: function () { return this.summary; },
+  });
+}
+
+LRUCache.defaultMaxToDump = 20;
 
 /**
- * Convenience known methods.
+ * Provide a reasonably-sized view of the object.
+ *
+ * @param  {number}   [depth]   - When < 0, only the toString() summary is returned
+ * @param  {object}   [options = {}]  - settings for output
+ * @param  {boolean}  [options.all = false] - When true, returns the object with all properties, ignoring limits
+ * @param  {number}   [options.maxToDump = 20] - When size > maxToDump, lists only the
+ *                      youngest `maxToDump - 2`, a placeholder with the number
+ *                      omitted, and the single oldest item. The secret variable
+ *                      LRUCache.defaultMaxToDump determines the default limit.
+ * @return {Map}
+ *
  */
-LRUCache.prototype.inspect = function() {
+LRUCache.prototype.inspect = function(depth, options = {}) {
+  if (arguments.length <= 1) { options = depth || {}; depth = 2; }
+  if (options.all) { var ret = {}; Object.assign(ret, this); return ret; }
+  if (depth < 0) { return this.toString(); }
+  var maxToDump = options.maxToDump || LRUCache.defaultMaxToDump;
   var proxy = new Map();
 
-  var iterator = this.entries(),
-      step;
+  var last = [this.K[this.tail], this.V[this.tail]];
+  snipToLast(this.entries(), proxy, {maxToDump, size: this.size, last});
 
-  while ((step = iterator.next(), !step.done))
-    proxy.set(step.value[0], step.value[1]);
-
-  // Trick so that node displays the name of the constructor
+  // Trick so that node displays the name of the constructor (does not work in modern node)
   Object.defineProperty(proxy, 'constructor', {
     value: LRUCache,
     enumerable: false
   });
+  if (typeof Symbol !== 'undefined') {
+    var self = this;
+    Object.defineProperty(proxy, Symbol.toStringTag, {
+      get: function () { return `${self.constructor.name}:${self.size}/${self.capacity}`; },
+    });
+  }
 
   return proxy;
 };

lru-map-with-delete.js

@@ -35,8 +35,21 @@ function LRUMapWithDelete(Keys, Values, capacity) {
 
 for (var k in LRUMap.prototype)
   LRUMapWithDelete.prototype[k] = LRUMap.prototype[k];
-if (typeof Symbol !== 'undefined')
+
+/**
+ * If possible, attaching the
+ * * #.entries method to Symbol.iterator (allowing `for (const foo of cache) { ... }`)
+ * * the summaryString method to Symbol.toStringTag (allowing `\`${cache}\`` to work)
+ * * the inspect method to Symbol.for('nodejs.util.inspect.custom') (making `console.log(cache)` liveable)
+ */
+if (typeof Symbol !== 'undefined') {
   LRUMapWithDelete.prototype[Symbol.iterator] = LRUMap.prototype[Symbol.iterator];
+  Object.defineProperty(LRUMapWithDelete.prototype, Symbol.toStringTag, {
+    get: function () { return `${this.constructor.name}:${this.size}/${this.capacity}`; },
+  });
+  LRUMapWithDelete.prototype[Symbol.for('nodejs.util.inspect.custom')] = LRUMap.prototype.inspect;
+}
+Object.defineProperty(LRUMapWithDelete.prototype, 'summary', Object.getOwnPropertyDescriptor(LRUMap.prototype, 'summary'));
 
 /**
  * Method used to clear the structure.

lru-map.js

@@ -211,17 +211,27 @@ LRUMap.prototype.forEach = LRUCache.prototype.forEach;
 LRUMap.prototype.keys = LRUCache.prototype.keys;
 LRUMap.prototype.values = LRUCache.prototype.values;
 LRUMap.prototype.entries = LRUCache.prototype.entries;
+LRUMap.prototype.summaryString = LRUCache.prototype.summaryString;
 
 /**
- * Attaching the #.entries method to Symbol.iterator if possible.
+ * Inherit methods
  */
-if (typeof Symbol !== 'undefined')
-  LRUMap.prototype[Symbol.iterator] = LRUMap.prototype.entries;
+LRUMap.prototype.inspect = LRUCache.prototype.inspect;
 
 /**
- * Convenience known methods.
+ * If possible, attaching the
+ * * #.entries method to Symbol.iterator (allowing `for (const foo of cache) { ... }`)
+ * * the summaryString method to Symbol.toStringTag (allowing `\`${cache}\`` to work)
+ * * the inspect method to Symbol.for('nodejs.util.inspect.custom') (making `console.log(cache)` liveable)
  */
-LRUMap.prototype.inspect = LRUCache.prototype.inspect;
+if (typeof Symbol !== 'undefined') {
+  LRUMap.prototype[Symbol.iterator] = LRUMap.prototype.entries;
+  Object.defineProperty(LRUMap.prototype, Symbol.toStringTag, {
+    get: function () { return `${this.constructor.name}:${this.size}/${this.capacity}`; },
+  });
+  LRUMap.prototype[Symbol.for('nodejs.util.inspect.custom')] = LRUCache.prototype.inspect;
+}
+Object.defineProperty(LRUMap.prototype, 'summary', Object.getOwnPropertyDescriptor(LRUCache.prototype, 'summary'));
 
 /**
  * Static @.from function taking an arbitrary iterable & converting it into

test/lru-cache.js

@@ -7,6 +7,7 @@ var assert = require('assert'),
     LRUMap = require('../lru-map.js'),
     LRUCacheWithDelete = require('../lru-cache-with-delete.js'),
     LRUMapWithDelete = require('../lru-map-with-delete.js');
+var NodeUtil = require('util');
 
 function makeTests(Cache, name) {
   describe(name, function() {
@@ -62,7 +63,7 @@ function makeTests(Cache, name) {
       assert.strictEqual(cache.peek('two'), 5);
       assert.deepStrictEqual(Array.from(cache.entries()), [['three', 3], ['four', 4], ['two', 5]]);
 
-      if (name === 'LRUCache' || name === 'LRUCacheWithDelete')
+      if (/LRUCache/.test(name))
         assert.strictEqual(Object.keys(cache.items).length, 3);
       else
         assert.strictEqual(cache.items.size, 3);
@@ -222,7 +223,7 @@ function makeTests(Cache, name) {
       assert.deepStrictEqual(entries, Array.from(cache.entries()));
     });
 
-    if ((name === 'LRUCacheWithDelete') || (name === 'LRUMapWithDelete')) {
+    if (/With/.test(name)) {
 
       it('should be possible to delete keys from a LRU cache.', function() {
         var cache = new Cache(3);
@@ -302,6 +303,7 @@ function makeTests(Cache, name) {
         assert.equal(dead, missingMarker);
 
         cache.set('one', 'uno');
+
         cache.set('two', 'dos');
         cache.set('three', 'tres');
 
@@ -488,6 +490,121 @@ function makeTests(Cache, name) {
       });
 
     }
+
+    describe('inspection', function() {
+      function makeExercisedCache(capacity) {
+        var cache = new Cache(capacity), ii;
+        cache.set(1, 'a'); cache.set(2, 'b'); cache.set('too old', 'c'); cache.set('oldest', 'd');
+        for (ii = 0; ii < capacity - 3; ii++) { cache.set(ii * 2, ii * 2); }
+        cache.set(4, 'D'); cache.set(2, 'B'); cache.get(1);
+        cache.set(5, 'e'); cache.set(6, 'f');
+        return cache;
+      }
+
+      it('toString() states the name size and capacity', function () {
+        var cache = new Cache(null, null, 200);
+        cache.set(0, 'cero'); cache.set(1, 'uno');
+        assert.deepStrictEqual(cache.toString(), `[object ${name}:2/200]`);
+        if (typeof Symbol !== 'undefined') {
+          assert.deepStrictEqual(cache[Symbol.toStringTag], `${name}:2/200`);
+        }
+        assert.deepStrictEqual(cache.summary, `${name}:2/200`);
+        cache.set(2, 'dos'); cache.set(3, 'tres');
+        assert.deepStrictEqual(cache.toString(), `[object ${name}:4/200]`);
+        cache = makeExercisedCache(200);
+        assert.deepStrictEqual(cache.toString(), `[object ${name}:200/200]`);
+      });
+
+      if (typeof Symbol !== 'undefined') {
+        it('registers its inspect method for the console.log and friends to use', function () {
+          var cache = makeExercisedCache(7);
+          assert.deepStrictEqual(cache[Symbol.for('nodejs.util.inspect.custom')], cache.inspect);
+        });
+
+        it('attaches the summary getter to the magic [Symbol.toStringTag] property', function () {
+          var cache = makeExercisedCache(7);
+          assert.deepStrictEqual(cache[Symbol.toStringTag], cache.summary);
+        });
+      }
+
+      it('accepts limits on what inspect returns', function () {
+        var cache = new Cache(15), inspectedItems;
+        // empty
+        inspectedItems = Array.from(cache.inspect({maxToDump: 5}).entries());
+        assert.deepStrictEqual(inspectedItems, []);
+        //
+        cache.set(1, 'a');
+        inspectedItems = Array.from(cache.inspect({maxToDump: 5}).entries());
+        assert.deepStrictEqual(inspectedItems, [[1, 'a']]);
+        //
+        cache.set(2, 'b');
+        inspectedItems = Array.from(cache.inspect({maxToDump: 5}).entries());
+        assert.deepStrictEqual(inspectedItems, [[2, 'b'], [1, 'a']]);
+        //
+        cache.set(3, 'c');
+        inspectedItems = Array.from(cache.inspect({maxToDump: 5}).entries());
+        assert.deepStrictEqual(inspectedItems, [[3, 'c'], [2, 'b'], [1, 'a']]);
+        //
+        cache.set(4, 'd');
+        inspectedItems = Array.from(cache.inspect({maxToDump: 5}).entries());
+        assert.deepStrictEqual(inspectedItems, [[4, 'd'], [3, 'c'], [2, 'b'], [1, 'a']]);
+        //
+        cache.set(5, 'e');
+        inspectedItems = Array.from(cache.inspect({maxToDump: 5}).entries());
+        assert.deepStrictEqual(inspectedItems, [[5, 'e'], [4, 'd'], [3, 'c'], [2, 'b'], [1, 'a']]);
+        //
+        cache.set(6, 'f');
+        inspectedItems = Array.from(cache.inspect({maxToDump: 5}).entries());
+        assert.deepStrictEqual(inspectedItems, [[6, 'f'], [5, 'e'], [4, 'd'], ['_...', 2], [1, 'a']]);
+        //
+        var ii;
+        for (ii = 0; ii < 20; ii++) { cache.set(ii * 2, ii * 2); }
+        inspectedItems = Array.from(cache.inspect({maxToDump: 5}).entries());
+        assert.deepStrictEqual(inspectedItems, [[38, 38], [36, 36], [34, 34], ['_...', 11], [10, 10]]);
+      });
+
+      it('puts a reasonable limit on what the console will show (large)', function () {
+        var cache = makeExercisedCache(600);
+        var asSeenInConsole = NodeUtil.inspect(cache);
+        // we're trying not to depend on what a given version of node actually serializes
+        var itemsDumped = /6 => 'f',\s*5 => 'e',\s*1 => 'a',(.|\n)+1168 => 1168,\s*'_\.\.\.' => 581,\s*'oldest' => 'd'/;
+        assert.deepStrictEqual(itemsDumped.test(asSeenInConsole), true);
+        assert.deepStrictEqual(new RegExp(`${name}:\[600/600\]`).test(asSeenInConsole), true);
+        assert.deepStrictEqual(asSeenInConsole.length < 800, true);
+      });
+
+      it('puts a reasonable limit on what the console will show (small)', function () {
+        var cache = makeExercisedCache(7);
+        var asSeenInConsole = NodeUtil.inspect(cache);
+        var itemsDumped = /6 => 'f',\s*5 => 'e',\s*1 => 'a',\s*2 => 'B',\s*4 => 'D',\s*0 => 0,\s*'oldest' => 'd'/;
+        assert.deepStrictEqual(itemsDumped.test(asSeenInConsole), true);
+        assert.deepStrictEqual(new RegExp(`${name}:7/7`).test(asSeenInConsole), true);
+      });
+
+      it('listens to advice about maximum inspection depth', function () {
+        var cache = makeExercisedCache(7);
+        var asSeenInConsole = NodeUtil.inspect({foo: {bar: cache}});
+        var itemsDumped = /6 => 'f',\s*5 => 'e',\s*1 => 'a',\s*2 => 'B',\s*4 => 'D',\s*0 => 0,\s*'oldest' => 'd'/;
+        assert.deepStrictEqual(itemsDumped.test(asSeenInConsole), true);
+        assert.deepStrictEqual(asSeenInConsole.length > 150, true);
+        // Cannot depend on this existing in older versions.
+        // asSeenInConsole = NodeUtil.inspect({foo: {bar: [cache]}});
+        // deepStrictEqual(asSeenInConsole.length < 70, true);
+        // deepStrictEqual(new RegExp(`\\[ \\[object ${name}:7/7\\] \\]`).test(asSeenInConsole), true);
+      });
+
+      it('allows inspection of the raw item if "all" is given', function () {
+        var cache = makeExercisedCache(250);
+        var inspected = cache.inspect({maxToDump: 8, all: true});
+        var kk;
+        for (kk of ['items', 'K', 'V', 'size', 'capacity']) {
+          assert.deepStrictEqual(inspected[kk] === cache[kk], true);
+        }
+        assert.deepStrictEqual(Object.keys(cache), Object.keys(inspected));
+      });
+
+    });
+
   });
 }
 

utils/snip.js

@@ -0,0 +1,24 @@
+
+function snipToLast(iterator, proxy, {maxToDump = 20, size = Infinity, last = []}) {
+  var step;
+
+  var ii = 0;
+  while ((step = iterator.next(), !step.done)) {
+    if (ii >= maxToDump - 2) {
+      if (ii >= size - 1) {
+        proxy.set(step.value[0], step.value[1]);
+      } else if (ii === size - 2) {
+        proxy.set(step.value[0], step.value[1]);
+        proxy.set(last[0], last[1]);
+      } else {
+        proxy.set('_...', size - ii - 1);
+        proxy.set(last[0], last[1]);
+      }
+      break;
+    }
+    proxy.set(step.value[0], step.value[1]);
+    ii += 1;
+  }
+}
+
+module.exports.snipToLast = snipToLast;