util: compact inspect() for sparse arrays

This prevents `console.log(arr)` from crashing node when given a sparse
array with large length. Fixes nodejs#4905

lib/util.js

@@ -501,20 +501,39 @@ function formatObject(ctx, value, recurseTimes, visibleKeys, keys) {
 
 function formatArray(ctx, value, recurseTimes, visibleKeys, keys) {
   var output = [];
-  for (var i = 0, l = value.length; i < l; ++i) {
-    if (hasOwnProperty(value, String(i))) {
-      output.push(formatProperty(ctx, value, recurseTimes, visibleKeys,
-          String(i), true));
-    } else {
-      output.push('');
+  // if less than a tenth of the array's slots are populated, consider it sparse
+  // and return a compact representation. this prevents node from crashing when
+  // calling console.log() on a small array with large length.
+  // see https://github.com/nodejs/node/issues/4905
+  var isSparse = value.length > 10 && keys.length * 10 < value.length;
+  if (!isSparse) {
+    // output a normal dense array, eg. [ , , , "foo", "bar"]
+    for (var i = 0, l = value.length; i < l; ++i) {
+      if (hasOwnProperty(value, String(i))) {
+        output.push(formatProperty(ctx, value, recurseTimes, visibleKeys,
+            String(i), true));
+      } else {
+        output.push('');
+      }
     }
   }
+  // output sparse arrays as eg. [10: "foo", 1000000: "bar"]
+  // also handle additional properties on an array, for example [1, 2, 3, foo:4]
   keys.forEach(function(key) {
+    var str = formatProperty(ctx, value, recurseTimes, visibleKeys, key, true);
     if (typeof key === 'symbol' || !key.match(/^\d+$/)) {
-      output.push(formatProperty(ctx, value, recurseTimes, visibleKeys,
-          key, true));
+      output.push(str); // str in format "foo: bar"
+    } else if (isSparse) {
+      output.push(key + ': ' + str); // str in format "bar"
     }
   });
+  if (isSparse) {
+    if (keys.length === 0) {
+      output.push('<empty array, length ' + value.length + '>');
+    } else {
+      output.push('<sparse array, length ' + value.length + '>');
+    }
+  }
   return output;
 }
 

test/parallel/test-util-inspect.js

@@ -42,6 +42,19 @@ assert.equal(util.inspect(Object.create({},
   '{ visible: 1 }'
 );
 
+// verify short output, no crash for long sparse arrays
+var arr = [];
+arr[2] = 1;
+assert.equal(util.inspect(arr), '[ , , 1 ]'); // dense
+arr[1000] = 1;
+assert.equal(util.inspect(arr),
+  '[ 2: 1, 1000: 1, <sparse array, length 1001> ]'); // sparse
+arr['foo'] = 'bar';
+assert.equal(util.inspect(arr),
+  '[ 2: 1, 1000: 1, foo: \'bar\', <sparse array, length 1001> ]');
+arr = new Array(1000000);
+assert.equal(util.inspect(arr), '[ <empty array, length 1000000> ]');
+
 for (const showHidden of [true, false]) {
   const ab = new ArrayBuffer(4);
   const dv = new DataView(ab, 1, 2);