console,util: avoid pair array generation in C++

Use a plain `[key, value, key, value]`-style list instead
of an array of pairs for inspecting collections.

This also fixes a bug with `console.table()` where
inspecting a non-key-value `MapIterator` would have
led to odd results.

PR-URL: https://github.com/nodejs/node/pull/20831
Refs: https://github.com/nodejs/node/pull/20719#discussion_r189342513
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

1 files changed, 20 insertions, 8 deletions

diff --git a/lib/util.js b/lib/util.js
index afd3cae724..5413cc0a33 100644
--- a/lib/util.js
+++ b/lib/util.js
@@ -981,7 +981,7 @@ function formatMap(ctx, value, recurseTimes, keys) {
 
 function formatWeakSet(ctx, value, recurseTimes, keys) {
   const maxArrayLength = Math.max(ctx.maxArrayLength, 0);
-  const entries = previewEntries(value).slice(0, maxArrayLength + 1);
+  const [ entries ] = previewEntries(value).slice(0, maxArrayLength + 1);
   const maxLength = Math.min(maxArrayLength, entries.length);
   let output = new Array(maxLength);
   for (var i = 0; i < maxLength; ++i)
@@ -998,14 +998,16 @@ function formatWeakSet(ctx, value, recurseTimes, keys) {
 
 function formatWeakMap(ctx, value, recurseTimes, keys) {
   const maxArrayLength = Math.max(ctx.maxArrayLength, 0);
-  const entries = previewEntries(value).slice(0, maxArrayLength + 1);
-  const remainder = entries.length > maxArrayLength;
-  const len = entries.length - (remainder ? 1 : 0);
+  const [ entries ] = previewEntries(value).slice(0, (maxArrayLength + 1) * 2);
+  // 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++) {
-    output[i] = `${formatValue(ctx, entries[i][0], recurseTimes)} => ` +
-      formatValue(ctx, entries[i][1], recurseTimes);
+  for (var i = 0; i < maxLength; i++) {
+    const pos = i * 2;
+    output[i] = `${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).
@@ -1017,9 +1019,19 @@ function formatWeakMap(ctx, value, recurseTimes, keys) {
   return output;
 }
 
+function zip2(list) {
+  const ret = Array(list.length / 2);
+  for (var i = 0; i < ret.length; ++i)
+    ret[i] = [list[2 * i], list[2 * i + 1]];
+  return ret;
+}
+
 function formatCollectionIterator(ctx, value, recurseTimes, keys) {
   const output = [];
-  for (const entry of previewEntries(value)) {
+  var [ entries, isKeyValue ] = previewEntries(value);
+  if (isKeyValue)
+    entries = zip2(entries);
+  for (const entry of entries) {
     if (ctx.maxArrayLength === output.length) {
       output.push('... more items');
       break;