errors: improve formatList in errors.js

benchmark/error/format-list.js

@@ -0,0 +1,44 @@
+'use strict';
+
+const common = require('../common.js');
+
+const bench = common.createBenchmark(main, {
+  n: [1e7],
+  input: [
+    '',
+    'a',
+    'a,b',
+    'a,b,c',
+    'a,b,c,d',
+  ],
+  type: [
+    'undefined',
+    'and',
+    'or',
+  ],
+}, {
+  flags: ['--expose-internals'],
+});
+
+function main({ n, input, type }) {
+  const {
+    formatList,
+  } = require('internal/errors');
+
+  const list = input.split(',');
+
+  if (type === 'undefined') {
+    bench.start();
+    for (let i = 0; i < n; ++i) {
+      formatList(list);
+    }
+    bench.end(n);
+    return;
+  }
+
+  bench.start();
+  for (let i = 0; i < n; ++i) {
+    formatList(list, type);
+  }
+  bench.end(n);
+}

lib/internal/errors.js

@@ -902,8 +902,14 @@ function determineSpecificType(value) {
  * @returns {string}
  */
 function formatList(array, type = 'and') {
-  return array.length < 3 ? ArrayPrototypeJoin(array, ` ${type} `) :
-    `${ArrayPrototypeJoin(ArrayPrototypeSlice(array, 0, -1), ', ')}, ${type} ${array[array.length - 1]}`;
+  switch (array.length) {
+    case 0: return '';
+    case 1: return `${array[0]}`;
+    case 2: return `${array[0]} ${type} ${array[1]}`;
+    case 3: return `${array[0]}, ${array[1]}, ${type} ${array[2]}`;
+    default:
+      return `${ArrayPrototypeJoin(ArrayPrototypeSlice(array, 0, -1), ', ')}, ${type} ${array[array.length - 1]}`;
+  }
 }
 
 module.exports = {

test/parallel/test-error-format-list.js

@@ -11,7 +11,7 @@ if (!common.hasIntl) common.skip('missing Intl');
   const and = new Intl.ListFormat('en', { style: 'long', type: 'conjunction' });
   const or = new Intl.ListFormat('en', { style: 'long', type: 'disjunction' });
 
-  const input = ['apple', 'banana', 'orange'];
+  const input = ['apple', 'banana', 'orange', 'pear'];
   for (let i = 0; i < input.length; i++) {
     const slicedInput = input.slice(0, i);
     strictEqual(formatList(slicedInput), and.format(slicedInput));