Add stableSort function

test/stableSort/stableSort.js

@@ -0,0 +1,8 @@
+var stableSort = (arr, compare) =>
+arr
+.map((item, index) => ({ item, index }))
+.sort((a, b) =>
+((result = compare(a.item, b.item)) => (result !== 0 ? result : a.index - b.index))()
+)
+.map(({ item }) => item);
+module.exports = stableSort;

test/stableSort/stableSort.test.js

@@ -0,0 +1,25 @@
+const test = require('tape');
+const stableSort = require('./stableSort.js');
+
+test('Testing stableSort', (t) => {
+  //For more information on all the methods supported by tape
+  //Please go to https://github.com/substack/tape
+  t.true(typeof stableSort === 'function', 'stableSort is a Function');
+  //t.deepEqual(stableSort(args..), 'Expected');
+  //t.equal(stableSort(args..), 'Expected');
+  //t.false(stableSort(args..), 'Expected');
+  //t.throws(stableSort(args..), 'Expected');
+  
+  // test if js engine's Array#sort implementation is stable
+  // https://gist.github.com/leeoniya/5816476
+  var str = 'abcdefghijklmnopqrstuvwxyz';
+  var compare = (a, b) => ~~(str.indexOf(b) / 2.3) - ~~(str.indexOf(a) / 2.3);
+
+  var input = str.split('');
+  var output = stableSort(input, compare);
+
+  t.equal(output.join(''), 'xyzvwtursopqmnklhijfgdeabc');
+  t.notDeepEqual(input, output);
+
+  t.end();
+});