Fixes #2741. _.first() and _.last() should return an empty array when requesting a specific number of elements.

Author: jashkenas

test/arrays.js

@@ -15,6 +15,9 @@
     result = _.map([[1, 2, 3], [1, 2, 3]], _.first);
     assert.deepEqual(result, [1, 1], 'works well with _.map');
     assert.strictEqual(_.first(null), void 0, 'returns undefined when called on null');
+    assert.deepEqual(_.first([], 10), [], 'returns an empty array when called with an explicit number of elements to return');
+    assert.deepEqual(_.first([], 1), [], 'returns an empty array when called with an explicit number of elements to return');
+    assert.deepEqual(_.first(null, 5), [], 'returns an empty array when called with an explicit number of elements to return');
 
     Array.prototype[0] = 'boo';
     assert.strictEqual(_.first([]), void 0, 'return undefined when called on a empty array');
@@ -71,6 +74,10 @@
     assert.deepEqual(result, [3, 3], 'works well with _.map');
     assert.strictEqual(_.last(null), void 0, 'returns undefined when called on null');
 
+    assert.deepEqual(_.last([], 10), [], 'returns an empty array when called with an explicit number of elements to return');
+    assert.deepEqual(_.last([], 1), [], 'returns an empty array when called with an explicit number of elements to return');
+    assert.deepEqual(_.last(null, 5), [], 'returns an empty array when called with an explicit number of elements to return');
+
     var arr = [];
     arr[-1] = 'boo';
     assert.strictEqual(_.last(arr), void 0, 'return undefined when called on a empty array');

underscore.js

@@ -497,7 +497,7 @@
   // values in the array. Aliased as `head` and `take`. The **guard** check
   // allows it to work with `_.map`.
   _.first = _.head = _.take = function(array, n, guard) {
-    if (array == null || array.length < 1) return void 0;
+    if (array == null || array.length < 1) return n == null ? void 0 : [];
     if (n == null || guard) return array[0];
     return _.initial(array, array.length - n);
   };
@@ -512,7 +512,7 @@
   // Get the last element of an array. Passing **n** will return the last N
   // values in the array.
   _.last = function(array, n, guard) {
-    if (array == null || array.length < 1) return void 0;
+    if (array == null || array.length < 1) return n == null ? void 0 : [];
     if (n == null || guard) return array[array.length - 1];
     return _.rest(array, Math.max(0, array.length - n));
   };