Adding _.count to count truthy values in an iterator. _.count([1, 2, …

…3, 4, 5, 6], function(num){ return num % 2 == 0; }) = 3

index.html

@@ -137,12 +137,13 @@ <h2>Table of Contents</h2>
       <span class="methods"><a href="#each">each</a>, <a href="#map">map</a>,
       <a href="#reduce">reduce</a>, <a href="#reduceRight">reduceRight</a>,
       <a href="#detect">detect</a>, <a href="#select">select</a>,
-      <a href="#reject">reject</a>, <a href="#all">all</a>,
-      <a href="#any">any</a>, <a href="#include">include</a>,
-      <a href="#invoke">invoke</a>, <a href="#pluck">pluck</a>,
-      <a href="#max">max</a>, <a href="#min">min</a>,
-      <a href="#sortBy">sortBy</a>, <a href="#sortedIndex">sortedIndex</a>,
-      <a href="#toArray">toArray</a>, <a href="#size">size</a></span>
+      <a href="#reject">reject</a>, <a href="#count">count</a>,
+      <a href="#all">all</a>, <a href="#any">any</a>, 
+      <a href="#include">include</a>, <a href="#invoke">invoke</a>, 
+      <a href="#pluck">pluck</a>, <a href="#max">max</a>, 
+      <a href="#min">min</a>, <a href="#sortBy">sortBy</a>, 
+      <a href="#sortedIndex">sortedIndex</a>, <a href="#toArray">toArray</a>, 
+      <a href="#size">size</a></span>
     </p>
 
     <p>
@@ -337,6 +338,16 @@ <h2>Collection Functions (Arrays or Objects)</h2>
       <pre>
 var odds = _.reject([1, 2, 3, 4, 5, 6], function(num){ return num % 2 == 0; });
 =&gt; [1, 3, 5]
+</pre>
+
+      <p id="count">
+        <b class="header">count</b><code>_.count(list, [iterator], [context])</code>
+        <br />
+        Returns a count of elements which pass the truth test (<b>iterator</b>).
+      </p>
+      <pre>
+var count = _.count([1, 2, 3, 4, 5, 6], function(num){ return num % 2 == 0; });
+=&gt; 3
 </pre>
 
       <p id="all">

test/collections.js

@@ -122,6 +122,13 @@ $(document).ready(function() {
     equals(odds.join(', '), '1, 3, 5', 'rejected each even number');
   });
 
+  test('collections: count', function() {
+    var count = _.count([1, 2, 3, 4, 5, 6], function(num){ return num % 2 == 0; });
+    equals(count, 3, 'counted even numbers');
+    var count = _.count([0, 1, 2, 3, 4, 5, 6]);
+    equals(count, 6, 'counted all truthy numbers');
+  });
+
   test('collections: all', function() {
     ok(_.all([]), 'the empty set');
     ok(_.all([true, true, true]), 'all true values');

underscore.js

@@ -164,6 +164,16 @@
     return results;
   };
 
+  // Returns a count of elements which pass a truth test.
+  _.count = function(obj, iterator, context) {
+    var count = 0;
+    iterator = iterator || _.identity;
+    each(obj, function(value, index, list) {
+      if (iterator.call(context, value, index, list)) count += 1;
+    });
+    return count;
+  };
+
   // Determine whether all of the elements match a truth test.
   // Delegates to **ECMAScript 5**'s native `every` if available.
   // Aliased as `all`.