allow grouping by multiple keys at once

Readme.md

@@ -51,6 +51,28 @@ groupBy(users, function(user){
 })
 ```
 
+### groupBy(array, [prop1, prop2])
+### groupBy(array, prop1, prop2)
+### groupBy(array, [fn1, fn2])
+### groupBy(array, fn1, fn2)
+
+  Also groups by multiple keys
+
+```js
+groupBy(nodes, 'type', 'version');
+```
+
+yields:
+
+```js
+{ log: 
+   { '1.0.0': [ { type: 'log', version: '1.0.0' } ],
+     '1.1.0': [ { type: 'log', version: '1.1.0' } ],
+     '1.3.0': [ { type: 'log', version: '1.3.0' } ] },
+  app: { '1.0.0': [ { type: 'app', version: '1.0.0' } ],
+         '1.1.0': [ { type: 'app', version: '1.1.0' } ] } }
+```
+
 ## License
 
   MIT

index.js

@@ -9,21 +9,29 @@ var toFunction = require('to-function');
  * Group `arr` with callback `fn(val, i)`.
  *
  * @param {Array} arr
- * @param {Function|String} fn or prop
+ * @param {Array|Function|String} fn or prop, or array of fn or prop
  * @return {Array}
  * @api public
  */
 
-module.exports = function(arr, fn){
+module.exports = function(arr, fns){
   var ret = {};
   var prop;
-  fn = toFunction(fn);
+  var obj;
+  var fn;
+  fns = {}.toString.call(fns) === '[object Array]' ? fns : [].slice.call(arguments, 1);
+  fns = fns.map(toFunction);
 
   for (var i = 0; i < arr.length; ++i) {
-    prop = fn(arr[i], i);
-    ret[prop] = ret[prop] || [];
-    ret[prop].push(arr[i]);
+    obj = ret;
+    for (var j = 0; j < fns.length; ++j) {
+      fn = fns[j];
+      prop = fn(arr[i], i);
+      obj[prop] = obj[prop] || (j === fns.length - 1 ? [] : {});
+      obj = obj[prop];
+    }
+    obj.push(arr[i]);
   }
 
   return ret;
-};
+};

test/index.js

@@ -18,6 +18,24 @@ describe('groupBy(arr, prop)', function(){
   })
 })
 
+describe('groupBy(arr, prop1, prop2)', function(){
+  it('should group by multiple properties', function () {
+    var ret = groupBy(nodes, 'type', 'version');
+    Object.keys(ret.log).should.have.length(3);
+    ret.log['1.0.0'].should.have.length(1);
+    ret.app['1.1.0'].should.have.length(1);
+  })
+})
+
+describe('groupBy(arr, [prop1, prop2])', function(){
+  it('should group by multiple properties', function () {
+    var ret = groupBy(nodes, ['type', 'version']);
+    Object.keys(ret.log).should.have.length(3);
+    ret.log['1.0.0'].should.have.length(1);
+    ret.app['1.1.0'].should.have.length(1);
+  })
+})
+
 describe('groupBy(arr, fn)', function(){
   it('should group by the returned property', function(){
     var ret = groupBy(nodes, function(node){
@@ -28,4 +46,31 @@ describe('groupBy(arr, fn)', function(){
     ret['1.1.0'].should.have.length(2);
     ret['1.3.0'].should.have.length(1);
   })
-})
+})
+
+describe('groupBy(arr, fn1, fn2)', function(){
+  it('should group by the returned properties', function () {
+    var ret = groupBy(nodes, function(node){
+      return node.type;
+    }, function(node){
+      return node.version;
+    });
+    Object.keys(ret.log).should.have.length(3);
+    ret.log['1.0.0'].should.have.length(1);
+    ret.app['1.1.0'].should.have.length(1);
+  })
+})
+
+describe('groupBy(arr, [fn1, fn2])', function(){
+  it('should group by the returned properties', function () {
+    var ret = groupBy(nodes, function(node){
+      return node.type;
+    }, function(node){
+      return node.version;
+    });
+    Object.keys(ret.log).should.have.length(3);
+    ret.log['1.0.0'].should.have.length(1);
+    ret.app['1.1.0'].should.have.length(1);
+  })
+})
+