add push/unshift helper methods

- add `update.push`, `update.unshift` helper methods. `push` is an
alias for existing `add` helper method, `unshift` is a new one with
an alias of `prepend`

README.md

@@ -90,19 +90,36 @@ Notes on object lookup:
 
 ### Array Update Helpers
 
-#### `update.add`
+#### `update.unshift`
 
 Adds item to the array.
 
 ```js
 import update from 'update-js';
 
 const obj = { foo: { bar: [1, 2] } };
-const upd = update.add(obj, 'foo.bar', 3);
+const upd = update.unshift(obj, 'foo.bar', 3);
+
+upd.foo.bar // => [3, 1, 2];
+```
+
+_Alias:_ `update.prepend`
+
+#### `update.push`
+
+Adds item to the array.
+
+```js
+import update from 'update-js';
+
+const obj = { foo: { bar: [1, 2] } };
+const upd = update.push(obj, 'foo.bar', 3);
 
 upd.foo.bar // => [1, 2, 3];
 ```
 
+_Alias:_ `update.add`
+
 #### `update.remove`
 
 Removes item from the array by index or lookup key.

index.js

@@ -3,7 +3,12 @@
 var set = require('lodash.set');
 
 update.with = updateWith;
-update.add = updateAdd;
+update.unshift = updateUnshift;
+update.prepend = updateUnshift;
+update.shift = updateShift;
+update.push = updatePush;
+update.add = updatePush;
+update.pop = updatePop;
 update.remove = updateRemove;
 update.assign = updateAssign;
 update.del = updateDel;
@@ -38,12 +43,30 @@ function updateWith(obj, path, fn) {
   return updateInWith(current, path, fn);
 }
 
-function updateAdd(obj, path, item) {
+function updateUnshift(obj, path, item) {
+  return updateWith(obj, path, function(collection) {
+    return [item].concat(collection);
+  });
+}
+
+function updateShift(obj, path) {
+  return updateWith(obj, path, function(collection) {
+    return collection.slice(1);
+  });
+}
+
+function updatePush(obj, path, item) {
   return updateWith(obj, path, function(collection) {
     return collection.concat([item]);
   });
 }
 
+function updatePop(obj, path) {
+  return updateWith(obj, path, function(collection) {
+    return collection.slice(0, -1);
+  });
+}
+
 function updateRemove(obj, path) {
   var match = path.match(/^(.+)\.(?!\.)(.+)$/);
 

test.js

@@ -132,6 +132,33 @@ describe('update', function() {
     });
   });
 
+  describe('update.unshift', function() {
+    it('prepends new item to the collection', function() {
+      var obj = { foo: { bar: [1, 2] } };
+      var upd = update.unshift(obj, 'foo.bar', 3);
+
+      assert.deepEqual(upd.foo.bar, [3, 1, 2]);
+    });
+  });
+
+  describe('update.prepend', function() {
+    it('prepends new item to the collection', function() {
+      var obj = { foo: { bar: [1, 2] } };
+      var upd = update.prepend(obj, 'foo.bar', 3);
+
+      assert.deepEqual(upd.foo.bar, [3, 1, 2]);
+    });
+  });
+
+  describe('update.push', function() {
+    it('adds new item to the collection', function() {
+      var obj = { foo: { bar: [1, 2] } };
+      var upd = update.push(obj, 'foo.bar', 3);
+
+      assert.deepEqual(upd.foo.bar, [1, 2, 3]);
+    });
+  });
+
   describe('update.add', function() {
     it('adds new item to the collection', function() {
       var obj = { foo: { bar: [1, 2] } };