Add update(key, updater, cb)

README.md

@@ -87,6 +87,13 @@ Retrieves the value for the given `key`.
 Removes the record with the given `key`. This is identical to setting the `key`'s value
 to `undefined`.
 
+### dirty.update(key, updater, [cb])
+
+Updates the record of the given `key` with the given `updater` which is a function
+that is passed the current value of the key.
+
+The optional `cb` callback is passed to `dirty.set`.
+
 ### dirty.forEach(fn)
 
 Calls the given `fn` function for every document in the database. The passed

lib/dirty/dirty.js

@@ -36,7 +36,7 @@ module.exports = Dirty;
 /**
 * set() stores a JSON object in the database at key
 * cb is fired when the data is persisted.
-* In memory, this is immediate- on disk, it will take some time.
+* In memory, this is immediate - on disk, it will take some time.
 */
 Dirty.prototype.set = function(key, val, cb) {
   if (val === undefined) {
@@ -95,6 +95,15 @@ Dirty.prototype.forEach = function(fn) {
 
 };
 
+/**
+* Update the value stored at a key in the database.
+* This is synchronous since a cache is maintained in-memory
+* cb is passed as per Dirty.prototype.set
+*/
+Dirty.prototype.update = function(key, updater, cb) {
+  this.set(key, updater(this.get(key)), cb);
+};
+
 /**
  * Close dirty db file stream, release file handle
  */

test/test-system.js

@@ -116,3 +116,15 @@ describe('test-chaining-of-constructor', function() {
     });
   });
 });
+
+describe('test-update', function () {
+  it('should give the updater the value and then set the value to what updater returns', function() {
+    var db = dirty();
+    db.set("foo", "bar");
+    db.update("foo", function (bar) {
+      assert.strictEqual(bar, "bar");
+      return "baz";
+    });
+    assert.strictEqual(db.get("foo"), "baz");
+  });
+});