Added ignoreValue option

README.md

@@ -208,6 +208,36 @@ unflatten({
 // }
 ```
 
+### ignoreValue
+
+Ignore specific values from flattening.
+
+```javascript
+var flatten = require('flat')
+var unflatten = require('flat').unflatten
+
+flatten({
+    key1: {
+        keyA: 'valueI'
+    },
+    key2: {
+        keyB: 'valueII'
+    },
+    key3: { a: { b: { c: 2 } } }
+}, {
+    ignoreValue: function(value){
+      return !!value.c;
+    }
+})
+
+// {
+//   'key1.keyA': 'valueI',
+//   'key2.keyB__': 'valueII',
+//   'key3.a.b': {'c':2},
+// }
+
+```
+
 ## Command Line Usage
 
 `flat` is also available as a command line tool. You can run it with 

index.js

@@ -33,7 +33,8 @@ function flatten (target, opts) {
         : transformKey(key)
 
       if (!isarray && !isbuffer && isobject && Object.keys(value).length &&
-        (!opts.maxDepth || currentDepth < maxDepth)) {
+        (!opts.maxDepth || currentDepth < maxDepth) &&
+        (!opts.ignoreValue || !opts.ignoreValue(value))) {
         return step(value, newKey, currentDepth + 1)
       }
 

test/test.js

@@ -206,6 +206,30 @@ suite('Flatten', function () {
       'hello.0500': 'darkness my old friend'
     })
   })
+
+  test('Ignore Values', function () {
+    assert.deepStrictEqual(flatten({
+      hello: {
+        world: {
+          again: 'good morning'
+        }
+      },
+      lorem: {
+        ipsum: {
+          dolor: 'good evening'
+        }
+      }
+    }, {
+      ignoreValue: function (value) {
+        return !!value.again;
+      }
+    }), {
+      'hello.world': {
+        again: 'good morning'
+      },
+      'lorem.ipsum.dolor': 'good evening'
+    })
+  });
 })
 
 suite('Unflatten', function () {