From 541103321a9c908edc12d76c2dc300b8ac972e3b Mon Sep 17 00:00:00 2001
From: Angelos Chalaris <chalarangelo@gmail.com>
Date: Wed, 24 Jan 2018 11:49:03 +0200
Subject: [PATCH] Add differenceBy

---
 snippets/differenceBy.md | 17 +++++++++++++++++
 tag_database             |  1 +
 2 files changed, 18 insertions(+)
 create mode 100644 snippets/differenceBy.md

diff --git a/snippets/differenceBy.md b/snippets/differenceBy.md
new file mode 100644
index 000000000..626ba7cdc
--- /dev/null
+++ b/snippets/differenceBy.md
@@ -0,0 +1,17 @@
+### differenceBy
+
+Returns the difference between two arrays, after applying the provided function to each array element of both.
+
+Create a `Set` by applying `fn` to each element in `b`, then use `Array.filter()` in combination with `fn` on `a` to only keep values not contained in the previously created set.
+
+```js
+const differenceBy = (a, b, fn) => {
+  const s = new Set(b.map(v => fn(v)));
+  return a.filter(x => !s.has(fn(x)));
+};
+```
+
+```js
+differenceBy([2.1, 1.2], [2.3, 3.4], Math.floor); // [1.2]
+differenceBy([{ x: 2 }, { x: 1 }], [{ x: 1 }], v => v.x); // [ { x: 2 } ]
+```
diff --git a/tag_database b/tag_database
index b8afea8ab..231cf9379 100644
--- a/tag_database
+++ b/tag_database
@@ -35,6 +35,7 @@ defaults:object
 defer:function
 detectDeviceType:browser
 difference:array,math
+differenceBy:array,function
 differenceWith:array,function
 digitize:math,array
 distance:math