From 8377cda0a57b72738bfad39cd2d09f034e62b320 Mon Sep 17 00:00:00 2001
From: Gleb Bahmutov <gleb.bahmutov@gmail.com>
Date: Thu, 18 Nov 2021 09:48:12 -0500
Subject: [PATCH] feat: add Nth item filter (#20)

## Every Nth item

You can quickly take every Nth item from an array

```js
it.each(items, N)(...)
```

This is the same as taking the index of the item (zero-based) and doing `k % N === 0`

```js
const items = [1, 2, 3, 4, 5, 6, ...]
it.each(items, 3)(...)
// tests item 1, 4, 7, ...
```
---
 README.md                       | 16 ++++++++++++++++
 cypress/integration/nth-spec.js | 21 +++++++++++++++++++++
 src/index.js                    |  6 ++++++
 3 files changed, 43 insertions(+)
 create mode 100644 cypress/integration/nth-spec.js

diff --git a/README.md b/README.md
index c784e26..7516cd0 100644
--- a/README.md
+++ b/README.md
@@ -152,6 +152,22 @@ it.each([
 
 See [cypress/integration/title-function.js](./cypress/integration/ title-function.js) for more examples
 
+## Every Nth item
+
+You can quickly take every Nth item from an array
+
+```js
+it.each(items, N)(...)
+```
+
+This is the same as taking the index of the item (zero-based) and doing `k % N === 0`
+
+```js
+const items = [1, 2, 3, 4, 5, 6, ...]
+it.each(items, 3)(...)
+// tests item 1, 4, 7, ...
+```
+
 ## Chunking
 
 There is a built-in chunking helper in `describe.each` and `it.each` to only take a subset of the items. For example, to split all items into 3 chunks, and take the middle one, use
diff --git a/cypress/integration/nth-spec.js b/cypress/integration/nth-spec.js
new file mode 100644
index 0000000..1cc41c8
--- /dev/null
+++ b/cypress/integration/nth-spec.js
@@ -0,0 +1,21 @@
+// @ts-check
+/// <reference types="cypress" />
+
+import '../../src'
+
+describe('nth item', () => {
+  const items = [1, 2, 3, 4]
+
+  context('every 2nd', () => {
+    // take every 2nd item, same as taking the item's zero-index module 2
+    it.each(items, 2)('every 2nd item %K', (x) => {
+      expect(x, '1 or 3').to.be.oneOf([1, 3])
+    })
+  })
+
+  context('every 3nd', () => {
+    it.each(items, 3)('every 3nd item %K', (x) => {
+      expect(x, '1 or 4').to.be.oneOf([1, 4])
+    })
+  })
+})
diff --git a/src/index.js b/src/index.js
index c06c522..0a8b29d 100644
--- a/src/index.js
+++ b/src/index.js
@@ -66,6 +66,12 @@ if (!it.each) {
       if (typeof totalChunks === 'number' && typeof chunkIndex === 'number') {
         // split all items into N chunks and take just a single chunk
         values = getChunk(values, totalChunks, chunkIndex)
+      } else if (
+        typeof totalChunks === 'number' &&
+        typeof chunkIndex === 'undefined'
+      ) {
+        // take every Nth item
+        values = values.filter((_, k) => k % totalChunks === 0)
       }
 
       values.forEach(function (value, k) {