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

@@ -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

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])
+    })
+  })
+})

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) {