Add cartesianProduct

snippets/cartesianProduct.md

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+---
+title: cartesianProduct
+tags: array,beginner
+---
+
+Calculates the cartesian product of two arrays.
+
+- Use `Array.prototype.reduce()`, `Array.prototype.map()` and the spread operator (`...`) to generate all possible element pairs from the two arrays.
+
+```js
+const cartesianProduct = (a, b) =>
+  a.reduce((p, x) => [...p, ...b.map(y => [x, y])], []);
+```
+
+```js
+cartesianProduct(['x', 'y'], [1, 2]);
+// [['x', 1], ['x', 2], ['y', 1], ['y', 2]]
+```