Rename js snippets

snippets/js/s/function-based-array-element-equality.md

@@ -0,0 +1,26 @@
+---
+title: Check if array elements are equal based on function
+type: snippet
+language: javascript
+tags: [array]
+cover: orange-coffee-2
+dateModified: 2020-10-19T22:14:49+03:00
+---
+
+Checks if all elements in an array are equal, based on the provided mapping function.
+
+- Apply `fn` to the first element of `arr`.
+- Use `Array.prototype.every()` to check if `fn` returns the same value for all elements in the array as it did for the first one.
+- Elements in the array are compared using the strict comparison operator, which does not account for `NaN` self-inequality.
+
+```js
+const allEqualBy = (arr, fn) => {
+  const eql = fn(arr[0]);
+  return arr.every(val => fn(val) === eql);
+};
+```
+
+```js
+allEqualBy([1.1, 1.2, 1.3], Math.round); // true
+allEqualBy([1.1, 1.3, 1.6], Math.round); // false
+```