Add allEqualBy

snippets/allEqualBy.md

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+---
+title: allEqualBy
+tags: array,intermediate
+---
+
+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
+```