# Check if array elements are equal based on function

JavaScript, Array · Oct 19, 2020

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.

```
const allEqualBy = (arr, fn) => {
const eql = fn(arr[0]);
return arr.every(val => fn(val) === eql);
};
```

```
allEqualBy([1.1, 1.2, 1.3], Math.round); // true
allEqualBy([1.1, 1.3, 1.6], Math.round); // false
```