Does it mutate 😱

Array.prototype.reduceRight

No mutation

Description

The reduceRight() method of Array instances applies a function against an accumulator and each value of the array (from right-to-left) to reduce it to a single value.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/reduceRight

Example

const array1 = [
  [0, 1],
  [2, 3],
  [4, 5],
];

const result = array1.reduceRight((accumulator, currentValue) =>
  accumulator.concat(currentValue),
);

console.log(result);
// Expected output: Array [4, 5, 2, 3, 0, 1]