flattenDepth.js

const flattenDepth = (arr, depth = 1) =>
  arr.reduce(
    (acc, cv) => [
      ...acc,
      ...(depth === 0 || !Array.isArray(cv)
        ? [cv]
        : flattenDepth(cv, depth - 1))
    ],
    []
  );

// flattenDepth([1, [2, [3, [4]], 5]], 3) => [1, 2, 3, 4, 5]