will flatten an array of arbitrarily nested arrays of primitives into a flat array of primitives

snippet-deep-flatten-array.js

// functional programming using recursion in ES2015 syntax
const flatten = (arr = []) => arr.reduce(
  (prev, curr) => prev.concat(Array.isArray(curr) ? flatten(curr) : curr), []
);

Run sample
flatten([undefined, null, [true, false], [1, 2, [3, , , , , , , 4, 5], 6], 'seven'])
or
flatten()