This code will flatten an array of arbitrarily nested arrays of integers into a flat array of integers

SampleFlatten.js

//e.g. [[1,2,[3]],4] -> [1,2,3,4]
let complexArray = [[1,2,[3]],4];

//Legacy Approach
function flatten(arr) {
  return arr.reduce(function (flat, toFlatten) {
    return flat.concat(Array.isArray(toFlatten) ? flatten(toFlatten) : toFlatten);
  }, []);
}

console.log(flatten(complexArray));

//Modern Approach
//let modernFlat = complexArray.flat(Infinity);
//console.log(modernFlat);