Flatten's an array any level deep. (for Citrus Byte) [written in ES6 JS]

flatten.js

//Mutable version
const flatten = (arr) =>
	arr.reduce((c, v) =>
      (Array.isArray(v))
        ? c.concat(flatten(v))
        : [...c, v]
  , []);

//or more effecient, in place
const flatten_in = (arr) =>
	arr.reduce((c, v) => {
      (Array.isArray(v))
        ? c.push(...flatten(v))
        : c.push(v)
      
      return c;
    }, []);

console.log(flatten([[1,2,[3]],4]));

export default flatten;
export flatten_in;