Fastest way to flatten / un-flatten nested JSON objects

Question

I threw some code together to flatten and un-flatten complex/nested JSON objects. It works, but it\'s a bit slow (triggers the \'long script\' warning).

For the flat

Answer 1

ES6 version:

const flatten = (obj, path = '') => {        
    if (!(obj instanceof Object)) return {[path.replace(/\.$/g, '')]:obj};

    return Object.keys(obj).reduce((output, key) => {
        return obj instanceof Array ? 
             {...output, ...flatten(obj[key], path +  '[' + key + '].')}:
             {...output, ...flatten(obj[key], path + key + '.')};
    }, {});
}

Example:

console.log(flatten({a:[{b:["c","d"]}]}));
console.log(flatten([1,[2,[3,4],5],6]));