SOURCE

const input = {
    a: {
        b: 'hello',
        c: {
            d: 'world'
        }
    },
    e: 'hello world'
}

function flatternObject(obj,parentKey='',result={}){
    for(const key in obj){
        const newKey = parentKey ? `${parentKey}.${key}` :key;

        if(typeof obj[key] === 'object' && obj[key] !== null){
            flatternObject(obj[key], newKey, result);

        } else {
            result[newKey] = obj[key];
        }
    }
    return result
}

const output = flatternObject(input);
console.log(output)