Flatten javascript object

index.js

let o = {
    k: 10,
    a: {
        b: {
            c: ["10", "20"]
        },
    },
    d: {
        e: 20
    },
    f: "30",
    g: {
        h: {
            i: {
                j: "40"
            }
        }
    }
}
let flatObject = {};
let originalObject = null;
function flattenObject(o, key = "") {
    for (let prop in o) {
        const type = typeof o[prop];
        if (type !== "object" || Array.isArray(o[prop])) {
            // if the value is string add it to flat object as a.b: 10
            flatObject[key + prop] = o[prop];
        } else {
            // if its a nested object modify the key and recurse
            key += prop + ".";
            flattenObject(o[prop], key);
        }
    }
    return flatObject;
}

console.log(flattenObject(o));