Convert javascript flatten function into typescript

I am stuck in converting the flatten function into typescript. Code sample is given below. I am stuck at giving type to function argument ob.

// Declare an object
let ob = {
    Company: "GeeksforGeeks",
    Address: "Noida",
    contact: +91-999999999,
    mentor: {
        HTML: "GFG",
        CSS: "GFG",
        JavaScript: "GFG"
    }
};

// Declare a flatten function that takes
// object as parameter and returns the
// flatten object
const flattenObj = (ob) => {

    // The object which contains the
    // final result
    let result = {};

    // loop through the object "ob"
    for (const i in ob) {

        // We check the type of the i using
        // typeof() function and recursively
        // call the function again
        if ((typeof ob[i]) === 'object' && !Array.isArray(ob[i])) {
            const temp = flattenObj(ob[i]);
            for (const j in temp) {

                // Store temp in result
                result[i + '.' + j] = temp[j];
            }
        }

        // Else store ob[i] in result directly
        else {
            result[i] = ob[i];
        }
    }
    return result;
};

console.log(flattenObj(ob));

Any help in converting this function into typescript will be highly appreciated.