Penumpang Angkot JS

let angkot = [];
let count = 0;

tambahPenumpang('Andre', angkot);
tambahPenumpang('Kevin', angkot);
hapusPenumpang('Kevin', angkot);
tambahPenumpang('Matthew', angkot);
tambahPenumpang('Joko', angkot);
tambahPenumpang('Matthew', angkot);
hapusPenumpang('Andre2', angkot);

function tambahPenumpang(namaPenumpang, angkot) {
    //check if there is undefined element in array
    let isUndefined = (element) => element == undefined;
    let checkIdx = angkot.findIndex(isUndefined);

    //check if there is duplicate element in array
    let isDuplicate = (element) => element == namaPenumpang;
    let checkName = angkot.findIndex(isDuplicate);

    //duplicate element  check
    if (checkName >= 0) {
        console.log('- ' + namaPenumpang + ' sudah ada di dalam Angkot');
    }
    //check if first seat is empty or not
    else if (angkot.length == 0) {
        angkot[0] = namaPenumpang;
    }
    //undefined check
    else if (checkIdx >= 0) {
        angkot[checkIdx] = namaPenumpang;
    }
    //add passenger to array
    else {
        angkot.forEach(function(val, i, arr) {
            arr[count + 1] = namaPenumpang;
        });

        count++;
    };

    return angkot;
};

//remove passenger from array
function hapusPenumpang(namaPenumpang, angkot) {
    //check is the passenger already seat or not
    let isAvailable = (element) => element == namaPenumpang;
    let checkPenumpang = angkot.findIndex(isAvailable);

    //check amount of passenger
    if (angkot.length == 0) {
        console.log('Angkot masih kosong!');
    }
    //remove the passenger
    else if (checkPenumpang >= 0) {
        angkot[checkPenumpang] = undefined;
    }
    //passenger isn't in the transportation
    else {
        console.log('- ' + namaPenumpang + ' tidak ada di dalam Angkot');
    };

    return angkot;
}

console.log(angkot.join(' - '));
console.log("\nJumlah Penumpang Angkot : " + angkot.length);