index.js

function bankAccount(accNo, accName, balance) {
    this.accNo = accNo;
    this.accName = accName;
    this.balance = balance;
}

bankAccount.prototype.deposit = function (dAmount) {
    if (isNaN(parseFloat(dAmount))) {
        return console.log('please enter the amount in numbers')
    } else if (Math.sign(dAmount) === -1) {
        return console.log('please enter a positive number')
    } else {
        return console.log(`you have deposited ${dAmount} and this is your balance ${this.balance += dAmount}`)
    }
}

bankAccount.prototype.withdraw = function (wAmount) {
    if (wAmount > this.balance) {
        return console.log(`you do not have sufficient balance in your account. Your account balance is ${this.balance}`)
    } else {
        return console.log(`you have withdrawn ${wAmount} and this is your balance ${this.balance -= wAmount}`)
    }
}
bankAccount.prototype.getBalance = function () {
    return console.log(`your balance is ${this.balance}`)
}
bankAccount.prototype.generateIBAN = function() {
    const IBAN =  Math.random().toString(36).substring(3, 8).toUpperCase()
    
    return console.log(` your IBAN number is ${IBAN + this.accNo}`)
}

user1 = new bankAccount(001, 'jane', 10000)
console.log(user1.deposit(5000))
console.log(user1.withdraw(20000))
console.log(user1.getBalance())
console.log(user1.generateIBAN())