Fibonacci Series

// Take input a number from user
var numberRegEx = /[+-]?([0-9]*[.])?[0-9]+/;
var bool = true;
var ahead = true;
while (bool) {
    var userInput = Number(prompt('Enter number of terms to print Fibonacci Series...'));
    if (numberRegEx.test(userInput)) {
        bool = false;
    } else {
        window.alert('Number is not valid!!');
        bool = window.confirm('Do you want to enter again..');
        ahead = bool;
    }
}
if (ahead) {

    // Displaying number which is entered by user
    document.write('Entered number by you = <strong>' + userInput + '</strong><br><br>');

    // Displaying Fibonacci series
    var firstTerm = 0;
    var secondTerm = 1;
    var temp;
    for (var i = 0; i <= userInput; i++) {
        document.write(firstTerm); // here print the term of fibonacci series
        if (i < userInput) {
            document.write(', '); // here print "," (comma) if next term is exist otherwise not
        }
        temp = firstTerm; // here first term is stored in temporary variable
        firstTerm = secondTerm; // here second term stored in first term
        secondTerm = temp + firstTerm; // here add previous values of first and second term in second term
    }
}