Joey Javas

To add an option to a dropdown select box with JavaScript, first create the option that will be added. Then add the option, as shown in the following example.

If this is the select list:

<select name='dropdown' id='dropdown'>
  <option value='1'>1</option>
  <option value='2'>2</option>
</select>

This JavaScript will add an option at the end of the list with a value of 3 and a label of 3:

var objDropdown = document.getElementById('dropdown');
var objOption = new Option('3','3');
document.write(objOption.value);
objDropdown.options[objDropdown.length] = objOption;

The two arguments to the Option method are text and value. Text is what shows in the dropdown box and is the first argument. Value is what is sent with the form submission and is the second argument. Note also that the length of the dropdown box is used to put the new option at the end of the list. A value could be hard-coded here, if desired.

Related articles:
How to Remove All Options from a Dropdown Select Box with JavaScript
How to Remove an Option from a Dropdown Select Box with JavaScript

To remove an option from a dropdown select box with JavaScript, use the JavaScript remove method. Pass it the index of the option that should be removed. The index is 0-based, so to remove the first item in the select list pass 0, to remove the second pass 1 and so forth.

If this is the select list:

<select name='dropdown' id='dropdown'>
  <option value='1'>1</option>
  <option value='2'>2</option>
  <option value='3'>3</option>
</select>

The second option can be removed as follows:

document.getElementById('dropdown').remove(1);

Related articles:
How to Remove All Options from a Dropdown Select Box with JavaScript

A function can be created in JavaScript by using the function statement, as follows:

function CalculateTotal(intInput)
{
  var intTotal = 0;
  intTotal = intInput*2;
  return intTotal;
}

To call this function in JavaScript code, simply do this:

CalculateTotal(4);

The curly braces are required as part of the function statement.

To remove all options from a dropdown select box with JavaScript, simply set the length property of the dropdown box to 0. For instance, if a dropdown select box is defined as follows:

<select name='Dropdown' id='Dropdown'>
  <option value='1'>1</option>
  <option value='2'>2</option>
</select>

Use this JavaScript to remove all the options in the dropdown select box.

document.getElementById('Dropdown').length = 0;

To remove all the elements from a JavaScript array, simply set the array length to 0, as shown in the following example:

var aryTest = [1,2,3,4];
document.write(aryTest.length);

aryTest.length = 0;
document.write(aryTest.length);

Related articles:
JavaScript Arrays Summary and Reference
JavaScript Array length Property

At work recently the need came up to determine the number of days in a month for a given month and year. This can be accomplished by creating a date object for the given month and year. Then do a while loop. In each iteration of the loop, set the day of the month, starting with 29, which is one more than the fewest days that can occur in a month. After setting the day, check if setting that day changed the month. If it did, then the number of days has been found. Otherwise, the loop iterates again.

For example, for September of 2007, the loop would use setDate() to set a date for September 29th. After doing this, the month would still be 8 (JavaScript dates are zero-relative). The loop would run again. setDate() would set the 30th and the month would still be 8. The loop would run again. setDate() would set the 31st. Since there is no 31st in September, setDate() actually sets October 1st, thus making the month 9. Since the month has changed, we know there are 30 days in September.

Code:

var intMonth = 7;//given month
var intYear = 2007;//given year

var dteMonth = new Date(intYear,intMonth);//
var intDaysInMonth = 28;//the fewest number of days in a month
var blnDateFound = false;//Set a variable to check on the while loop

while (!blnDateFound)
{
dteMonth.setDate(intDaysInMonth+1);//create the next possible day
var intNewMonth = dteMonth.getMonth();//new month date

if (intNewMonth != intMonth)//if the month has changed
  blnDateFound = true;
else
  intDaysInMonth++;
}
document.write(intDaysInMonth);

To include an external JavaScript file in a file, use the script tag.

<script src='../filename.js'></script>

The src attribute refers to the path where the included file resides. The file specified is a file that contains JavaScript only and has no script tags.

Note that the end script tag immediately follows the beginning script tag.

Why use the script tag to include a JavaScript file instead of embedding the JavaScript directly within the HTML file? A number of advantages are listed below:

1) Allows for the separation of presentation and scripting.
2) Files can be included in an HTML page from other web servers.
3) Provides for modular and reusable code. For instance, a function can be created once, placed in the .js file and then used among multiple pages.
4) Placing JavaScript in it’s own file causes the browser to cache the JavaScript when it is used among multiple pages.

To determine the operating system of the client machine on which a browser is executing JavaScript, use the appVersion property of the JavaScript navigator object, as demonstrated in the following example:

document.write(navigator.appVersion);

The information given in this property can be extrapolated, albeit in a somewhat rudimentary way, to determine whether the operating system is Windows or Macintosh, as shown below:

var strOS = navigator.appVersion;

if (strOS.toLowerCase().indexOf('win') != -1)
document.write('Windows');
else if (strOS.toLowerCase().indexOf('mac') != -1)
document.write('Macintosh');

Related articles:
JavaScript String Object indexOf Method