JavaScript Function to Validate Zip codes (valid zipcode)

This function returns True if a Zipcode is a validly formatted U.S. Zip code, false if it is not.

/*

	ValidZipCode - true for valid zip codes, false for invalid ones

*/

function ValidZipCode(ZipCode)

{

	//Your zip code must contain 5 or 9 digits.

        // 9 digit zip codes should contain no spaces and a

        // hyphen before the last 4 digits.

	

	var stringValue = new String(ZipCode);

	var stringLength = stringValue.length;

	

	if ((stringLength!=5)&&(stringLength!=9)&&(stringLength!=10))

	{

		return false;

	}



	if(stringLength==5)

	{

		

		for (var i = 0; i < stringLength; i++)

		{

			value = stringValue.charAt(i)

			if (!((value >= 0) && (value <=9)))

			{

				return false;

			}

		}

	}

	

	if(stringLength==9)

	{

		for (var i = 0; i < stringLength; i++)

		{

			value = stringValue.charAt(i)

			if (!((value >= 0) && (value <=9)))

			{

				return false;

			}

		}

	}



	if(stringLength==10)

	{

		var zip=stringValue.substring(0,5)

		var symbol=stringValue.substring(5,6)

		var plus4=stringValue.substring(6,10)

	

		if(symbol!="-")

		{

			return false;

			

		}

			

			

		for (var i = 0; i < zip.length; i++)

		{

			value = zip.charAt(i)

			if (!((value >= 0) && (value <=9)))

			{

				return false;

			}

		}

		for (var i = 0; i < plus4.length; i++)

		{

			value = plus4.charAt(i)

			if (!((value >= 0) && (value <=9)))

			{

				return false;

			}

		}

		

	}

	return true;

}