How do i take user input and convert the string into an integer that i can use mathematically in a function?

Here are three common ways:

var myNumber = +myString;            // coerce string to number
var myFloat = parseFloat(myString);  // parse as floating point number
var myInt = parseInt(myString, 10);  // parse as a base-10 integer

Documentation:

+ operator - mdn msdn
parseFloat() method - mdn msdn
parseInt() method - mdn msdn

Each of these techniques have their own quirks. Because of the dynamic nature of JavaScript, type conversion is not simple.

parseInt()

parseInt() produces either an integer or a NaN by reading the digits from left to right. If your string contains only numeric digits, and you are using a browser that supports ECMAScript 5 (IE8 and older do not), It is very straightforward - you get those digits as a number in base 10. In IE8 or older, if the first digit is 0, you'll get the number in octal. Eg, parseInt("010") returns 10 in modern browsers and 8 in older browsers. To correct for this, always specify the radix in the second parameter. parseInt("010", 10) always returns 10, regardless of browser version.

Parsing stops at the first non-digit character. So, parseInt("4 donuts") returns 4 as does parseInt("4,000"). If the first character in the string is not a digit or + or -, you will get NaN, so you can't parse "$5".

If the string starts with 0x it is treated as Hexadecimal (base 16). parseInt("0x10") returns 16. Same thing if you explicitly specify base 16 with the radix: parseInt("10", 16).

parseFloat()

parseFloat() is similar to parseInt(). The differences are that parseFloat() only works with base 10, and, obviously, it produces a floating point number.

Addition operator (+)

In JavaScript, a string can be used as a number. When a JavaScript construct accepts a number and gets something else (such as a string), it very typically tries to "coerce" it to a number. One case where we see this behavior is the + operator. In some ways, coercion is more strict than parsing -- The entire string must be numeric, not just the beginning, or else it returns NaN. But, in another way, it's less strict -- an empty or whitespace string returns 0 when coerced to a number, but returns NaN when parsed.

Besides the addition operator, other ways to coerce a string to a number include:

• Use the Number() constructor as a function
• Pass a string to a method that accepts a number, eg Math.floor("10")
• Use a string with an arithmetic operator:

var string = "10";
var num = string * 1;
var num = string / 1;

Get the full details about how a string is coerced to a number in the ECMAScript Spec, Section 9.3.1.

There is no such thing as 'Integer', that's Java not JavaScript.

To parse a string into an integer, you can use parseInt, but if you don't care whether it's an integer or floating-point there's no need to do anything - it will automatically convert when you try to use it.

However, you cannot access a field value just by using its id. Instead you must use the DOM - 'document.getElementById("getTVLdate").value'

Use parseInt('value', 10 ) to convert a string to a number

To read the value use this syntax .. Where it selects the element based on the id and then use the value property on it to get the value.

Remember this is JavaScript and not Java

var TLVday = document.getElementById.('getTLVdate').value

Change your code to

var button = document.getElementById('calculate');

button.addEventListener('click', TLV);

function TLV() {
    var TLVday = parseInt(document.getElementById('getTLVdate').value, 10),
        OBday = parseInt(document.getElementById('getOBdate').value, 10),
        DD = 0;
    if (TLVday < OBday) {
        DD = OBday + 10;
    } else {
        DD = OBday + 30;
    }
    document.getElementById("TLVoutput").innerHTML = DD;
}

Also avoid binding events inline. Always a better idea to separate your concerns.

Check Fiddle

If you have a string you need converted to a number you can use parseInt , parseFloat if you need a floating point.

parseInt('84945');

You can give another parameter parseInt('84945',10); to specify the base, bas 10 in this case