Question

How can I check if a number has a -, +, x or nothing in front of it?

Options to check would be:

20
+20
-20
x20

20 being any number.

Answer 1

If you wanted to be a little more flexible and allow whitespace so you were looking for the first non-whitespace character that comes before a digit, you could use a regular expression:

var str = " + 20";

var match = str.match(/^\s*([^\d])/);
if (match) {
    switch(match[1]) {
        case "+":
            // put code here to handle this operator
            break;
        case "-":
            // put code here to handle this operator
            break;
        case "x":
            // put code here to handle this operator
            break;
        default:
            // put code here to handle unknown operator
            break;
    }
}

Working example here: http://jsfiddle.net/jfriend00/fJcQJ/

Or, if you wanted a function that would return prefix and number, you could do this:

function parseNum(str) {
    var match = str.match(/^\s*([\+\-x]?)\s*(\d+)/);
    if (match) {
        return({
            num: parseInt(match[2], 10),
            prefix: match[1] ? match[1] : ""
        });
    }
    // nothing found of the right format
    return(null);
}

And, a working example with test cases: http://jsfiddle.net/jfriend00/DCxcb/