Determining character frequency in a string (Javascript)

Question

I'm working on a solution for determining character frequency in a string. The characters are being added to my object properly but all counts end up as NaN. (I think I'm taking a less efficient approach by splitting the string into an array of characters, but I'd like to solve this approach nonetheless.)

var charFreq = function (frequencyString) {
    var stringArray = frequencyString.split("");
    var frequencies = {};
    for (var k in stringArray) {
        var nowLetter = stringArray[k];
        if (stringArray.hasOwnProperty(k)) {
            frequencies[nowLetter] += 1;
        }
    }
    return frequencies;
}

charFreq("what is the reason for this"); 

Answer 1

Your frequencies is an object and when you access

frequencies[nowLetter] += 1;

you are accessing a previously unavailable property like frequencies.a which will be undefined. Hence you are getting NaN.

See http://jsfiddle.net/xbUtR/ for the fix.

if(frequencies[nowLetter] === undefined)
                frequencies[nowLetter] = 0;
            frequencies[nowLetter] += 1;

Answer 2

Because the property values in frequencies have an initial value of undefined and undefined + 1 == NaN

Try code like this:

var charFreq = function (frequencyString) {
    var stringArray = frequencyString.split("");
    var frequencies = {};
    for (var k in stringArray) {
        var nowLetter = stringArray[k];
        if (stringArray.hasOwnProperty(k)) {

            // One way to initialize the value -- not the only way.
            if (!frequencies[nowLetter]) {
              frequencies[nowLetter] = 0;
            }

            frequencies[nowLetter] += 1;
        }
    }
    return frequencies;
}