Approximating the width of a string in JavaScript

textWidth.js

var textWidth = (function() {
    function charW(w, c) {
        if (c == 'W' || c == 'M') w += 15;
        else if (c == 'w' || c == 'm') w += 12;
        else if (c == 'I' || c == 'i' || c == 'l' || c == 't' || c == 'f') w += 4;
        else if (c == 'r') w += 8;
        else if (c == c.toUpperCase()) w += 12;
        else w += 10;
        return w;
    }

    return function(s) {
        return _.reduce(s.split(''), charW, 0);
    };
})();