Get all unique characters on a webpage

unique-chars.js

(function(){
    var chars = [];

    var logCharacters = function(str) {
        var letters = str.split('');
        letters.forEach(function(l){
            l = l.trim(); // we don't need spaces
            if (chars.indexOf(l) === -1) {
                chars.push(l);
            }
        });
    };

    var walkDOM = function () {
        var elements = document.getElementsByTagName('*');
        var l = elements.length;
        for (var i = 0; i < l; i++) {
            var element = elements[i];

            for (var j = 0; j < element.childNodes.length; j++) {
                var node = element.childNodes[j];

                if (node.nodeType === 3) {
                    var text = node.nodeValue;
                    logCharacters(text);
                }
            }
            console.log('Processed element ' + i + '/' + l);
        }
        console.log('Found ' + chars.length + ' unique characters');
        console.log(chars);
    };

    walkDOM();
})();