Generate generic HTML table from JSON

jsonToTable.js

function generateTable(arr) {

    var content = {},
        headers = [],
        longest = 0;
    
    function setEmpies(content) {
        for (var key in content) {
            if (!content[key].affected) {
                content[key].push('');
            } else {
                content[key].affected = false;
            }
        }
    }
    
    function addNulls(arr, num) {
        var i = 0;
        for (i; i<=num; i++) {
            arr.push('');
        }
    }
    
    function generateHTMLstring(headers, content, longest) {
        var i = 0,
        table = '';
        for (i; i < longest; i++) {
            var row = [];
            headers.forEach(function(val) {
                row.push(content[val][i]);
            });
            table += '<tr><td>' + row.join('</td><td>') + '</td></tr>';
        }
        return '<table><thead><tr><td>' + headers.join('</td><td>') + '</td></tr></thead><tbody>' + table + '</tbody></table>';
    }
    
    arr.forEach(function(val, idx) {
        for (var key in val) {
            if (!(key in content)) {
                content[key] = [];
                addNulls(content[key], idx);
            }
            content[key].push(val[key]);
            content[key].affected = true;
        }
        setEmpies(content);
    });
    
    for (var key in content) {
        headers.push(key);
        if (content[key].length > longest) {
            longest = content[key].length;
        }
    }
    
    return generateHTMLstring(headers, content, longest);
}

//TEST
var arr = [{a: 'a1', b: 'b1'},
           {a: 'a2', b: 'b2'},
           {a: 'a3', b: 'b3'},
           {a: 'a4', b: 'b4', c: 'c4'},
           {a: 'a5', b: 'b5'},
           {a: 'a6', b: 'b6', c: 'c6', d: 'd6'}];

document.body.innerHTML = generateTable(arr);