repl.js

var logged;
var logPre;
var returned;
var $0;
var p = prompt('Enter expression to evaluate');
do {
    try {
        logged = [];
        returned = $0 = eval(p);
        try {
            if ([RegExp, Function].indexOf(returned.constructor) > -1) {
                returned = 'Returned (' + returned.constructor.toString().match(/[A-Z]\w+/)[0] + '):\n' + returned;
            } else {
                returned = 'Returned:\n' + JSON.stringify($0, null, '    ');
            }
        } catch (f) {}
    } catch (e) {
        returned = 'Errored: ' + e.message;
    }
} while (p && (p = prompt(
    (logged.length ? 'logged:\n' + logged.join('\n') + '\n' : '') +
    returned
, p)));
setTimeout(function() {
    document.body.removeChild(logPre)
}, 10000);

function log(arg) {
    try { arg = JSON.stringify(arg, null, '    '); } catch (e) {}
    if (!logPre) {
        logPre = document.createElement('pre');
        logPre.style.position = 'fixed';
        logPre.style.top = logPre.style.left = '10px';
        logPre.style.zIndex = 999;
        logPre.style.background = 'wheat';
        logPre.style.padding = '5px';
        document.body.appendChild(logPre);
    }
    logPre.innerText = arg;
    logged.push(arg);
}