Speed test: in versus ===

Because you can never know too many micro-optimizations...

In JavaScript, when you need to know if a property is present on an object, you have a couple of options. For a long time, I've used the following idiom:

if ('undefined' !== typeof obj['prop']) { /* do something */ }

Recently, I came upon some code using the following:

if ('prop' in obj) { /* do something */ }

I like the clarity of the syntax. Which is faster? Let's find out!

function eq_(obj, prop, iters) {
    var start, result;
    start = new Date;

    do {
        result = ('undefined' !== typeof obj[prop]);
    } while (--iters);

    $('#duration-eq').html(new Date - start);
    $('#result-eq').html(result.toString());
}

Result: ---
Duration: ---

function in_(obj, prop, iters) {
    var start, result;
    start = new Date;

    do {
        result = (prop in obj);
    } while (--iters);

    $('#duration-in').html(new Date - start);
    $('#result-in').html(result.toString());
}

Result: ---
Duration: ---