Strange behavior of the event object in EventHandler

[igrimpe]

I have an event handler:

 

function evthandler(event)
{
    if (event.name === 'input' && (typeof (event.delta) !== 'undefined'))
    {
           console.log('event: ' + JSON.stringify(event));
           console.log('event.delta: ' + JSON.stringify(event.delta));
           let b = event.delta.state;
           console.log(b);
    }
}

When the event fires, the console.log statements are done, but the script breaks with an unnamed error (see image).

NOW ... when I change the code a bit:
 

function evthandler(event)
{
    if (event.name === 'input' && (typeof (event.delta) !== 'undefined'))
    {
           console.log('event: ' + JSON.stringify(event));
           console.log('event.delta: ' + JSON.stringify(event.delta));
           let d = event.delta;
           let b = d.state;
           console.log(b);
    }
}

This code does NOT break!

If I try something like:

 

let a = JSON.parse('{ "foo" : "bar", "baz" : { "k" : 42 }}');
console.log(a.baz.k);

Then this code also works without problem.

So could anybody please explain what is so "special" about the event object and where this behavior is documented?