elinks/test/ecmascript/assert/customEvent.html

<script>
var e = new CustomEvent('message', { cancelable: true, detail: {name:"cat"}});

console.error('customEvent.html');
console.assert(e.cancelable, 'cancelable')
console.assert(!e.defaultPrevented, 'defaultPrevented false');
e.preventDefault();
console.assert(e.defaultPrevented, 'defaultPrevented true');
console.assert(e.type === 'message', 'message');
console.assert(e.detail.name === 'cat', 'cat');
console.exit(0);
</script>