Is their JavaScript “not in” operator for checking object properties?

To get only the else part, just negate, using ! the operator in JavaScript. You can try to run the following code to achieve this −

var arr = {};
if (!(id in arr)) {
}
else {
}

In addition, if you want the existence of a property in an object, then use hasOwnProperty −

if (!arr.hasOwnProperty(id)) {
}
else {
}