There are several things at play here.
First, to check if the object exists in the current scope, you should use the typeof operator:

let objectExists = typeof myObject !== 'undefined'

this way, the interpreter won't throw an error.

Second, to check if the object has a specific property, it's still best to use the old in operator

let propertyExists = 'myProperty' in myObject

Both of these expressions return a boolean value, true or false.
So, for safe checking, you might use:

if(typeof myObject !== 'undefined' && myProperty in myObject) {
  // do your stuff
}

if you try to use the newer form of myObject?.myProperty it will still throw a ReferenceError if you haven't declared myObject

and if you use myObject.hasOwnProperty(myProperty) you might get a misleading result, if your object inherits from another and the property belongs to the ancestor

if myObject doesn't exist at all, use try/catch

let exists;
try {
exists = myObject.hasOwnProperty('myProperty');

}catch(err){
console.log(err.message) 
}

if it's null or undefined, Use ?. (optional chaining) operator:

let exists = myObject?.hasOwnProperty('myProperty');

Since the myObject variable might not be defined (as per your question), you must wrap your code with try/catch in order for the flow not to break:

let exists = false;

try{
  // try to access "a" property of myObject
  exists = myObject?.a;
}
catch{
  // because myObject is not defined, the catch block is invoked
  console.log("myObject", "is not defined")
}

console.log({exists})