let a = {
	name: 'Sergey',
	surname: 'Honcharenko',
	age: 28,
	isMarried: false,
}

let b = {
	name: 'Andrew',
	surname: 'Klymchuk',
	middleName: 'Anatoliyovych',
	haveAJob: true,
}

let c = {
	name: 'Alexander',
	surname: 'Slobodian',
	sex: 'male',
	hasAPet: false,
}

if('age' in a) {
	alert('age');
}

if('isMarried' in a) {
	alert('isMarried');
}

if('middleName' in b) {
	alert('middleName');
}

if('haveAJob' in b) {
	alert('haveAJob');
}

if('sex' in c) {
	alert('sex');
}

if('hasAPet' in c) {
	alert('hasAPet');
}