[destus]

Какая разница?

let obj = {id: 123},
	s;
	
	try{
		s = 'keeeek' + obj[id];
		alert(s);
	}
	catch(e){
		alert(e);
	}

	try{
		s = 'blablabla' + obj['id'];
		alert(s);
	}
	catch(e){
		alert(e);
	}