objectSample1.js

var obj = {
		mon : 0,
		tue	: 1,
		wed	: 2,
		thu	: undefined,
		time : {
			morning : "5-11",
			night	: "19-22"
		}
	}
	
	// ||を使ってデフォルト値
	var daytime = obj.time.daytime; 
	console.log(daytime); //undefined

	var daytime = obj.time.daytime || "11-17"; 
	console.log(daytime); //11-17

	// &&を使ってTypeErrorを防ぐ
	console.log(obj.fri);  //undefined
	console.log(obj.fri.detail);  //TypeError !!
	console.log(obj.fri && obj.fri.detail);  //undefined