[Malleys]

crabkilla,

var albumInfoObj = {
	"title": "Ray of Light",
	"singer": "Madonna",
	"year": 1998,
	"songs": ["Swim", "Little Star", "Frozen", "Skin", "The Power of Good-Bye"],
	"recorded": {
		"studioName": "Larrabee North Studio",
		"months": 4.5
	},
	"singles": [{
		"name": "Frozen",
		"year": 1998
	}, {
		"name": "Nothing Really Matters",
		"year": 1999
	}, {
		"name": "Ray of Light",
		"year": 1998
	}],
}
var propList = [];
for (var prop in albumInfoObj.singles[0]) {
	propList.push(prop);
}
console.log(propList.join("\n"));

Требуется написать цикл for in,

Можно обойтись статичным методом Object.keys

var albumInfoObj = {
	"title": "Ray of Light",
	"singer": "Madonna",
	"year": 1998,
	"songs": ["Swim", "Little Star", "Frozen", "Skin", "The Power of Good-Bye"],
	"recorded": {
		"studioName": "Larrabee North Studio",
		"months": 4.5
	},
	"singles": [{
		"name": "Frozen",
		"year": 1998
	}, {
		"name": "Nothing Really Matters",
		"year": 1999
	}, {
		"name": "Ray of Light",
		"year": 1998
	}],
}

console.log(Object.keys(albumInfoObj.singles[0]).join("\n"));