問題
var obj = {x:2, y:3};
Object.preventExtensions(obj);
obj.z = 4;
Object.keys(obj); //=>?
delete obj.y;
Object.keys(obj); //=>?
obj.x = 20;
obj.x; //=>?
答え
var obj = {x:2, y:3};
Object.preventExtensions(obj);
//プロパティ追加はできない
obj.z = 4;
Object.keys(obj); //=>["x", "y"]
//プロパティ削除は可能
delete obj.y;
Object.keys(obj); //=>["x"]
//プロパティ値変更は可能
obj.x = 20;
obj.x; //=> 20