jsオブジェクト存在チェック

オブジェクト存在チェック

if-in

var hoge = {aa : 'good'};
if ('aa' in hoge) {}

var myObj = {a: 1, b: 2, c: 3}, myKeys = [], i=0;
for (myKeys[i++] in myObj);
console.log(myKeys);

上記のやり方は賢いw、以下のリンクをご参照
Exploring JavaScript for-in loops

hasOwnProperty()

obj.hasOwnProperty();

上記の違いは以下のストックをご参照
in と hasOwnProperty() の違い

注意点

for .. inの場合はhasOwnPropertyの判断を入れた方が安全、原因は

Some frameworks (e.g. Prototype.js and Mootools) add a lot of custom prototype augmentation and using for-in to iterate Arrays and Strings is generally considered a bad idea. Using a regular for loop is a good alternative for Array and String iteration. In addition, ES5 defines a bunch of custom Array iterators (forEach, map etc). Unfortunately non of these alternate iteration strategies work with regular Objects – which is why its considered very bad practice to augment Object.prototype.

例：

var arr = ['a','b','c'], indexes = [];
Array.prototype.each = function() {/*blah*/};

for (var index in arr) {
    indexes.push(index);
}

indexes; //["0", "1", "2", "each"] whoops!

if

var hoge = {aa : 'good'};
if (hoge['aa'])

ifもfor .. inと同じくhasOwnPropertyで判断必要あり

結論

Arrayの場合はfor文かArray.forEach(every,some)で使うベキ
Objectの場合はfor .. in + hasOwnPropertyでやる