• MDN : JavaScript Operator - in

index in Array
attribute in Object

in操作符的计算结果是boolean类型。

• 判断指定的数字是否为数组元素的索引。
• 判断给定的字符串是否为对象的属性。

var names = ["Kent Beck", "Erich Gamma", "James Gosling", "Doug Lea", "Bob Lee"];
if (10 in names) {
    console.log('length enough!');
} else {
    //TODO
}

var person = {
    name : "Kent Beck",
    age : 56,
    gender : "male",
    address : "美国俄勒冈州科瓦利斯市"
};
if ("address" in person) {
    console.log(person["address"]);
} else {
    console.log("地址不详");
}