JavaScript 判断值是否在数组中的方法：in 与 indexOf

> var b = ["aa", "bb"]
> "aa" in b

>>> a = ["aa" , "bb"]
>>> "aa" in a
True
>>>

// Arraysvar trees = ['redwood', 'bay', 'cedar', 'oak', 'maple'];
0 in trees // returns true
3 in trees // returns true
6 in trees // returns false
'bay' in trees // returns false (you must specify the 
   // index number, not the value at that index)
'length' in trees // returns true (length is an Array property)
Symbol.iterator in trees // returns true (arrays are iterable, works only in ES2015+)

// Predefined objects'PI' in Math  // returns true

// Custom objectsvar mycar = {make: 'Honda', model: 'Accord', year: 1998};
'make' in mycar // returns true
'model' in mycar // returns true

> var b = ["aa", "bb"]
undefined
> "aa" in b
false
> b.indexOf("aa")
0
> b.indexOf("aaa")