First initialize the two variables, please pay attention to the subtle differences:
typeof a[0]; // 'object'
typeof b[0]; // 'object'
IE(6|7 |Performance under 8):
typeof a[0] ; // 'undefined'
typeof b[0]; // 'object'
It should be noted that under IE(6|7|8), a is only missing than b [0] is an object member.
I have submitted this bug to the official. Before getting the fix, I’d better add the backslash honestly.