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var b,d;
if(!('a' in window)){
a = 1
}
if(!('b' in window)){
//'b' in window == true 所以此处赋值失败
var b = 1
}
if(!('c' in window)){
var d = 1
}
if(!('e' in window)){
f = 1
}
alert(a) //1
alert(b) //undefined
alert(d) //1
alert(f) //1