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var a;
a = a?a+1:0;
alert(a)
if (a) 如果a未定义或是undefined a = 0 否则a+1
look:
if (undefined) {
a = 0;
} else {
a = 1
}
if (!undefined) {
a = 3;
}
只需要理解alert(!undefind); alert(!!undefined);
还有
if (undefined) {
a = 0;
} else {
a = 1
}