Javascript 条件订单评估
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Javascript conditional order evaluation
提问by atp
Can I count on Javascript failing immediately when one condition in an expression results to false?
当表达式中的一个条件结果为 false 时，我可以指望 Javascript 立即失败吗？
f = {'a':'b'};
if (f.a !== undefined || f.a === 'b') {
// Is this OK to use, because the second condition will never be evaluated?
}
回答by alex
Yes, this is known as short circuit evaluation.
是的，这就是所谓的短路评估。
With an ANDlogical operator, if the first evaluates to false, then the second is never evaluated, because the condition knows enough already to be met.
对于AND逻辑运算符，如果第一个计算结果为false，则永远不会计算第二个，因为条件已经足够满足。
With the ORlogical operator, if the first one is false, it will evaluate the second one. Otherwise if the first is trueit won't evaluate the second (no need to).
使用OR逻辑运算符，如果第一个是false，它将评估第二个。否则，如果第一个是true它不会评估第二个（不需要）。
This is also why you see...
这也是为什么你看到...
var a = function(b) {
b = b || 7;
}
