js callback && callback()

js 中有时会这么写：
callback && callback();

解释：

t's a shorthand conditional.

If the left of the && is truth-y, then whatever is on the right side of the && is executed.

This statement says that if callback is defined and truth-y (not null, false, or 0), execute it.

要是callback被定义了，那么执行callback()

 
http://stackoverflow.com/questions/19120317/what-does-callback-callback-mean-in-javascript