什么时候直接用函数名调用函数，什么时候要用"函数名（）"？
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以我的理解，给你举个例子，参考参考：
function ab(){ return c; } var me = ab(); // (1) var me = ab; // (2)
(1)是调用；(2)是赋值。
【(1)中，等价于 var me = c。那么这是赋值还是调用？不要被误导，这是调用。先调用了函数ab，调用完返回了c，赋值给变量me。】
那么在什么情况下是赋值？
比如 btn.onclick = ab;这种情况，就是把函数给系统，让系统去调用。为什么不是btn.onclick = ab(); 如果是这样，文件加载到这一行，他就会直接执行，执行完，如果有返回值，就会给btn.onclick返回值，如举例中返回的是c。如果没有返回值，则返回null，这样相当于btn.onclick = null。
后者才是函数的调用吧，函数名只是代表了一个函数，单独写出来并不能执行，函数名（）才可以执行函数，自己的一点理解，不知道能不能帮到你