使用括号调用方法因为括号的，该调用的结果将存储在before_add中。

With parenthesis the method is invoked because of the parenthesis, the result of that invocation will be stored in before_add.

如果没有括号，则将引用（或指针）存储到变量中的函数中。这样，只要有人调用before_add（），就会调用它。

Without the parenthesis you store a reference (or "pointer" if you will) to the function in the variable. That way it will be invoked whenever someone invokes before_add().

如果那不清楚的话;也许这会有所帮助：

If that didn't clear things up; maybe this will help:

function Foo() {
    return 'Cool!';
}

function Bar(arg) {
    console.log(arg);
}

// Store the >>result of the invocation of the Foo function<< into X
var x = Foo();
console.log(x);

// Store >>a reference to the Bar function<< in y
var y = Bar;
// Invoke the referenced method
y('Woah!');

// Also, show what y is:
console.log(y);

//Now, try Bar **with** parenthesis:
var z = Bar('Whut?');

//By now, 'Whut?' as already been output to the console; the below line will
//return undefined because the invokation of Bar() didn't return anything.
console.log(z);

如果您再看一下浏览器的控制台窗口，您应该看到：

If you then take a look at your browsers' console window you should see:

Cool!
Woah!
function Bar(arg)
Whut?
undefined

第1行是调用 Foo（）的结果，

第2行是调用 Bar（） via y ，

第3行是 y 的内容，

第4行是 var z = Bar（'Whut？'）; 行的结果; Bar函数调用，

第5行显示调用 Bar（）并将结果赋值给 z 没有返回任何内容（因此： undefined ）。

Line 1 is the result of invoking Foo(),
Line 2 is the result of invoking Bar() "via" y,
Line 3 is the "contents" of y,
Line 4 is the result of the var z = Bar('Whut?'); line; the Bar function is invoked,
Line 5 shows that invoking Bar() and assigning the result to z didn't return anything (thus: undefined).