javascript中return function与return function()的区别

参考https://stackoverflow.com/questions/7629891/functions-that-return-a-function-javascript

问题：唯一的区别是return中的函数是否带括号

输入：

function a() {

    alert('A!');

    function b(){
        alert('B!');
    }

    return b();
}

var s = a();
alert('break');
s();

输出：

A!
B!
break

输入：

function a() {

    alert('A!');

    function b(){
        alert('B!');
    }

    return b;
}

var s = a();
alert('break');
s();

输出：

A!
break
B!

回答1：

Assigning a variable to a function (without the parenthesis) copies the reference to the function. Putting the parenthesis at the end of a function name, calls the function, returning the functions return value.

http://jsfiddle.net/kaleb/Las6w/

function a() {
    alert('A');// A未在此作用域定义
}
//alerts 'A', returns undefined

function b() {
    alert('B');
    return a; //返回的是一个函数
}
//alerts 'B', returns function a

function c() {
    alert('C');
    return a();//返回函数执行结果
}
//alerts 'C', alerts 'A', returns undefined

alert("Function 'a' returns " + a());
alert("Function 'b' returns " + b());
alert("Function 'c' returns " + c());

回答2：

Returning the function name without () returns a reference to the function, which can be assigned as you've done with var s = a(). s now contains a reference to the function b(), and calling s() is functionally equivalent to calling b().

// Return a reference to the function b().
// In your example, the reference is assigned to var s
return b;

Calling the function with () in a return statement executes the function, and returns whatever value was returned by the function. It is similar to calling var x = b();, but instead of assigning the return value of b() you are returning it from the calling function a(). If the function b() itself does not return a value, the call returns undefined after whatever other work is done by b().

Returning the function name without () returns a reference to the function, which can be assigned as you've done with var s = a(). s now contains a reference to the function b(), and calling s() is functionally equivalent to calling b().

// Return a reference to the function b().
// In your example, the reference is assigned to var s
return b;
Calling the function with () in a return statement executes the function, and returns whatever value was returned by the function. It is similar to calling var x = b();, but instead of assigning the return value of b() you are returning it from the calling function a(). If the function b() itself does not return a value, the call returns undefined after whatever other work is done by b().

// Execute function b() and return its value
return b();
// If b() has no return value, this is equivalent to calling b(), followed by
// return undefined;

回答3：

return b(); calls the function b(), and returns its result.

return b; returns a reference to the function b, which you can store in a variable to call later.