调用函数和传递函数有什么区别？

Question

This conversation came up at work today and I wasn't completely able to grasp the difference, and my google search so far has been fruitless.

The example that was used at work was the difference between

DoSomethingFunction().then(function(){
                 DoSomethingElse();
          });

and

DoSomethingFunction().then(DoSomethingElse());

To me, I look at the first, and it's an anonymous function, so that would just get called and inside it, is the function we want to call, but that's why I used the second version, which just calls the function without wrapping it in a separate function.

Can somebody explain the difference to me?

Answer 1

Functions in JavaScript are objects, and can be passed around just like any other object. DoSomethingElse is a function object that can be executed with DoSomethingElse() .

.then expects a function object, so the following just executes the function and returns its value or undefined if it returns nothing:

 // this is wrong, unless `DoSomethingElse` returns a function
DoSomethingFunction().then(DoSomethingElse());

The following passes an anonymous function (this is what .then expects, a function), then executes DoSomethingElse inside.

DoSomethingFunction().then(function(){
  DoSomethingElse();
});

The following passes the function object to then , and does the same as the above:

DoSomethingFunction().then(DoSomethingElse);

Answer 2

Passing a function means, that a function is passed as an argument, and it can be called in the called function. Calling a function means that the function body is executed, and it returns a value. In your example

DoSomethingFunction().then(DoSomethingElse());

you pass the result of a function call, so the result of calling DoSomethingElse (the return value). In case of

DoSomethingFunction().then(DoSomethingElse);

you pass a function, which then needs to be executed in the then function to get it's result.