为什么我不能从node和express.js中的app.get函数中提取回调？

Question

I'm trying to do this in my server.js file:

// doesn't work... don't know why.
var findBooks = function (err, books) {
    if (!err) {
        return response.send(books);
    } else {
        console.log(err);
    }
}

// Get all books
app.get('/api/books', function (request, response, err, books) {
    return BookModel.find( findBooks(err, books));
});

and it gives me a 404 error when I run a jQuery xhr in the console, and the same error when I request the page in the browser. It works if I put the callback in the usual place, moving the findBooks function into the BookModel.find function where it's called and deleting the last two arguments of app.get . I'm working though Chapter 5 of Addy Osmani's Backbone.js Applications book to give you some context. Why doesn't this work? Is doing this a good thing, or am I trying something that isn't recommended?

Answer 1

You have two problems:

1. findBooks tries to access response , but response is not in scope at that point.
2. You're calling findBooks with err and books and passing the return value to BookModel.find , when you probably want to pass the function for find to call.

Let's first solve the second problem. Rather than calling the function, just pass it:

return BookModel.find(findBooks);

That was easy. Now, to solve the first problem, we'll need to add an extra argument to findBooks . I'll assume you put it at the start for reasons I'll detail later. Then we can change the usage to look like this:

return BookModel.find(function (err, books) {
    return findBooks(response, err, books);
});

It turns out, though, that there's a more concise way to do that, using bind . bind is best known for its use in binding the value of this , but it can also be used to bind arguments:

return BookModel.find(findBooks.bind(null, response));

All together, we get:

function findBooks (response, err, books) {
    if (!err) {
        return response.send(books);
    } else {
        console.log(err);
    }
}

// Get all books
app.get('/api/books', function (request, response) {
    return BookModel.find( findBooks.bind(null, response) );
});