Javascript：将参数传递给回调函数

Question

I want to call a function on the returned data from this database query, as in the snippet below.

function connectDB(query, data, callback) {
    pg.connect(process.env.DATABASE_URL, function (err, client) {
        if (err) {
            console.log(err);
        } else {
                client
                    .query(query, data, function (err, result) {
                        callback(result.rows);                            
                    });
        }
     }
}

function foo(data) {
  connectDB("SELECT * FROM mytable WHERE id=$1", someID, callbackfn(data));
}

I don't know how to write callbackfn so that I can use both data from the original function and rows from the db result.

function callbackfn(data) {
  return function(rows) {
   // Do something with rows and data
  }
}

Answer 1

You don't accept any params with the callback function. How about:

function callback(rows) {
  /* not sure what your intention was here? nested functions? */
  return function () {
    return function () {
        // Do something with rows
    }
  }
}

And to be clear, can you post where you call the connectDB function? As callback is name of the variable after it is passed to connectDB .

Answer 2

Why are you defining callback function? Please elaborate on this. Also, try calling connectDB like this:

 connectDB(query, data, function(rows) {
       // Do something with rows
 });

Read this answer on callbacks.

Answer 3

You have an incredible level of nested functions. Moreover, you are defining callback as parameter, but it won't take the callback function you defined, if you are not calling the base function passing again "callback" as argument. If you want to pass always the same callback, it doesn't make sense that you provide it as argument, you can directly call "callback" inside your "connectDB" function. Moreover your callback returns a function, so it should be called again using again () . Then your main callback function needs to accept rows as parameter from outside. I kept your code using the callback parameter because I "hope" that the same name was just an example to explain that you want to provide every time a function in order to manipulate as you want the rows. You code should look like this:

function connectDB(query, data, callback) {
    pg.connect(process.env.DATABASE_URL, function (err, client) {
        if (err) {
            console.log(err);
        } else {
                client
                    .query(query, data, function (err, result) {
                        // this returns a function so it needs to be called again
                        callback(result.rows)();                            
                    });
        }
     }
}

// third inner function was useless
function callback(rows) {
  return function () {
    // Do something with rows
  }
}

// the third parameter HAS TO BE callback, otherwise you will not pass the function you defined. 
connectDB(query, data, callback);