How to make a proper Node.js error callback?

Question

I'm trying to make a proper asynchronous function which contains a callback but apparently my way of doing this doesn't work.

Here's my code:

var fs = require('fs');
var path = require('path');

var tmp = 0;
var i = 0;

function getFilesByExtArgs(dir, ext, function(err)){
    if (err){
        console.log("Error: " + err.code + " (" + err.message + ")");
        return;
    }
    else{
        fs.readdir(dir, function (err, data){
                while (i <= data.length){
                    if (path.extname(data[i]) == ('.' + ext))
                        console.log(data[i]);
                    i++;
                }
            });
    }
}
module.exports = getFilesByExtArgs;

When I try to launch that, I get the following error:

function getFilesByExtArgs(dir, ext, function(err)){
                                     ^^^^^^^^
SyntaxError: Unexpected token function

How can I make it "the node way" ?

Venemo · Accepted Answer · 2015-06-02 15:14:58Z

2

You just need a parameter for your function and then call it. You can then expect the caller of the function to provide a callback function to that parameter.

Another mistake is your use of the while loop. You're better off if you use a for loop and declare the i variable inside it. That way, you can make sure that no other function will touch the i variable.

Also, instead of i <= data.length you need i < data.length. If you access data[data.length] you will get out of range.

Of course inside the getFilesByExtArgs function you may want to check if the parameter really is a function. You can do it with the typeof operator. For example:

if (typeof(callback) !== "function") {
    throw new Error("The callback parameter must be a function");
}

Here's your code, with a fixed syntax: I assume you want a callback not just for errors, but for actual results as well? Then you can do it like this:

var fs = require('fs');
var path = require('path');

function getFilesByExtArgs (dir, ext, callback) {
    if (typeof(callback) !== "function") {
        throw new Error("The callback parameter must be a function");
    }
    fs.readdir(dir, function (err, data) {
        if (err) {
            return callback(err);
        }
        var results = [];
        for (var i = 0; i < data.length; i++) {
            if (path.extname(data[i]) == ('.' + ext)) {
                results.push(data[i]);
            }
        }
        callback(null, results);
    });
}

You can call it like this:

getFilesByExtArgs('/my/file/path', 'txt', function (err, results) {
    if (err) {
        // Something bad happened
        console.log("an error occoured!");
        return;
    }

    // You can use the results array
    console.log("printing the results:");
    for (var i = 0; i < results.length; i++) {
        console.log(i, results[i]);
    }
});

Hope this helps.

edited Jun 2, 2015 at 15:14

answered Jun 2, 2015 at 14:00

Venemo

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7 Comments

Mehdi Over a year ago

I don't understand what should I right after the if when I call my function, right now, when I call the function I have:

getFilesByExtArgs(process.argv[2], process.argv[3], function(results){         if (results instanceof Error)             console.log("Error"); });

Venemo Over a year ago

@Mehdi You should put some error handling in there. What do you want to do if something bad happens?

Mehdi Over a year ago

I just want to right an error message if something is wrong, otherwise display my list normally.

andrezsanchez Over a year ago

This is not "the node way", the callback should have separate parameters for the error and for the return values.

Venemo Over a year ago

@andrezsanchez I think that's only a matter of opinion, but I edited my answer to suit your way.

|

James · Accepted Answer · 2015-06-02 13:59:50Z

You don't define functions as parameters in this way, you simply provide a named parameter which you then invoke as a function e.g.

function getFilesByExtArgs(dir, ext, callback){
    if (/* something bad */) {
        callback('Error thrown');
    } else {
        callback();
    }
}
...
getFilesByExtArgs('/my/file/path', '.txt', function(err) {
    if (err) throw err;
    ...
});

In your example, I presume what you are trying to do is something like

function getFilesByExtArgs(dir, ext, callback){
    fs.readdir(dir, function(err, data) {
        if (err) {
            callback(err);
        } else {
           while (i <= data.length) {
               if (path.extname(data[i]) == ('.' + ext))
                   console.log(data[i]);
               i++;
           }
           callback();
        }
    });
}

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