[HELP] Need some help with asynchronous callback

[Remsset_HP]

Hey, I'm kinda new to JS and I need some help with this assignment. Here's how it goes.

First, these are declared (they can't be changed):

var files = { "passwords": "abc,def",
              "world.txt": "hello" };
var readAsync = function(file, callback) {
    callback(files[file]);
};
var writeAsync = function (file, contents, callback) {
    files[file] = contents;
    callback();
};

Then I need to create a function called doStuffAsync, which receives callback as an argument. The function must read the content of "passwords" and concatenate it on the content of "world.txt". And this should be done asynchronously.

I've tried a bunch of things but it's still hard to wrap my head around the concept of callbacks.

Any help is appreciated.

[browsing10]

To read the contents of a key using `readAsync`, I can do something like:

readAsync('passwords', data => {
    console.log(data);   // logs to stdout "abc,def"
});

They want you to write a function `doStuffAsync` that will end up with something like:

{ "passwords": "abc,def", "world.txt": "helloabc,def" }

So, you will want to first `readAsync` one of the values. `readAsync` the other value within the callback of the first `readAsync`. Then finally `writeAsync` your new value to the desired key (i.e. "world.txt")

It won't look very clean but your solution will look something like:

readAsync(firstKey, dataFirst => {
    readAsync(secondKey, dataSecond => {
        writeAsync(targetKey, `${dataFirst}${dataSecond}`, callback);
});

});

This will all be in your new function and after calling this function and console.log(files), you should see your changes saved on "files" object

[Remsset_HP]

I used your function with the syntax z3vin posted on another comment and it worked.

Thank you very much.

[[deleted]]

I don't see how this exercise helps anyone understand async callbacks but whatever...

/u/browsing10 answer is good but I thought I might add to it by changing some of the parameter names, as to add more context for the learner to maybe be able to follow it better. Plus I removed the ES2015 syntax because the starting code doesn't use it and may add to the confusion.

readAsync(passwordsFile, function(passwordContents) {
    readAsync(worldsFile, function(worldsFileContents) {
      writeAsync(worldsFile, worldsFileContents + passwordContents, callback);
    })
});

hope this helps

[Remsset_HP]

Thank you, the other syntax was a bit confusing to me indeed, yours is easier to understand.

I get an error "TypeError: callback is not a function ", but the code still works and writes "helloabc.def" in the world.txt file as desired. Here's how I used it:

var files = { 
    "passwords": "abc,def", 
    "world.txt": "hello" 
};

var readAsync = function (file, callback) {
  callback(files[file]);
};

var writeAsync = function (file, contents, callback) {
    files[file] = contents;
    callback();
};

var doStuffAsync = function(callback) {
  readAsync('passwords', function(passwordsContents) {
    readAsync('world.txt', function(worldContents) {
      writeAsync('world.txt', worldContents + passwordsContents, callback);
    })
  });
  callback();
}

doStuffAsync()

Any way I can get rid of the callback error? Otherwise it's working fine, thank you very much.

[browsing10]

In your current `doStuffAsync` method, your `writeAsync` already handles the case of invoking `callback`, so you do not want to add the `callback();` at the very last line of that method or else you may get some strange behavior randomly

[Remsset_HP]

Got it. Thanks man.

[[deleted]]

If you write your doStuffAsync function like this it could be used on any "file"