Functional programming issue regarding passing functions with parameters

Question

I am currently writing an API to unzip files to the web browser sandbox filesystem. I have a basic need to pass a parameter to a function that in turn is itself passed as a parameter. Here is some code to illustrate the issue:

//Request a permanent filesystem quota to unzip the catalog.
function requestFilesystem(size){
    window.webkitStorageInfo.requestQuota(PERSISTENT, size*1024*1024, function(grantedBytes) {
        window.requestFileSystem(PERSISTENT, grantedBytes, function(fs) {
            filesystem = fs;
            removeRecursively(filesystem.root, unzip(url), onerror);
        }, onerror);
    }, function(e) {
        console.log('Error', e);
    });
}

//unzip method can be changed, API remains the same.
//URL of zip file
//callback oncomplete
function unzip(URL) {
    importZipToFilesystem(URL, function(){
        console.log("Importing Zip - Complete!");
    });
}

//remove removeRecursively a folder from the FS
function removeRecursively(entry, onend, onerror) {
    var rootReader = entry.createReader();
    console.log("Remove Recursive"+entry.fullPath);
    rootReader.readEntries(function(entries) {
        var i = 0;

        function next() {
            i++;
            removeNextEntry();
        }

        function removeNextEntry() {
            var entry = entries[i];
            if (entry) {
                if (entry.isDirectory)
                    removeRecursively(entry, next, onerror);
                if (entry.isFile)
                    entry.remove(next, onerror);
            } else
                onend();
**Uncaught TypeError: undefined is not a function**
        }

        removeNextEntry();
    }, onerror);
}

If I try to use

function removeRecursively(entry, onend(URL), onerror) {

there is an error, ao my issue is how to pass around the URL value for the unzip function, this unzip function is used as a callback function on the onsuccess of removeRecursively

just pass unzip, don't call it directly. removeRecursively(filesystem.root, unzip, onerror); not removeRecursively(filesystem.root, unzip(url), onerror);. Also, writing function removeRecursively(entry, onend(URL), onerror) { makes no sense at all, how could you execute inside a param list? — Frances McMullin
– Frances McMullin, Commented May 27, 2013 at 13:38
@DavidMcMullin Indeed it was a dumb thing to do, I was misinterpreting that passing unzip I was passing it as a function, a function can take parameters, so let's put some parameters in this. I can't that is why I asked the question, it wasn't making sense to me. Thanks for the reply! — Astronaut
– Astronaut, Commented May 27, 2013 at 13:57

Ry- · Accepted Answer · 2013-12-23 21:04:23Z

3

You're passing the result of unzip to removeRecursively, which is undefined.

What you probably want to do is

removeRecursively(filesystem.root, function() { unzip(url); }, onerror);

Here you pass a function as a parameter, this function calls unzip with the parameter you want.

edited Dec 23, 2013 at 21:04

Ry-♦

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answered May 27, 2013 at 13:38

Jad

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1 Comment

Astronaut Over a year ago

Duh... I have to pass it inside a function, I was misinterpreting the code that by passing unzip was already passing it as a function. I was mislead because if I pass onerror("Error unzipping") as a parameter it works. //error logging function onerror(message) { console.error(message); } removeRecursively(entry, unzip, onerror("Error Removing")); - Works. Going to test this solution.

Ry- · Accepted Answer · 2013-12-23 21:04:55Z

1

You are calling

removeRecursively(filesystem.root, unzip(url), onerror);

but unzip doesn't return anything

function unzip(URL) {
    importZipToFilesystem(URL, function(){
        console.log("Importing Zip - Complete!");
    });
}

So the second argument of removeRecursively (onend) becomes undefined, which probably causes the error when you trying to use it as a function.

If you want to use unzip function as a callback, you should just pass unzip (instead of unzip(url)) without calling it and then call onend(URL) inside removeRecursively.

edited Dec 23, 2013 at 21:04

Ry-♦

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answered May 27, 2013 at 13:31

Paweł Chorążyk

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1 Comment

Astronaut Over a year ago

The issue is that i want to pass it different values for the URL, So I want unzip to have as a parameter a URL.

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