CODEWITHSUNDEEP
node.js
drhanlau
drhanlau

Reputation: 2527

Is this the right behaviour of NodeJS?

I obtained a list of files using fs.readdir, and take the first 5 files trying to read their content.

files.slice(0,5).forEach(function(item){
                        console.log(item);
                        readContent(path+"/"+item);
                    }
                );


function readContent(filename)
{
    var fs= require("fs");
    fs.readFile(filename, 'utf8', function(err, data)
    {
       console.log(data);
    });
}

But it prints the filename first, then only print the content, instead of "filename -> content" as I expected. I am new to NodeJS, is this part of the asynchronous feature ? Or I did something wrong?

28965131362770944.txt
28965131668946944.txt
28965131803168769.txt
28965131991912448.txt
28965132189040641.txt 


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Upvotes: 1

Views: 69

Answers (1)

sergej shafarenka
sergej shafarenka

Reputation: 20426

Yes, this is expected behavior. All IO operations in Node.js are done asynchronously. This means console.log(item) will be executed immediately, but reading content will happen a tick later, because of it's asynchronous nature. Thus main thread prints all file names first, and then their content.

If you want to print "filename-content", "filename-content" etc sequence, you need to give file name to the readContent() method and then log it inside readFile() method. Like this.

files.slice(0,5).forEach(function(item){
    readContent(path+"/"+item, item);
});

function readContent(filename, item) {
    var fs = require("fs");
    fs.readFile(filename, 'utf8', function(err, data) {
        console.log(item);
        console.log(data);
    });
}

P.S. There is an option to call fs.readFileSync() which reads file synchronously. I would strongly discourage you from using it in productive code, because for the time a file is read it will stop processing of other incoming requests. Synchronous methods are normally used during application initialization, but never at runtime, when application serves requests.

Upvotes: 1

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