Node.JS module.exports for passing parameter between two functions?

Question

I am new to Node.js so this is probably a fundamental question and I am missing something obvious. In the code below, I am trying to set the sql_file name from foo but I keep getting an error on the file not existing because the variable is not working. If I hard code the file name in sql_util.js it works fine.

So how do I pass a parameter or any object from one js file into the function of another?

foo.js

var misc = require('./sql_get');
console.log(misc.sql_file("on.sql"));

sql_util.js

fs = require('fs');
file = 'on.sql'
function sql_file(cb) {
    var fileName = "./SQLs/" + sql_file;
    fs.readFile(fileName, function(err, buffer) {
        if (err) return cb(err);
        return cb(null, buffer.toString());
    });
}
sql_file(function(err, sqlstatement) {
    if (err) throw err;
    console.log('sql statement is: ' + sqlstatement);
});
};
module.exports.x = x;
module.exports.sql_fil = sql_file;

This code is full of typos, undefined variable references and incorrect file names. I suggest cleaning up the code first and see if it works, then worry about parameter passing. — Brandon
– Brandon, Commented Mar 7, 2014 at 14:12
Like I said, I am new to this so I appreciate the feedback, thanks! — justZito
– justZito, Commented Mar 7, 2014 at 14:13

Brandon · Accepted Answer · 2014-03-07 14:24:50Z

3

Let's go through this line by line because I see a lot of errors, both syntax and semantic.

foo.js

var misc = require('./sql_get');
console.log(misc.sql_file("on.sql"));

You defined in the function below sql_file to be asynchronous, so it does not return a value, but takes a callback that it passes the result to.

sql_util.js

fs = require('fs');
file = 'on.sql'

You have an unused module-global variable file. Remove this. It's going to cause confusion.

function sql_file(cb) {
    var fileName = "./SQLs/" + sql_file;

sql_file is a function. I can tell because this code lives in a function called sql_file. Therefore, fileName will be "./SQLs/" + the .toString() result of the function, which is the source of the function. I think perhaps you want a parameter?

    fs.readFile(fileName, function(err, buffer) {
        if (err) return cb(err);
        return cb(null, buffer.toString());
    });
}

This seems ok.

sql_file(function(err, sqlstatement) {
    if (err) throw err;
        console.log('sql statement is: ' + sqlstatement);
    });
};

I am not sure what you are trying to do here. Why are you calling the function?

module.exports.x = x;
module.exports.sql_fil = sql_file;

Both of these lines have a problem. There is no symbol defined named x. I am surprised this code runs without throwing an error on that line. Second, you are exporting sql_file as sql_fil, missing the letter e.

The Solution

Since what you asked about is parameter passing and you only have a single function, let's simplify this. You never want to use global variables. If a function needs a particular variable, just pass it as an argument.

foo.js

var misc = require('./sql_get');
misc.sql_file('on.sql', function (err, contents) {
    console.log(contents);
});

sql_get.js (notice the file is renamed)

var fs = require('fs');

function sql_file(sqlFilename, cb) {
    var fileName = "./SQLs/" + sqlFilename;
    fs.readFile(fileName, function(err, buffer) {
        if (err) return cb(err);
        return cb(null, buffer.toString());
    });
}

module.exports.sql_file = sql_file;

answered Mar 7, 2014 at 14:24

Brandon

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2 Comments

justZito Over a year ago

Brandon, I am coming from a Visual Basic world and it is just such a learning curve, I have been reading Node.ja in Action, it is good but a bit over my head. Today I have Sams learn Node.Js. Really its the fundamentals that you point out that I need to focus on. I have only been working in js for about a week now so your feedback was great, thanks for taking the time! It there is any other good literature or exercises you can recommend I would be grateful.. Thanks again

Brandon Over a year ago

Sure. JavaScript and node.js in particular are single-threaded and the trick to scaling concurrency is asynchronous I/O with callbacks. I suggest studying callbacks since there is probably no exposure to it in VB.

Matt · Accepted Answer · 2014-03-07 14:16:09Z

3

A few problems:

You're requiring sql_get but naming the other file sql_util
```
var misc = require('./sql_util');
```
You're exporting module.exports.sql_fil = sql_file; (see the missing e). You probably mean;
```
module.exports.sql_file = sql_file;
```

While calling sql_file, you are passing a string but expecting a cb in the function itself -

misc.sql_file("on.sql", function(err, fileContent) {
    if(err) return console.log(err);
    console.log('File content: ', fileContent);
});

function sql_file(sqlFileName, cb) {
    var fileName = "./SQLs/" + sqlFileName;
    fs.readFile(fileName, function(err, buffer) {
        if (err) return cb(err);
        return cb(null, buffer.toString());
    });
}

And I don't know what you are doing with calling sql_file function in that file itself. Remove that.

edited Mar 7, 2014 at 14:16

Matt

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answered Mar 7, 2014 at 14:14

Mukesh Soni

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1 Comment

justZito Over a year ago

I want to put the call back into var reader = connection.reader("SELECT * FROM ON", []) to keep the code neat so it looks like connection.reader(sql_file("on.sql")). Reading the raw sql file and use the select right from the sql file.

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