Undefined Variable Inside Looped Async Function in Node.js

Question

I'm building a function that will automatically scan a folder on server start or on folder change. The script will assemble a widgets folder for installed widgets, then concatenate and build a new partial file for inclusion in the front end app.

The script looks like:

concat.readMethodContents = function(concat) {

    var widget;
    for (widget in concat.widgets) {

        var method;
        for (method in concat.widgets[widget]) {

            if (concat.widgets[widget][method].ready) {
                concat.widgets[widget][method].ready = false;

                fs.readFile('./lib/widgets/' + widget + '/methods/' +  concat.widgets[widget][method].handle, 'utf8', function(err, data) {
                    if (err) {
                        console.log(err);
                    } else {

                        concat.widgets[widget][method].contents = data;

                        concat.writeJsPartialFile(concat);
                    }
                });
            }
        }
    }
}

Before this method is called, the concat object looks like:

{
    "widgets": {
        "hello_world": [{
            "handle": "method1.js",
            "ready": true
        }, {
            "handle": "method2.js",
            "ready": true
        }, {
            "handle": "method3.js",
            "ready": true
        }]
    }
}

The problem with the script is that when method is called, it is defined as done because the loop has finished and the async function is running a callback.

How can I encapsulate or otherwise preserve the variable through that part of the operation?

Welcome to the land of promises.

Sterling Archer
– Sterling Archer

2014-10-31 18:22:30 +00:00
Commented Oct 31, 2014 at 18:22 — Sterling Archer
– Sterling Archer, Commented Oct 31, 2014 at 18:22

Community · Accepted Answer · 2017-05-23 12:28:18Z

I just answered this exact same problem here: Accessing a MongoDB value from a query within a query

You use a self-executing function to wrap your closure to ensure the value doesn't change underneath you due to the loop. Try something like:

concat.readMethodContents = function(concat) {
    var widget;
    for (widget in concat.widgets) {
        var method;
        for (method in concat.widgets[widget]) {
            (function(widgetMethod) {
                if (widgetMethod.ready) {
                    widgetMethod.ready = false;
                    fs.readFile('./lib/widgets/' + widget + '/methods/' +  widgetMethod.handle, 'utf8', function(err, data) {
                        if (err) {
                            console.log(err);
                        } else {
                            widgetMethod.contents = data;
                            concat.writeJsPartialFile(concat);
                        }
                    });
                }
            })(concat.widgets[widget][method]);
        }
    }
}

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