在Node.js中与单例同步问题readdir

Question

First app with Node.js, trying to make a file explore to get contents from them, within a singletoned class, but the order is not that i expected. Surely a knowledge missing from me, can you tell me why..

Singleton class :

var Singleton = (function()
{
    var _instance = null;

    return new function()
    {
        this.Instance = function()
        {
            if (_instance == null)
            {
                _instance = new Foo();
            }
            return _instance;
        }
    };
})();

The Foo class :

var Foo= function Foo()
{
    this._filesDir= "./core/files/";
    this._storedFiles = {};

    this.method1();
console.log("call constructor");
};

Foo.prototype = {
    method1: function()
    {
        console.log("call method1");
        var that = this;

        var c = 0;

        fs.readdirSync(this._filesDir).forEach(function(fileName)
        {
            console.log("iterating file"+ c);

            c++;
            fs.readFile(that._filesDir + fileName, 'utf-8', function(err, content)
            { 
                var clean_FileName = fileName.replace(".txt", "");
                console.log( clean_fileName );
                that._storedFiles[ clean_fileName ] = content;
            });
        });
    },

    method2: function( fileName )
    {
        console.log('call method2');
        return ( fileName in this._storedFiles);
    }
};

The calling :

console.log( Singleton.Instance().method2("myfile") );

In the directory, there is only this myfile.txt

But, the console displaying me that :

call method1
iterating file0
call constructor
call method2
false
GET /test 304 11ms
myfile

So my response is false and is this normal constructor called at the third position ? I need the class construct, store, and finally execute the method2(). What i'm doing bad ?

Answer 1

The root of your problem is that fs.readFile is asynchronous. method1 returns before you've read the contents of the file. A simple fix is to change it to fs.readFileSync .

The reason "call constructor" is third is because you invoke method1() first.

this.method1();
console.log("call constructor");

Everything in method1 gets run before console.log("call constructor") happens. You can simply swap the two if you want the order to be correct.

From a high-level, using synchronous calls (readdirSync, readFileSync) is usually a bad idea because they block Node from doing anything else while they're running. I would recommend studying callbacks, control flow, and the asynchronous nature of Node.js. There are lots of great tutorials out there.