Question

I am using node.js. I have this handlers.js file:

exports.Handlers = function(prefix) {
    this.prefix = prefix;
    this.db = new DBInstance();
};

exports.Handlers.prototype.getItemById = function(id) {
    var item = this.db.getItemById(id, function(error, item) {
        item.name = this.prefix + item.name;
        ...
        ...
    });
};

When I call:

var h = new Handlers();
h.getItemById(5);

I get an error since the context isn’t the Handlers and this.prefix doesn’t exists. I can fix it using this:

exports.Handlers.prototype.getItemById = function(id) {
    var scope = this;
    var item = this.db.getItemById(id, function(error, item) {
        item.name = scope.prefix + item.name;
        ...
        ...
    });
};

Is there any better way to pass the context to the callback?
What is the nodejs common way to pass the context to the callback?

Answer 1

Node implements ECMAScript 5, which has Function.bind().

I think that is what you are looking for.

exports.Handlers.prototype.getItemById = function(id) {
    var item = this.db.getItemById(id, (function(error, item) {
        item.name = this.prefix + item.name;
        ...
        ...
    }).bind(this)); //bind context to function
};

This works, however when using a closure as a callback, like you are doing, the most common way this is done is by storing the context in a variable that can be used in the closure.

This method is more common because many times callbacks can be deep and calling bind on each callback can be heavy; whereas defining self once is easy:

SomeObject.prototype.method = function(id, cb) {
    var self = this;
    this.getSomething(id, function(something) {
        //self is still in scope
        self.getSomethingElse(something.id, function(selse) {
            //self is still in scope and I didn't need to bind twice
            self.gotThemAll(selse.id, cb);
        });
    });
};