Nodejs calling function from within a function

Question

I have 3 methods

exports.getImageById = function (resultFn, id) {
 ...
}

exports.getCollectionById = function (resultFn, id) {
}

in the third method I want to call both methods

exports.getCollectionImages = function (resultFn, collectionId) {

var arr = new Array();

this.getCollectionById(   // fine, 1st call
    function (result) {
        var images = result.image;
        for (i = 0; i < images.length; i++) {
            this.getImageById(function (result1) {   // error, 2nd call
                    arr[i] = result1;
                }, images[i]
            );

         }
    }
    , collectionId
);

resultFn(arr);
}

I can call first function this.getCollectionById but it fails to call this.getImageById, it says undefined function, whats the reason for that?

Ruan Mendes · Accepted Answer · 2012-12-07 18:21:08Z

4

When you call this.getCollectionById passing it a callback, the callback doesn't have access to the same this

The simplest solution is to save this as a local variable.

exports.getCollectionImages = function (resultFn, collectionId) {    
    var arr = new Array();        
    var me = this; // Save this
    this.getCollectionById(   // fine, 1st call
        function (result) {
            var images = result.image;
            for (var i = 0; i < images.length; i++) {
                // Use me instead of this
                me.getImageById(function (result1) {   // error, 2nd call
                    arr[i] = result1;
                }, images[i]);
            }
        }, collectionId);
    resultFn(arr);
}

answered Dec 7, 2012 at 18:21

Ruan Mendes

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3 Comments

Sleiman Jneidi Over a year ago

Thank you for this, it seems that javascript is pure instance oriented language

Ruan Mendes Over a year ago

A more accurate description is that methods in JavaScript are not bound to an instance, it depends on how they are called.

Ruan Mendes Over a year ago

@sleimanjneidi is your getImageById asynchronous? If so, the value of i will be wrong (the last value of i in the loop) in the callback to me.getImageById

bfavaretto · Accepted Answer · 2012-12-07 18:40:26Z

The value of this inside the inner function is not the same object as outside, because it's determined depending on how the function is called. You can find a detailed explanation in the MDN article on this.

One of the ways to solve it is by keeping a reference to the outer this in another variable such as that:

var that = this;
this.getCollectionById(   // fine, 1st call
    function (result) {
        var images = result.image;
        for (i = 0; i < images.length; i++) {
            that.getImageById(function (result1) {   // 2nd call
                    arr[i] = result1;
                }, images[i]
            );

        }
    }
    , collectionId
);

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