OOP Javascript 'this' keyword in a nested private method

Question

Here is some example code:

ExampleClass = function()
{
    this.initiate();
};

ExampleClass.prototype.initiate = function()
{
    var connect = function()
    {
        this.sendNOP();
    };

    connect();
};

ExampleClass.prototype.sendNOP = function()
{
    console.info('Sending NOP...');
    var callback = function()
    {
        console.info('Server responded to NOP. ZzzZzzzZzz...');
    };
    setTimeout(callback, 1500);
};

I am very curious why I can't call this.sendNOP() in ExampleClass.initiate so that ExampleClass.initiate._connect() will pass the instanceof ExampleClass as this to ExampleClass.sendNOP(), it seems to pass window as this. Why?

EDIT:

The problem is when we call ExampleClass.initiate._connect() we only use connect() which does not specify any context. Calling ExampleClass.initiate._connect() with .apply(this) works! .apply(this) sets the context to ExampleClass.

ExampleClass.prototype.appliedInitiate = function()
{
    var connect = function()
    {
        this.sendNOP();
    };

    connect.apply(this);
};

Final code

ExampleClass = function()
{  
    this.appliedInitiate();
};

ExampleClass.prototype.sendNOP = function()
{
    console.info('Sending NOP...');
    var callback = function()
    {
        console.info('Server responded to NOP. ZzzZzzzZzz...');
    };
    setTimeout(callback, 1500);
};

ExampleClass.prototype.initiate = function()
{
    var connect = function()
    {
        this.sendNOP();
    };

    connect(); // Won't work. connect() is not called from any context (ie. obj.connect() )
};

ExampleClass.prototype.appliedInitiate = function()
{
    var connect = function()
    {
        this.sendNOP();
    };

    connect.apply(this); // Will work, we are calling connect with apply, which sets the context to ExampleClass
};

score 2 · Accepted Answer · answered May 20 '14 at 10:47

2

You can't calling this.sendNOP in ExampleClass.initiate. You are calling it in connect. That the call to connect is inside the initiate function is irrelevant.

You haven't called connect with any context, so the context is the default object (window).

answered May 20 '14 at 10:47

Quentin

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+1 and as addition one of my favorite SO javascript questions: http://stackoverflow.com/questions/3127429/javascript-this-keyword – mrak May 20 '14 at 10:52
Calling connect with `.apply()` doesn't seem to change a thing. How am I supposed to elegantly fight this problem? – Axel Latvala May 20 '14 at 13:20
It should do. How are you using it? – Quentin May 20 '14 at 13:22
How are you calling `ExampleClass`? – Quentin May 20 '14 at 13:31
@Quentin I am doing it through the constructor; `exc = new ExampleClass()` – Axel Latvala May 20 '14 at 13:35
@Quentin paste the codes in a js console (chrome, ff, w/e) and try it out – Axel Latvala May 20 '14 at 13:36
You don't have the same problem. `this` is no longer `window`. The new problem is that `sendNOP` is a method of **ExampleClass** and not the prototype. You need `ExampleClass.prototype.sendNOP = ...` – Quentin May 20 '14 at 13:38
@Quentin ah yes I discovered that a little after I wrote the last messages and started correcting the code. The solution is in the initial post now, with explanations. – Axel Latvala May 20 '14 at 14:10

OOP Javascript 'this' keyword in a nested private method

1 Answers1