how to access "this" object in javascript?

Question

How come in the code below, the second line is giving me an undefined error?

function DAO()
{
        this.arrVariable = new Array();
        this.getItem = getItem;
        this.getItemQuery = getItemQuery;
}

function getItem(key)
{
        dao.arrVariable[key]();
}

function getItemQuery(key, url, options, pollfrequency)
{
        alert('hey');
}


var dao = new DAO();
dao.arrVariable['var1'] = function() { this.getItemQuery('a','b','c','d'); };

dao.arrVariable['var1']();

I want to be able to access the dao's getItemQuery as an object call. How do I do this?

Why do you need to use "this"? My assumption is that getItemQuery actually does more than alert 'hey'? Perhaps you could elaborate? — John Hartsock
– John Hartsock, Commented Dec 6, 2011 at 2:03
The DAO object will have many more properties. I'd like to be access them when I'm in the function() {this.getItemQuery} — learningtech
– learningtech, Commented Dec 6, 2011 at 2:04
Refer to Michael's answer "this" is not needed for what you are doing. — John Hartsock
– John Hartsock, Commented Dec 6, 2011 at 2:05
Since dao.addVariable is declared as an array, do you really intend to add the object property var1 to it? In JavaScript, syntax like arrVariable['var1'] refers to an object property, not an array key. — Michael Berkowski
– Michael Berkowski, Commented Dec 6, 2011 at 2:07
Just as an aside, if you want an "associative array", i.e., you want to access elements based on key names rather than numeric indexes, then you should be using a JavaScript object: x = {}; x["key1"] = "A"; — nnnnnn
– nnnnnn, Commented Dec 6, 2011 at 2:07

Michael Berkowski · Accepted Answer · 2011-12-06 02:27:04Z

4

In that context, this refers to arrVariable. You can instead refer to it as dao.getItemQuery() inside the function:

dao.arrVariable['var1'] = function() { dao.getItemQuery('a','b','c','d'); };

edited Dec 6, 2011 at 2:27

answered Dec 6, 2011 at 2:04

Michael Berkowski

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1 Comment

Bergi Over a year ago

To be exact, this refers to dao.arrVariable, not to the property var1 (which would be the function itself)

Kevin Ji · Accepted Answer · 2011-12-06 02:04:56Z

1

You can use apply or call here.

So, instead of

dao.arrVariable['var1']();

Use either

dao.arrVariable['var1'].apply(dao, /* array of arguments here */);

or

dao.arrVariable['var1'].call(dao, /* arguments here, separated by commas */);

answered Dec 6, 2011 at 2:04

Kevin Ji

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Comments

fuyou001 · Accepted Answer · 2011-12-06 02:05:22Z

1

dao.getItemQuery can access dao's property

answered Dec 6, 2011 at 2:05

fuyou001

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Comments

Ben L · Accepted Answer · 2011-12-06 02:05:47Z

0

THe this in function() { this.getItemQuery('a','b','c','d'); }; refers to function() not to DAO. You need to access DAO by:

dao.arrVariable['var1'] = function() { dao.getItemQuery('a','b','c','d'); };

answered Dec 6, 2011 at 2:05

Ben L

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1 Comment

Bergi Over a year ago

No, this refers to the object the function is called on. This might be the array in Johns secont-to-last line, or the window object if the function is called "directly".

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