javascript getting and setting property

Question

I'm trying to work with javascript objects, but not getting very far. Any ideas why this doesn't work?

function Tracking(){ var choices = new Array(); } Tracking.prototype.getChoice = function (key){ return this.choices[key]; } Tracking.prototype.setChoice = function (choice){ this.choices.push[choice]; } function TrackingChoice(key, choice){ this.key = key; this.choice = choice; }

.....

 var tracking = new Tracking(); var choices = new Array(); choices.push(new TrackingChoice("purchase", true)); choices.push(new TrackingChoice("listing", false)); choices.push(new TrackingChoice("offers", false)); choices.push(new TrackingChoice("messages", false)); tracking.setChoice(choices); var a = tracking.getChoice(0);

var a is empty as the choices array in the Tracking object are still null. This confuses me.

power_scriptor · Accepted Answer · 2013-11-09 07:10:13Z

try like this

 function Tracking(){ this.choices = []; var count = 0; this.getChoice = function (key){ return this.choices[key]; }; this.setChoice = function (choice){ this.choices[count] = choice;count++; }; } function TrackingChoice(key, choice){ this.key = key; this.choice = choice; } var tracking = new Tracking(); var choices = new Array(); tracking.setChoice(new TrackingChoice("purchase", true)); tracking.setChoice(new TrackingChoice("listing", false)); tracking.setChoice(new TrackingChoice("offers", false)); tracking.setChoice(new TrackingChoice("messages", false)); var a = tracking.getChoice(0);

sweet thanks mate but I'm not sure why the .push() didn't work as i use that all the time

James Montagne · Accepted Answer · 2013-11-09 07:01:45Z

You've got a few issues here.

function Tracking(){ var choices = new Array(); }

Should be

function Tracking(){ this.choices = new Array(); }

Then, push is a function, use () not []:

this.choices.push(choice);

Then your test code is passing an entire array to setChoice when that function looks like it sets one item at a time. So you would push 1 element into the array, which is the entire choices array. You would probably want to call setChoice for each element rather than once. Though really your setChoice should be named addChoice as it adds an element, doesn't set the whole array. I would expect setChoice to just reassign the entire array. It really just depends what you intended that function to really do.

http://jsfiddle.net/unw3R/

nkron · Accepted Answer · 2013-11-09 07:05:43Z

The problem is here:

Tracking.prototype.setChoice = function (choice){ this.choices.push[choice]; }

You are reading the choice property off the push method of the choices array which doesn't really do anything - it will read the property and return it, but it will return undefined and that value is not stored or used anywhere.

You instead should set the choices property on this:

Tracking.prototype.setChoice = function (choice){ this.choices = choice; }

Check out the Array.push() method for more examples on using the push method correctly.

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