How come an array inside an instanciated object returns undefined?

Question

function test() { var test_array = []; } var arrayList = new Array(); arrayList[0] = new test(); arrayList[0].test_array[0] = "test";

(try it out)

I'm trying to create an array, initialize an object as a member of that array, and then assign a text value to an array inside that object. However the array inside the object gives

TypeError: arrayList[0].test_array is undefined

when I try to assign a string to the inner array. I'm still pretty new to javascript so any tips to best practice in this case would be greatly appreciated.

The test_array variable is only accessible within test(). You have to expose it with "this" — Silkster
– Silkster, Commented Jan 13, 2014 at 18:52
See possible duplicate Javascript: Do I need to put this.var for every variable in an object? for the difference between variables and properties — Bergi
– Bergi, Commented Jan 13, 2014 at 18:52

aga · Accepted Answer · 2014-01-13 18:51:12Z

4

Statement var test_array = []; creates a local variable which will not be visible outside the scope it is defined within. What you need here is the variable defined as the property of object instance. Use this reference inside your test function:

function test() { this.test_array = []; }

For more information on scopes in JavaScript read the documentation.

answered Jan 13, 2014 at 18:51

aga

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3 Comments

William Martin Over a year ago

When I make this change the I get SyntaxError: missing variable name jsfiddle.net/5t62z/4

cookie monster Over a year ago

@WilliamMartin: There's no syntax error in this answer. Read and type carefully and double check your work. You're doing var this.test_array = [];, which isn't valid.

aga Over a year ago

@WilliamMartin strange, I've modified your fiddle as I've described and printed the value of arrayList[0].test_array[0] via console.log(arrayList[0].test_array[0]) - it worked as expected.

Jonathan Lonowski · Accepted Answer · 2014-01-13 18:52:19Z

var declarations within a constructor don't actually modify the instance. They're just locally-scoped within the constructor function itself.

To define a property of the instance, you have to modify this:

function test() { this.test_array = []; }

Neil Kistner · Accepted Answer · 2014-01-13 18:53:39Z

Updated Fiddle: http://jsfiddle.net/5t62z/3/

Your function was not returning anything. If you notice my update, I now will return an object with the test_array property.

function test() { return { test_array: [] }; }

"Your function was not returning anything..." Yes it does. It returns a new object because it was invoked using new. jsfiddle.net/5t62z/5

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