Use the in keyword to test if a attribute is defined in a object

if (assoc_var in assoc_pagine)

OR

if (home in assoc_pagine)

There are quite a few issues here.

Firstly, is var supposed to a variable has the value home, work or about? Or did you mean to inspect actual property called var?

If var is supposed to be a variable that has a string value, please note that var is a reserved word in JavaScript and you will need to use another name, such as assoc_var.

var assoc_var = home;

assoc_pagine[assoc_var] // equals 0 in your example

If you meant to inspect the property called var, then you simple need to put it inside of quotes.

assoc_pagine[var]

Then, undefined is not the same as undefined. You will need typeof to get the string representation of the objects type.

This is a breakdown of all the steps.

var assoc_var = home; 

var value = assoc_pagine[assoc_var]; // 0

var typeofValue = typeof value; // number

So to fix your problem

if (typeof assoc_pagine[assoc_var] != undefined) 

update: As other answers have indicated, using a array is not the best sollution for this problem. Consider using a Object instead.

var assoc_pagine = new Object();

assoc_pagine[home]=0;

assoc_pagine[about]=1;

assoc_pagine[work]=2;