Note that objects (with named properties) and associative arrays are the same thing in javascript.

You can use hasOwnProperty to check if an object contains a given property:

o = new Object();  

o.prop = 'exists'; // or o['prop'] = 'exists', this is equivalent 



function changeO() {  

  o.newprop = o.prop;  

  delete o.prop;  

}  



o.hasOwnProperty('prop');   //returns true  

changeO();  

o.hasOwnProperty('prop');   //returns false  

Alternatively, you can use:

if (prop in object)

The subtle difference is that the latter checks the prototype chain.