Strange behavior array in object JavaScript -

why b.first[0] return "t" , how can avoid this?

i need safe "q" in b.first[0]

var extend = function(o,p){     for(prop in p){         o[prop] = p[prop];     }     return o; };  var = {first:['q','w']}; var b = {};  extend(b,a);  document.write(a.first[0]); //q document.write(b.first[0]); //q  a.first[0] = 't';  document.write(a.first[0]); // t document.write(b.first[0]); // t ????????????????????? 

this issue relating concept extending b doesn't recreate data a. if of data object (like array), instead "points" array instead of creating new, identical array. you're storing 2 pointers same array, when change one, change other.

here answer discusses idea of "cloning" object in javascript in more detail.

https://stackoverflow.com/a/728694/1570248