arrays - JavaScript: How to create new object properties/members and assign values inside a For Loop? -

i trying create function create , assign value object property/member. i'm getting error:

uncaught typeerror: cannot read property 'id' of undefined

the function suppose create property/member called 'id', , suppose assign value newly created property/member.

    var add_ids = function(array,index) {         for(var iii = 1; iii <= 3; iii++) {             array[index+iii].id = (array[index].id * 4)+iii;         }         array[index].id *= 4;     }; 

array array pass function.

index position in array start on loop

p.s. loop not overwriting important. element corresponds "array[index+iii]" empty.

array[index+iii].id = (array[index].id * 4)+iii; 

in that, if array[index+iii] undefined before, array[index+iii].id give error you've mentioned.

so before accessing id check whether property exists.

if(!array[index+iii]) array[index+iii] = {}; // initializing if not present. array[index+iii].id = (array[index].id * 4)+iii;