/*
Computed Property Names is ES6 feature which allows 
the names of object properties in JavaScript OBJECT LITERAL NOTATION
to be determined dynamically, i.e. computed.
*/

let propertyname = 'c';

let obj ={
    a : 11,
    b : 12,
    [propertyname] : 13
};

obj; // result is  {a:11 , b:12 , c:13}

//or incase if you want a as your object you can set in this way

let a_value = {
    [obj.a] = obj // a_value's key name as (a) and the complete (obj) present above itself will act as a value
};