JSFiddle - React, Tailwind, and code Playground

JavaScript

var data = [
    { a: 10 },
    { a: 20 },
    { a: 30 }
];

var large_array = _.map(data, function(element) {
   element.b = element.a;
   delete element.a;
   return element;
});

console.log(large_array.length);