const arr = [], length = 10000;

  let i = 0;

  for(; i<length; ++i) {

    arr.push(5);

  }

  function test(x) {

    return x*x;

  }

const newArr = arr.map(test);

const arr = [], length = 10000;

  let k = 0;

  for(; k<length; ++k) {

    arr.push(5);

  }

  function test(x) {

    return x*x;

  }

let updatedItems = [];

            const len = arr.length;

            let p = 0;

            for(; p < len; ++p) 

                updatedItems.push(test(p));

​