Заставьте плавание только показать два десятичных разряда

const jsonData1=[{firstName:"Sam",age:"10"},{firstName:"John",age:"11"},{firstName:"Jack",age:"12"},{firstName:"Pam",age:"13"},{firstName:"Tom",age:"14"},{firstName:"Mitch",age:"15"}];
const jsonData2=[{firstName:"Sam",city:"London"},{firstName:"John",city:"New York"},{firstName:"Jack",city:"Paris"},{firstName:"Pam",city:"Moscow"},{firstName:"Roger",city:"Shanghai"},{firstName:"Don",city:"Jakarta"}];


/* 
Use Array#concat() to combine both input arrays before performing
the merge. This will ensure that all items in both arrays are
processed and accounted for during the merge (which is important in
situations where the input arrays differ in length).

Object.values() is used to transform the dictionary resulting from
reduce() to an array
*/
var result = Object.values(
[].concat(jsonData1, jsonData2
).reduce((dict, item) => {

  /* 
  We use Array#reduce is an intermediate step to construct a dictionary
  which maps each unique "item.firstName" to a corresponding object 
  that contains the merged (or yet to be merged) data for this first 
  name
  */
   
  var value = dict[item.firstName] || {}

  /*
  Use Object.assign() to merge existing data for "item.firstName" with
  current item being processed. Also pass default values as first
  argument to ensure all three key values are present in merge result
  */
  value = Object.assign({ firstName : '', age : '', city : ''} , value, item)

  /*
  Update the dictionary with merged data
  */
  dict[item.firstName] = value

  return dict

}, {}))

console.log(result);