The map() method needs to return a value.

Try this

var filteredData=data.map(x => {
   // get the original values to avoid repeating x. 
   const {month, veryLate, Late, onTime} = x;
   
   // do your calculations
   const newVeryLate = veryLate / ( veryLate + Late + onTime) * 100;
   const newLate = Late / (veryLate + Late + onTime) * 100;
   const newOnTime = onTime / (veryLate + Late + onTime) * 100;
   
   // return the new object
   return {month, veryLate: newVeryLate, Late: newLate, onTime: newOnTime}
});

x.veryLate wont work in x it should be veryLate itself same for the others

const data=[
{
  month:"nov",
  veryLate:3,
  Late:5,
  onTime:2
},
{
  month:"dec",
  veryLate:1,
  Late:3,
  onTime:16
},
{
  month:"jan",
  veryLate:28,
  Late:1,
  onTime:1
},
]

var filteredData= data.map(x => (
   {
    ...x, 
    veryLate:  x.veryLate/(x.veryLate+x.Late+x.onTime)*100,         
    Late: x.Late/(x.veryLate+x.Late+x.onTime)*100, 
    onTime: x.onTime/(x.veryLate+x.Late+x.onTime)*100
  })
)

console.log(filteredData)

You also must wrap the returning object literal into parentheses. Currently the curly braces is being denoted as the function body.

var filteredData=data.reduce((a, v)=>{
  let obj = {};
  let sum = v.veryLate+v.Late+v.onTime;
  obj.month = v.month;
  obj.veryLate = v.veryLate/sum*100;
  obj.Late = v.Late/sum*100;
  obj.onTime = v.onTime/sum*100;
  a.push(obj)
  return a;
}, []);

Throwing error because:

1. The data array has no closing bracket
2. map method is not returning anything. if you want to return an object from it use return keyword.