Applying a function to each object in a JavaScript array

You could use Array.prototype.map:

var new_array = old_array.map(function(e) { 
  e.data = e.data.split(','); 
  return e;
});

As the comment said, this way changes the old_array. You could also return a new object in the callback function without changing the original array.

var data = [{
    name: 'hi',
    data: '1,2,3,4,5'
}, {
    name: 'hello',
    data: '5,4,3,2,1'
}];

You can use the Array.prototype.map on data to construct a new Array, like this

var result = data.map(function (currentObject) {
    return {
        name: currentObject.name,
        data: currentObject.data.split(",").map(Number)
    };
});

Here, we split the currentObject.data based on , and then we call Number function on all the split strings, so that you will get the result object's data as numbers, as you wanted in the question.

Output

[{
    name: 'hi',
    data: [1, 2, 3, 4, 5]
}, {
    name: 'hello',
    data: [5, 4, 3, 2, 1]
}]

let data = [{
    name: 'hi',
    data: '1,2,3,4,5'
  }, {
    name: 'hello',
    data: '5,4,3,2,1'
  }],
  result = data.map(function(currentObject) {
    return {
      name: currentObject.name,
      data: currentObject.data.split(",").map(Number)
    };
  });
console.log(result);