pie.value()

pie.value([value])

If value is specified, sets the value accessor to the specified function or number and returns this pie generator. If value is not specified, returns the current value accessor, which defaults to:

function value(d) {
  return d;
}

When a pie is generated, the value accessor will be invoked for each element in the input data array, being passed the element d, the index i, and the array data as three arguments. The default value accessor assumes that the input data are numbers, or that they are coercible to numbers using valueOf. If your data are not simply numbers, then you should specify an accessor that returns the corresponding numeric value for a given datum. For example:

var data = [
  {"number":  4, "name": "Locke"},
  {"number":  8, "name": "Reyes"},
  {"number": 15, "name": "Ford"},
  {"number": 16, "name": "Jarrah"},
  {"number": 23, "name": "Shephard"},
  {"number": 42, "name": "Kwon"}
];

var arcs = d3.pie()
    .value(function(d) { return d.number; })
    (data);

This is similar to mapping your data to values before invoking the pie generator:

var arcs = d3.pie()(data.map(function(d) { return d.number; }));

The benefit of an accessor is that the input data remains associated with the returned objects, thereby making it easier to access other fields of the data, for example to set the color or to add text labels.