Returns the sum of a list, after mapping each element to a number using the provided function.

Use Iterable.map() to map each element to the numeric value returned by fn, Iterable.reduce() to sum the values.

代码实现

num sumBy<T>(List<T> lst, num Function(T) fn) {
  return lst.map(fn).reduce((num a, num b) => a + b);
}

使用样例

sumBy([ {'n': 4}, {'n': 2}, {'n': 8}, {'n': 6} ], (o) => o['n']); // 20