Array-valued casts are flattened when grouping duplicate columns #547

group_columns_by_name: true flattens values returned as arrays by cast, rather than grouping each field value intact.

import { parse } from 'csv-parse/sync'

parse('1,2,3', {
  columns: ['value', 'value', 'value'],
  group_columns_by_name: true,
  cast: value => [Number(value)]
})
// Actual: [{ value: [1, 2, 3] }]
// Expected: [{ value: [[1], [2], [3]] }]

Empty arrays returned by cast disappear entirely. The parser uses Array.isArray to identify an existing column group, which also matches array-valued fields; concat then flattens later arrays as well.