Naive JS implementation of tidyr's gather function. Intended for use with JSON-styled tabular data... like you'd get from d3.dsv

gather.ts

function withFields<S>(record: S, fields: (keyof S)[]) {
  // Returns record with only properties specified in fields
  return fields.reduce((acc: Partial<S>, key) => {
    acc[key] = record[key]
    return acc
  }, {})
}

function splitRecord<S extends object>(record: S, ...fields: Array<keyof S>) {
  let withGivenFields = withFields(record, fields)
  let otherFields = keys(record).filter((key: keyof S) => !fields.includes(key))
  let withOtherFields = withFields(record, otherFields)
  return [withOtherFields, withGivenFields]
}

function keys<O extends object>(obj: O): Array<keyof O> {
  return Object.keys(obj) as Array<keyof O>
}

function lengthen<S extends object>(record: Partial<S>, keyLabel: string, valueLabel: string) {
  return keys(record).map(key => ({ [keyLabel]: key, [valueLabel]: record[key] }))
}

export function gather<S extends object = object>(
  data: S[],
  keyLabel: string,
  valueLabel: string,
  ...columns: (keyof S)[]
) {
  let result: any
  return data
    .map(record => {
      let [keptFields, wideFields] = splitRecord(record, ...columns)
      let longFields = lengthen<S>(keptFields, keyLabel, valueLabel)
      let nestedArrays = longFields.map(longField => Object.assign({}, wideFields, longField))
      return nestedArrays
    })
    .reduce((acc, arr) => acc.concat(arr), [])
}