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

gather-final.js

'use strict';
let R = require('ramda');

// lengthenRow :: String -> String -> [Object]
let lengthenRow = R.curry(function (keyLabel, valueLabel, row) {
  let customKV = key => ({[keyLabel]: key,
                          [valueLabel]: row[key]});
  return Object.keys(row).map(customKV);
});

// gatherRow :: String -> String -> [String] -> Object -> [Object]
let gatherRow = R.curry(function (keyLabel, valueLabel, columns, row) {
  // Convert wide JSON representation of CSV row into an array long format rows
  let pickWithout = R.pick(R.difference(R.keys(row), columns));
  let pickWith = R.pick(columns);
  let kept = pickWithout(row);
  let wide = pickWith(row);

  let lengthened = lengthenRow(keyLabel, valueLabel, wide);

  let mergeAll = R.map(R.merge(kept));
  return mergeAll(lengthened);
});

module.exports.gatherRow = gatherRow;

gather-ramda.js

'use strict';
let R = require('ramda');

// lengthenRow :: String -> String -> [Object]
let lengthenRow = R.curry(function (keyLabel, valueLabel, row) {
  // This point-free thing is kinda cute, but I think the original was clearer.
  // Note: lengthenRow is just the trivial gatherRow when no columns are kept!
  let customKV = key => ({[keyLabel]: key,
                          [valueLabel]: row[key]});
  let customEntries = R.map(customKV);
  let lengthen = R.compose(customEntries, R.keys);
  return lengthen(row);
});

// gatherRow :: String -> String -> [String] -> Object -> [Object]
let gatherRow = R.curry(function (keyLabel, valueLabel, columns, row) {
  // Convert wide JSON representation of CSV row into long format
  let pickWithout = R.pick(R.difference(R.keys(row), columns));
  let pickWith = R.pick(columns);
  let kept = pickWithout(row);
  let wide = pickWith(row);

  let lengthened = lengthenRow(keyLabel, valueLabel, wide);

  let mergeAll = R.map(R.merge(kept));
  return mergeAll(lengthened);
});

module.exports.gatherRow = gatherRow;

gather-vanilla.js

'use strict';

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

function splitRecord(record, ...fields) {
  let withGivenFields = withFields(record, fields);
  let otherFields = Object.keys(record)
    .filter(key => !(fields.includes(key)));
  let withOtherFields = withFields(record, otherFields);
  return [withOtherFields, withGivenFields];
}

function gather(data, keyLabel, valueLabel, ...columns) {
  // Convert wide JSON representation of CSV into long format
  let lengthen = record => Object.keys(record)
    .map(key => ({[keyLabel]: key,
                [valueLabel]: record[key]}));

  return data.map(record => {
    let [keptFields, wideFields] = splitRecord(record, ...columns);
    let longFields = lengthen(wideFields);
    let nestedArrays = longFields.map(
      longField => Object.assign({}, longField, keptFields)
    );
    return nestedArrays;
  }).reduce((acc, arr) => acc.concat(arr), []);
}

module.exports.gather = gather;

In the gather-vanilla.js the variables are changed in lines 25 and 27.

return data.map(record => {
   let [keptFields, wideFields] = splitRecord(record, ...columns);
   let longFields = lengthen(keptFields); //here
   let nestedArrays = longFields.map(
     longField => Object.assign({}, longField, wideFields) // and here
   );
   return nestedArrays;
 }).reduce((acc, arr) => acc.concat(arr), []);