JSONtoCSV

Converts an array of objects to a comma-separated values (CSV) string that contains only the columns specified.

• Use Array.prototype.join(delimiter) to combine all the names in columns to create the first row.
• Use Array.prototype.map() and Array.prototype.reduce() to create a row for each object, substituting non-existent values with empty strings and only mapping values in columns.
• Use Array.prototype.join('\n') to combine all rows into a string.
• Omit the third argument, delimiter, to use a default delimiter of ,.

const JSONtoCSV = (arr, columns, delimiter = ',') =>
  [
    columns.join(delimiter),
    ...arr.map(obj =>
      columns.reduce(
        (acc, key) =>
          `${acc}${!acc.length ? '' : delimiter}"${!obj[key] ? '' : obj[key]}"`,
        ''
      )
    ),
  ].join('\n');

JSONtoCSV(
  [{ a: 1, b: 2 }, { a: 3, b: 4, c: 5 }, { a: 6 }, { b: 7 }],
  ['a', 'b']
); // 'a,b\n"1","2"\n"3","4"\n"6",""\n"","7"'
JSONtoCSV(
  [{ a: 1, b: 2 }, { a: 3, b: 4, c: 5 }, { a: 6 }, { b: 7 }],
  ['a', 'b'],
  ';'
); // 'a;b\n"1";"2"\n"3";"4"\n"6";""\n"";"7"'