Tag Archives: TypeScript

JavaScript: In-browser export to CSV

This function will turn an array of data into a CSV file and get the browser to “download” it. It should also be wrapping and escaping values properly and, because of the BOM, be read correctly when opened in e.g. Excel.

It’s written in TypeScript ("lib": ["ES2017", "DOM"]), but should be easy to “downgrade” to regular JS if needed…

type GetValue <T> = <I extends T>(item: I) => any;
type FieldName <T> = keyof T;

export interface Columns<T> {
  [s: string]: FieldName<T> | GetValue<T>;
}

const COLUMN_SEPARATOR = ';';
const ROW_SEPARATOR = '\r\n';
const UNICODE_BOM = '\uFEFF';

const wrapValue = (value: string) => `"${value}"`;
const escapeValue = (value: string) => (value || '').replace(/"/, '""');

const toHeaderRow = <T> (columns: Columns<T>) => Object
  .keys(columns)
  .map(escapeValue)
  .map(wrapValue)
  .join(COLUMN_SEPARATOR);

const toRow = <T> (columns: Columns<T>, item: T) => Object
  .values(columns)
  .map(field => typeof field === 'function' ? field(item) : item[field])
  .map(String)
  .map(escapeValue)
  .map(wrapValue)
  .join(COLUMN_SEPARATOR);

export const exportToCsv = function <T> (data: T[], columns: Columns<T>, filename: string): void {
  const rows = [];

  rows.push(toHeaderRow(columns));

  for (const item of data) {
    rows.push(toRow(columns, item));
  }

  const csv = UNICODE_BOM + rows.join(ROW_SEPARATOR);
  const uri = `data:text/csv;charset=utf-8;header=present,${encodeURIComponent(csv)}`;

  const link = document.createElement('a');
  link.setAttribute('href', uri);
  link.setAttribute('download', filename);
  link.addEventListener('click', () => link.parentNode.removeChild(link));
  document.body.appendChild(link);

  link.click();
};

Usage

const users: User[] = [{id: 1, name: 'Alice', isCool: true}, ...];
const columns: Columns<User> = {
  'Id': 'id',
  'Name': 'name',
  'Is cool': user => user.isCool ? 'Yes' : 'No',
};
exportToCsv(users, columns, 'users.csv');