Simple CSV Parser

parseCSV.js

/**
 * Parses a string as if it was the contents of a CSV file.
 * @param {string} strCSV 
 * @param {?parseCSV__Options=} options
 * @return {*[][]}
 *   An array of arrays where each value comes from `strCSV`.
 */
function parseCSV(strCSV, options) {
  // Get and validate the delimiter.
  const delimiter = options?.delimiter ?? ',';
  if (/^(?:[\r\n]|[^]{2,}|)$/.test(delimiter)) {
    throw new Error('Delimiter must be exactly one character and cannot be the double-quote or the linefeed or the newline character.');
  }

  // Get the parseValue function.
  const parseValue = options?.parseValue;

  // Parse every part of the CSV.
  let row = [];
  const rows = [row];
  const rgx = new RegExp(`(${delimiter})|(\r\n?|\n)|"((?:[^"]+|"")*)"|([^\r\n${delimiter}]+)`, 'g');
  let lastWasDelim = 2;
  for (let {1: isDelim, 2: isNewRow, 3: quoted, 4: nonQuoted, index} of strCSV.matchAll(rgx)) {
    if (isNewRow || isDelim) {
      if (lastWasDelim) {
        row.push('');
      }
      if (isNewRow) {
        rows.push(row = []);
      }
      lastWasDelim = isNewRow ? 2 : 1;
    }
    else {
      if (!lastWasDelim) {
        throw new Error(`Unexpected cell value starting at character ${index}`);
      }
      if (!nonQuoted) {
        nonQuoted = nonQuoted ?? quoted.replaceAll('""', '"');
      }
      row.push(parseValue ? parseValue(nonQuoted, rows.length - 1, row.length) : nonQuoted);
      lastWasDelim = false;
    }
  }
  // If the last character was a comma then make sure to add an empty string
  // value and parse it if necessary.
  if (lastWasDelim === 1) {
    row.push(parseValue ? parseValue('', rows.length - 1, row.length) : '');
  }

  // The array of arrays of values.
  return rows;
}
/**
 * @typedef parseCSV__Options
 * @property {string} delimiter
 *   Defaults to `","`.  The character that delimits each cell in a row.
 * @property {?((string: string, rowIndex: number, colIndex: number) => any)=} parseValue
 *   If defined this function will be called for each cell's string to determine
 *   the appropriate value for this cell.
 */

toCSV.js

/**
 * Converts a an array of arrays of values into a CSV string.
 * @param {*[][]} rows
 *   The array of arrays of values that will be turned into a CSV string.
 * @param {?toCSV__Options=} options
 *   Optional options that alter how `rows` is turned into a CSV string.
 * @returns {string}
 *   The CSV string version of `rows`.
 */
function toCSV(rows, options) {
  const delimiter = options?.delimiter ?? ',';
  if (/^(?:[\r\n]|[^]{2,}|)$/.test(delimiter)) {
    throw new Error('Delimiter must be exactly one character and cannot be the double-quote or the linefeed or the newline character.');
  }

  const toValue = options?.toValue;

  const rgxNeedsQuoting = new RegExp(`[${delimiter}"\r\n]`);
  
  return rows.map(
    (row, rowIndex) => row.map((value, colIndex) => {
      if (toValue) {
        value = toValue(value, rowIndex, colIndex);
      }
      if ('string' !== typeof value) {
        value = '' + value;
      }
      return rgxNeedsQuoting.test(value)
        ? `"${value.replaceAll('"','""')}"`
        : value;
    }).join(delimiter)
  ).join('\n');
}
/**
 * @typedef toCSV__Options
 * @property {string} delimiter
 *   Defaults to `","`.  The character that delimits each cell in a row.
 * @property {?((string: string, rowIndex: number, colIndex: number) => any)=} toValue
 *   If defined this function will be called for each value to get the values
 *   that will appear in the CSV contents.
 */

toCSVTable.js

/**
 * Converts a an array of arrays of values into a CSV string.
 * @param {Record<string,*>[]} records
 *   The array of arrays of values that will be turned into a CSV string.
 * @param {?toCSVTable__Options=} options
 *   Optional options that alter how `rows` is turned into a CSV string.
 * @returns {string}
 *   The CSV string version of `rows`.
 */
function toCSVTable(records, options) {
  const toValue = options?.toValue;
  let headers = options?.headers;
  if (!Array.isArray(headers)) {
    const fixHeaders = headers;
    headers = [];
    for (const record of records) {
      for (const rowHeader of Object.keys(record)) {
        if (!headers.includes(rowHeader)) {
          headers.push(rowHeader);
        }
      }
    }
    if ('function' === typeof fixHeaders) {
      headers = fixHeaders(headers);
    }
  }

  const headerCount = headers.length;
  const recordCount = records.length;
  const newRows = [headers];
  for (let recordIndex = 0; recordIndex < recordCount; recordIndex++) {
    const record = records[recordIndex];
    const newRow = [];
    for (let headerIndex = 0; headerIndex < headerCount; headerIndex++) {
      const header = headers[headerIndex];
      const value = record[headers[headerIndex]];
      newRow.push(toValue ? toValue(value, recordIndex, headerIndex, header) : value);
    }
    newRows.push(newRow);
  }

  return toCSV(newRows, {delimiter: options?.delimiter});
}
/**
 * @typedef toCSVTable__Options
 * @property {(headers: string[]) => string[]|string[]} headers
 *   Defaults to `","`.  The character that delimits each cell in a row.
 * @property {string} delimiter
 *   Defaults to `","`.  The character that delimits each cell in a row.
 * @property {?((string: string, rowIndex: number, colIndex: number, header: string) => any)=} toValue
 *   If defined this function will be called for each value to get the values
 *   that will appear in the CSV contents.
 */

Parse fails with:

Lorem,"ipsum", dolor, sit,
Lorem,ipsum","dolor,""", sit

Hi @radiocity, I see what you mean. I think my issue with your example is that it really isn't valid CSV. To make that valid it should really look like this anyway:

Lorem,ipsum, dolor, sit
Lorem,"ipsum""","dolor,""", sit

With that said, I will think about making the solution a bit more robust to handle your example as well. Thanks.