Function tokenizeFormatString

src/adapter/messageFormat.ts:17–66

(format: string, formatterNames: string[])

export type Formatters<T> = Map<string, (a: T, context: IPreviewContext) => string>;

function tokenizeFormatString(format: string, formatterNames: string[]): FormatToken[] {
  if (!format.includes('%')) {
    return [{ type: 'string', value: format }]; // happy path, no formatting needed
  }

  const tokens: FormatToken[] = [];

  function addStringToken(str: string) {
    if (!str) return;
    const lastToken = tokens[tokens.length - 1];
    if (lastToken?.type === 'string') lastToken.value += str;
    else tokens.push({ type: 'string', value: str });
  }

  function addSpecifierToken(
    specifier: string,
    precision: number | undefined,
    substitutionIndex: number,
  ) {
    tokens.push({
      type: 'specifier',
      specifier: specifier,
      precision: precision,
      substitutionIndex,
    });
  }

  let textStart = 0;
  let substitutionIndex = 0;
  const re = new RegExp(`%%|%(?:(\\d+)\\$)?(?:\\.(\\d*))?([${formatterNames.join('')}])`, 'g');
  for (let match = re.exec(format); !!match; match = re.exec(format)) {
    const matchStart = match.index;
    if (matchStart > textStart) addStringToken(format.substring(textStart, matchStart));

    if (match[0] === '%%') {
      addStringToken('%');
    } else {
      const [, substitionString, precisionString, specifierString] = match;
      if (substitionString && Number(substitionString) > 0) {
        substitutionIndex = Number(substitionString) - 1;
      }
      const precision = precisionString ? Number(precisionString) : undefined;
      addSpecifierToken(specifierString, precision, substitutionIndex);
      ++substitutionIndex;
    }
    textStart = matchStart + match[0].length;
  }
  addStringToken(format.substring(textStart));
  return tokens;
}

export function formatMessage<T>(
  format: string,