targeted-feature-flag.ts

/**
 * `filteredFlagFactory` supports 3 states of a feature flag:
 * - True,
 * - False,
 * - and restricted by ID(s).
 *
 */
export function filteredFlagFactory(
  flagValue: string,
  defaultIdField = 'userId'
): (args?: any) => boolean {
  if (isTruthyString(flagValue)) return () => true;
  if (flagValue && flagValue.length >= 1) {
    const idSets = getTokenizedIds(flagValue, defaultIdField);
    return (args) => {
      const [lookupField, lookupId] = getHeadKeyValue(args);
      return idSets[lookupField].includes(lookupId);
    }
  }
  
  return (args) => false;
}

function isTruthyString(s: string) {
  s = `${s}`.toLowerCase()
  return s === 'true' || s === 'yes' || s === 'on' || s === '1'
}

function getHeadKeyValue(obj: { [key: string]: number | string }) {
  const [keyValue] = obj != null ? Object.entries(obj) : [];
  
  return keyValue;
}

const idToken = /^\w*:?\d+$/m;

function getTokenizedIds(idList: string, defaultIdField = 'userId') {
  const ids = `${idList}`.split(',');
  return ids
    .filter(id => idToken.test(id))
    .reduce((idSet: { [fieldName: string]: Array<number | string> }, currentId: string) => {
      // Extract ID and any prefix/field label
      let [prefix, id] = currentId.split(':');
      // If no prefix, use the defaultIdField
      if (!id && prefix) {
        id = prefix;
        prefix = defaultIdField;
      }
      // Add/append ID to the prefix array in the object to return
      idSet[prefix] = Array.isArray(idSet[prefix]) ? idSet[prefix] : [];
      idSet[prefix].push(parseInt(id));
      return idSet;
    }, {})
}