invertValueArrays

invertValueArrays.js

import * as _ from 'lodash';

//
// I couldn't figure out a clean way to write nor name this function.  It's
//   similar to lodash's "invertBy" except the values are arrays of strings.
//
// It's easiest examplained by example
//
// input
// {
//   key1: ['val1', 'val2'],
//   key2: ['val3', 'val1'],
// }
//
// output
// {
//   val1: ['key1', 'key2'],
//   val2: ['key1'],
//   val3: ['key2'],
//   val4: ['key2'],
// }
//
// I use this below to create a map of supported file extensions to parser
//   names, which creates a helpful error message to detect duplicate file
//   extensions handlers.
//
export const invertValueArrays = (
  result: { [propName: string]: string[] },
  valueArray: string[],
  key: string
): { [propName: string]: string[] } => {

  return valueArray.reduce((innerResult, value) => {
    if (innerResult[value]) {
      innerResult[value].push(key);
    } else {
      _.set(innerResult, value, [key]);
    }
    return innerResult;
  }, result);
};