redux-form/es/structure/immutable/splice.js

A higher order component decorator for forms using Redux and React

import { List } from 'immutable';

export default (function () {
  var list = arguments.length <= 0 || arguments[0] === undefined ? List.isList(list) || List() : arguments[0];
  var index = arguments[1];
  var removeNum = arguments[2];
  var value = arguments[3];

  if (index < list.count()) {
    if (value === undefined && !removeNum) {
      // inserting undefined
      // first insert null and then re-set it to undefined
      return list.splice(index, 0, null).set(index, undefined);
    }
    if (value != null) {
      return list.splice(index, removeNum, value); // removing and adding
    } else {
      return list.splice(index, removeNum); // removing
    }
  }
  if (removeNum) {
    // trying to remove non-existant item: return original array
    return list;
  }
  // trying to add outside of range: just set value
  return list.set(index, value);
});