compute-if-absent.js

import Ember from 'ember';

/**
 * If the specified keyName is not already associated with a value, attempts to compute its value using
 * the given value function and enters it into the object.
 *
 * @param obj The object to query (and modify)
 * @param keyName The property key to query (and set)
 * @param valueFunc The function that is called with the keyName if the keyName does not yet exist
 * @returns {boolean} The current (existing or computed) value associated with the specified key
 */
export default function computeIfAbsent(obj, keyName, valueFunc) {
  let value = Ember.get(obj, keyName);
  if (value !== undefined)
    return value;

  let newValue = valueFunc(keyName);
  if (newValue !== undefined)
    Ember.set(obj, keyName, newValue);

  return newValue;
}