1. 1 : /**
  2. 2 :  * Object.defineProperty but "lazy", which means that the value is only set after
  3. 3 :  * it retrieved the first time, rather than being set right away.
  4. 4 :  *
  5. 5 :  * @param {Object} obj the object to set the property on
  6. 6 :  * @param {string} key the key for the property to set
  7. 7 :  * @param {Function} getValue the function used to get the value when it is needed.
  8. 8 :  * @param {boolean} setter wether a setter shoould be allowed or not
  9. 9 :  */
  10. 10 : const defineLazyProperty = function(obj, key, getValue, setter = true) {
  11. 11 :   const set = (value) =>
  12. 12 :     Object.defineProperty(obj, key, {value, enumerable: true, writable: true});
  13. 13 : 
  14. 14 :   const options = {
  15. 15 :     configurable: true,
  16. 16 :     enumerable: true,
  17. 17 :     get() {
  18. 18 :       const value = getValue();
  19. 19 : 
  20. 20 :       set(value);
  21. 21 : 
  22. 22 :       return value;
  23. 23 :     }
  24. 24 :   };
  25. 25 : 
  26. 26 :   if (setter) {
  27. 27 :     options.set = set;
  28. 28 :   }
  29. 29 : 
  30. 30 :   return Object.defineProperty(obj, key, options);
  31. 31 : };
  32. 32 : 
  33. 33 : export default defineLazyProperty;