git.squeep.com Git - squeep-lazy-property/blob - index.js

   1 'use strict';
   2
   3 /**
   4  * Establish some immutable descriptor Shapes with our preferred defaults.
   5  * We want normally-behaving properties unless told otherwise.
   6  */
   7 function defaultShape(proto, ...objs) {
   8   const newObject = Object.create(proto);
   9   Object.assign(newObject, ...objs);
  10   return newObject;
  11 }
  12
  13 const defaultDataDescriptor = defaultShape(null, {
  14   configurable: true,
  15   enumerable: true,
  16   writable: true,
  17   value: undefined,
  18 });
  19
  20 const defaultAccessorDescriptor = defaultShape(null, {
  21   configurable: true,
  22   enumerable: true,
  23   get: undefined,
  24   set: undefined,
  25 });
  26
  27 /**
  28  * Create a new descriptor which inherits our preferred defaults.
  29  * @param {Object} defaultDescriptor
  30  * @param  {...any} objs
  31  * @returns {Object}
  32  */
  33 function descriptorFromDefault(defaultDescriptor, ...objs) {
  34   return Object.assign(Object.create(defaultDescriptor), ...objs);
  35 }
  36
  37 /**
  38  * Defer calling initializer to set value for name until name is read.
  39  * @param {Object} obj
  40  * @param {String} name
  41  * @param {() => {*}} initializer
  42  * @param {Object=} descriptor
  43  * @param {Boolean} descriptor.configurable
  44  * @param {Boolean} descriptor.enumerable
  45  * @param {Boolean} descriptor.writable
  46  */
  47 function lazy(obj, name, initializer, descriptor) {
  48   if (typeof initializer !== 'function') {
  49     throw new TypeError('initialize is not callable');
  50   }
  51
  52   // Establish the descriptor for the eventually-initialized property.
  53   const finalDescriptor = descriptorFromDefault(defaultDataDescriptor, descriptor);
  54
  55   // The descriptor defining the interim accessor property.
  56   const lazyDescriptor = descriptorFromDefault(defaultAccessorDescriptor, {
  57     // The accessor replaces itself with the value returned by invoking the initializer.
  58     get: function () {
  59       finalDescriptor.value = initializer.apply(obj);
  60       Object.defineProperty(obj, name, finalDescriptor);
  61       return finalDescriptor.value;
  62     },
  63
  64     // An explicit set overrides the lazy descriptor with a default data property.
  65     set: function (value) {
  66       Object.defineProperty(obj, name, descriptorFromDefault(defaultDataDescriptor, {
  67         value,
  68       }))
  69     },
  70
  71     configurable: true, // Ensure we can replace ourself.
  72     enumerable: finalDescriptor.enumerable, // Use requested visibility.
  73   });
  74
  75   Object.defineProperty(obj, name, lazyDescriptor);
  76 }
  77
  78 module.exports = {
  79   lazy,
  80 };