@kingjs/property-descriptor.make-lazy

Replaces a description of a function or accessor with a corresponding descriptor that delegates to the original descriptor and caches its result in a a corresponding property on this.

Usage

`use strict`;
var assert = require('assert');
var makeLazy = require('@kingjs/property-descriptor.make-lazy');

var DefaultValue = null;
var IsWriteOnce = true;

var count;
var next = () => count++;

function Type() { }
Object.defineProperties(
  Type.prototype, {
    lazyAccessor: makeLazy.call({ get: next }, 'lazyAccessor'),
    lazyFunction: makeLazy.call({ value: next }, 'lazyFunction'),

    writeOnceAccessor: makeLazy.call(
      { }, // defaults to `get: o => o`
      'writeOnceAccessor',
      IsWriteOnce, 
      DefaultValue
    ),
    
    resolve: makeLazy.call(
      { value: o => o }, 
      'resolve', 
      IsWriteOnce,
      DefaultValue
    ),
  }
)

var instance = new Type();

// lazy accessor
count = 0;
assert(instance.lazyAccessor == 0);
assert(instance.lazyAccessor == 0);

// lazy function
count = 0;
assert(instance.lazyFunction() == 0);
assert(instance.lazyFunction() == 0);

// writeOnce accessor
instance.writeOnceAccessor = 0; // if absent, then DefaultValue is returned
assert(instance.writeOnceAccessor == 0);
assert(Object.getOwnPropertyDescriptor(instance, 'writeOnceAccessor').writable === false);

// resolver function
instance.resolve = 0; // if absent, then DefaultValue is returned
assert(instance.resolve() == 0);
assert(Object.getOwnPropertyDescriptor(instance, 'resolve').writable === false);

API

makeLazy(this, name[, writeOnce[, argument[, isStatic]]])

Parameters

• this: A description of a get accessor or function. If neither, then get is set to the identify function.
• name: The name of the property described by this.
• writeOnce: If provided, then its assumed the get accessor or function described by this accepts a single argument. In this case, the returned promise descriptor may be set with a value which will be passed to the original descriptor when the promise is fulfilled.
• argument: Modifies writeOnce. If no value is set before the function or accessor is resolved then argument is set as a default.
• isStatic: If true then the returned promise descriptor is marked configurable. This allows the promise to replace itself as happens when the descriptor is defined on a target that is also the this at runtime.

Remarks

If the original descriptor returns undefined then the promise is not fulfilled and may be tried again later. This allows the debugger to evaluate a promise before it's ready to be fulfilled without preventing fulfillment at the nominal time.

Install

With npm installed, run

$ npm install @kingjs/property-descriptor.make-lazy

Source

https://repository.kingjs.net/property-descriptor/make-lazy

License

MIT