@kingjs/reflect.define-property

Extends Reflect.defineProperty to allow richer descriptors which can include callback, extends, and lazy properties. And lazy can be modified by writeOnce, argument, and static.

Usage

var assert = require('assert');
var defineProperty = require('@kingjs/reflect.define-property');

var target = null;

// create descriptor by harvesting name from named function
var { name, descriptor } = defineProperty(
  null, function foo() { }
)
assert(name == 'foo');
assert(descriptor.value.name == 'foo');

var target = { };

// harvest name from named function
defineProperty(
  target, function foo() { return 0; }
)
assert(target.foo() == 0);

// harvest name from named function in a descriptor
defineProperty(
  target, { value: function bar() { return 0; } }
)
assert(target.bar() == 0);

// harvest name from named get/set in a descriptor
defineProperty(
  target, { get: function baz() { return 0; } }
)
assert(target.baz == 0);

// wrap get/set in a function
defineProperty(
  target, 'lambda', { get: 'this' }
)
assert(target.lambda == target);

// a "readOnly" property can be set only once
defineProperty(
  target, 'readOnly', { 
    get: o => o, 
    lazy: true,
    seeded: true,
    static: true, // because target == this at runtime
    /* seed: 20 */ // un-comment to provide a default
  }
)
target.readOnly = 10;
assert.throws(() => target.readOnly = 20);
assert(target.readOnly == 10);

// wrap value in a function if descriptor is lazy
defineProperty(
  target, 'lazyLambda', { 
    value: 'this.i++', 
    lazy: true,
    static: true, // because target == this at runtime
  }
)
target.i = 0;
assert(target.lazyLambda() == 0);
assert(target.lazyLambda() == 0);

// wrap value in a function if descriptor is an extension
var GetLength = Symbol('getLength');
defineProperty(
  Object.prototype, GetLength, { 
    value: 'this.length', 
    extends: () => String 
  }
)
assert('foo'.length == 'foo'[GetLength]()); // extends String
assert.throws(() => [ ][GetLength]()) // does not extend Array

// defer creation of descriptor until target and name are known
defineProperty(
  target, 
  'extern', { 
    callback: (d, n, t) => ({
      ...d,
      get: () => ({ 
        target: t, name: n 
      }) 
    })
  }
)
assert(target.extern.name == 'extern');
assert(target.extern.target == target);

API

defineProperty(target, name, descriptor)

Overloads

defineProperty(target, namedFunction)
  => defineProperty(target, name, { value: namedFunction, ... })
  
defineProperty(target, { get|set|value: namedFunction, ... })
  => defineProperty(target, name, { get|set|value: namedFunction, ... })

defineProperty(target, name, nonObjectOrNull)
  => defineProperty(target, name, { value: nonObjectOrNull })

Parameters

• target: The target on which the property will be defined.
• name: The name of the property.
• descriptor: A descriptor which supports these additional properties:
• descriptor.extends: A callback that returns a type (function) representing the type being extended. If runtime this is not an instanceof the type, then an exception is thrown. An extension's name must be a symbol and its target must be Object.prototype.
  • Returns a function representing the type being extended.
• descriptor.lazy: Caches the result of the property on the runtime this.
• descriptor.seeded: Modifies lazy. Allows setting the property with a value that gets passed to the promise when resolved.
• descriptor.seed: Modifies seeded. If no value is set, then seed is used as a default.
• descriptor.static: Modifies lazy. Makes the stub configurable so, if runtime this and target are the same object, the stub can be replaced with the cached value.
• descriptor.callback: Called just before calling Object.defineProperty to allow the descriptor to configure itself given name and target. The resulting descriptor is passed to a recursive call of defineProperty.
  • descriptor: A copy of the descriptor.
  • name: The name of the property.
  • target: The target on which the property will be defined.
  • Returns an updated descriptor.

Returns

Returns target if the property was successfully created. Otherwise undefined is returned. If target is null or undefined then { name, descriptor } is returned.

Remarks

• Strings that appear where functions are expected will be wrapped into functions; String values for get or set, or value when extends or lazy is present are wrapped as functions.
• Transforms are applied in the order: lambdize > extends > lazy > callback.
• After applying transforms associated with the properties callback, extends, and lazy, the corresponding property is deleted from the descriptor. This can only be seen if no target is supplied causing the descriptor to be returned.

See Also

• @kingjs/reflect.define-function
• @kingjs/reflect.define-accessor
• @kingjs/reflect.define-value

Install

With npm installed, run

$ npm install @kingjs/reflect.define-property

Dependencies

Package	Version
@kingjs/is	^1.0.9
@kingjs/property-descriptor.lambdize	^1.0.2
@kingjs/property-descriptor.make-lazy	^1.0.3
@kingjs/property-descriptor.target-instance-of	^1.0.3

Source

https://repository.kingjs.net/reflect/define-property

License

MIT