JavaScript decorator function

console.txt

Foo > 1
Bar > 1
Bar (doubled) > 2
Bar (doubled, default message) > New value is: 2
Bar (doubled, custom message) > New custom value is: 2
Bar (doubled, default message, uppercase) > NEW VALUE IS: 2
Bar (doubled, custom message, uppercase) > NEW CUSTOM VALUE IS: 2

decorator.js

/**
 * Decorator function
 *
 * Decorates any object method, passing the last result to following decorations as the first argument
 * Additional arguments are passed after that, allowing all arguments to be passed to passed all decorations
 */
function decorate(obj, prop, fn) {
	var fnObj = obj[prop]
	obj[prop] = function () {
		var result = fnObj.apply(obj, arguments)
		return fn.apply(obj, [result, ...arguments])
	}
}


/**
 * Decorator example
 *
 * Will create two instances of a base class, and apply multiple decorations to the second 
 */

// base class
function Foo (value) {
	this.value = value || 0;
	this.getValue = function () {
		return this.value; 
	}
}


// create first instance
var foo = new Foo(1)
console.log('Foo >', foo.getValue())


// create second instance
var bar = new Foo(1)
console.log('Bar >', bar.getValue())


// decoration 1 (doubles the result)
decorate(bar, 'getValue', function (original) {
	return original * 2;
})
console.log('Bar (doubled) >', bar.getValue())

// decoration 2 (adds a message)
decorate(bar, 'getValue', function (original, message) {
	return (message || 'New value is: ') + original;
})
console.log('Bar (doubled, default message) >', bar.getValue())
console.log('Bar (doubled, custom message) >', bar.getValue('New custom value is: '))


// decoration 3 (converts to uppercase)
decorate(bar, 'getValue', function (original, message) {
	return original.toUpperCase();
})
console.log('Bar (doubled, default message, uppercase) >', bar.getValue())
console.log('Bar (doubled, custom message, uppercase) >', bar.getValue('New custom value is: '))