proxify-class

Simple library to modify method arguments before calling real method implementation.

Motivation

Library is a result of trying to solve a problem of obtaining database objects by helper methods that could be called in a different context (with different input types).

Depending on the context, helper was going to be called with a DB entity or only ID of the entity.

In order to make sure that the helper always operated on the Object and not on the String and to simplify the code of the helpers, we came up with ProxifyClass idea.

ProxifyClass allowed for separating parameter modifications and method logic, so the methods can always obtain standardised arguments (in our case arguments were DB objects) and not worry about type or structure of the arguments - the arguments had the same form from the method invocation perspective.

Browser/NodeJS support

!! IMPORTANT !!

The library supports ES5 and ES6 separately, due to the differences in class implementation.

By default, the version that is exported works with native ES6 classes (perfect for using with NodeJS).

However, as the browser support for native classes is inconsistent, another version of the library is exposed - supporting ES5 - it could be found in the subdirectory of the library.

To require ES5 version:

 const proxifyClassES5 = require('proxify-class/es5');

For now, to keep the transpiling process as simple as possible, ES6 transform assumes compatibility with recent NodeJS whereas ES5 version assumes compatibility with all recent browsers (using default babel-preset-env settings).

If Proxy object is available, library uses handler.apply() method of the Proxy. Otherwise, simple wrapping is performed, using function.apply.

Methods

proxify(originalFunction, modifier) => Function

Proxifying a single method is rather straightforward:

1. Pass the original function that should be proxified
2. Pass the modifier that supports the same signature as the original function (will be called with exactly the same parameters).

Modifier function has to return all the processed parameters as an Array or Array-like so they can be passed to an original function invocation later.

Modifier function should take a form of modifier(...args) => Array.

Note on immutability

Modifying original arguments passed to a modifier function is an extremely bad practice (immutability for the win!). The recommended approach is to return new list of arguments after processing original ones.

proxifyClass(originalClass, modifier, decide, options) => Class|Object

ProxifyClass allows for proxifying classes, class instances and plain objects.

It allows for proxifying any keys existing on the class prototype, apart from the constructor (if you want to proxify contructor, please use proxifyFunction separately).

As only some of the methods might need to be proxified in specific cases, optional decide function can be passed to proxifyClass to statically choose which class methods should be modified, based on their names.

Under the hood, proxifyClass calls proxifyFunction for every class method that should be proxified.

Options support following parameters:

• passGenerator - indicates that passed modifier is a modifier generator, taking propertyName as an only argument and returning modifier. Useful for any interaction that needs property (method) name, e.g. logging

decide(methodName) => Boolean

Decider has a simple purpose - to determine which class methods should be proxified, based on their names. This function is ran statically once on initial proxifying. It should return true for every method that should be proxified and any falsy value for the ones that should be left untouched.

modifier(...args) => Array

Modifier function takes all the original arguments of a proxified method, clones and modifies them and returns as an elements of the array, so the original method can be called with them. Check proxifyFunction for details.

Support for Promises and async/await

The library supports modifiers that are asynchronous. Please note that returning asynchronous value from the modifier will automatically cause whole proxified function to return asynchronous value. The original implementation HAS to be aware of this fact (so it's developer responsibility to be aware whether returned values should or should not be asynchronous).

To sum up, following three cases make perfect sense:

• Synchronous modifier and synchronous original implementation - will produce synchronously returned variable
• Synchronous modifier and asynchronous original implementation - will always produce asynchronously returned variable (promise)
• Asynchronous modifier AND asynchronous original implementation - will always produce asynchronously returned variable (promise)

Support for callbacks

As the callbacks are just a special functions passed as the last parameter of the function, their support is given (check the tests for an example). There are two scenarios for using callbacks:

• Modifying input arguments - modify arguments however you want and pass the callback as it is
• Modifying the callback (This is an interesting case!) - you can modify values AFTER they are processed in the original function. You just have to wrap the callback into another callback and perform any operation you want on returned values.

Examples

For examples, please refer to examples directory and examples README.