Identity Proxy

Proxy that is kind of an identity element.

Installation

npm install --save identity-proxy

Motivation

In a React project, I want to make a higher order component to accomplish following task:

import React from "react";
import { connect } from "react-redux";

function withSubscription(
  WrappedComponent,
  mapStateToProps,
  mapDispatchToProps
) {
  class WithSubscription extends React.Component {
    componentDidMount() {
      // for each prop injected by connect from mapDispatchToProps (someAction),
      // someAction();
    }

    componentDidUpdate(prevProps) {
      // for each prop injected by connect from mapStateToProps (someState),
      // if (prevProps.someState != this.props.someState) {
      //   someAction(someState);
      // }
    }

    render() {
      return <WrappedComponent {...this.props} />;
    }
  }

  return connect(
    mapStateToProps,
    mapDispatchToProps
  )(WithSubscription);
}

I wonder how to get the name of someAction and someState without explicitly provided an array of name strings. mapDispatchToProps is usually an object, so someAction can be easily retrieved by using Object.keys(mapDispatchToProps). But mapStateToProps is a function. How can I get the keys of the return object? My answer is this package.

Usage

Any action acting on the proxy will return the proxy itself.

import createIdentityProxy from "identity-proxy";

const identityProxy = createIdentityProxy();

const foo = identityProxy.foo;
const bar = identityProxy();

console.log(foo === identityProxy); // true
console.log(bar === identityProxy); // true
console.log(foo === bar); // true

With the above behaviour, my original question can be answered:

const mapStateToProps = state => ({
  bar: state.foo.bar,
  baz: state.foo.baz
});

const identityProxy = createIdentityProxy();

console.log(Object.keys(mapStateToProps(identityProxy))); // ['bar', 'baz']

The implementation of the function mapStateToProps does not matter. What's more, type can be perserved upon operation on a primitive type:

const identityProxy = createIdentityProxy();
console.log(typeof (identityProxy * 1 - 1 + 1)); // 'number'
console.log(typeof `${identityProxy}`); // 'string'

The goal of all the above behaviours is to not break any function executed with the identityProxy arguments.

Remark

This package relies on Proxy and Symbol.toPrimitive. Need to use suitable polyfills to target old browsers.