## React for the algebra enthusiast – Part 1

When I learned to use the react framework, I always had the feeling that it is written in a very mathy way. Since simple googling did not give me any hints if this was a consideration in the design, I thought it might be worth sharing my thoughts on that. I should mention that I am sure others have made the same observations, but it might help algebraist to understand react faster and mathy computer scientiests to remember some algebra.

## Free monoids

In abstract algebra, a monoid is a set M together with a binary operation “$\cdot$” satisfying these two laws:

• There is a neutral element “e”, such that: $\forall x \in M: x \cdot e = e \cdot x = e$
• The operation is associative, i.e. $\forall x,y,z \in M: x \cdot (y\cdot z) = (x\cdot y) \cdot z$

Here are some examples:

• Any set with exactly one element together with the unique choice of operation on it.
• The natural numbers $\mathbb{N}=\{0,1,2,\dots \}$ with addition.
• The one-based natural numbers $\mathbb{N}_1=\{1,2,3,\dots\}$ with multiplication.
• The Integers $\mathbb Z$ with addition.
• For any set M, the set of maps from M to M is a monoid with composition of maps.
• For any set A, we can construct the set List(A), consisting of all finite lists of elements of A. List(A) is a monoid with concatenation of lists. We will denote lists like this: $[1,2,3,\dots]$

Monoids of the form List(A) are called free. With “of the form” I mean that the elements of the sets can be renamed so that sets and operations are the same. For example, the monoid $\mathbb{N}$ with addition and List({1}) are of the same form, witnessed by the following renaming scheme:

$0 \mapsto []$

$1 \mapsto [1]$

$2 \mapsto [1,1]$

$3 \mapsto [1,1,1]$

$\dots$

— so addition and appending lists are the same operation under this identification.

With the exception of $\mathbb{N}_1$, the integers and the monoid of maps on a set, all of the examples above are free monoids. There is also a nice abstract definition of “free”, but for the purpose at hand to describe a special kind of monoid, it is good enough to say, that a monoid M is free, if there is a set A such that M is of the form List(A).

## Action monoids

A react-app (and by that I really mean a react+redux app) has a set of actions. An action always has a type, which is usally a string and a possibly empty list of arguments.

Let us stick to a simple app for now, where each action just has a type and nothing else. And let us further assume, that actions can appear in arbirtrary sequences. That means any action can be fired in any state. The latter simplification will keep us clear from more advanced algebra for now.

For a react-app, sequences of actions form a free monoid. Let us look at a simple example: Suppose our app is a counter which starts with “0” and has an increment (I) and decrement (D) action. Then the sequences of action can be represented by strings like

ID, IIDID, DDD, IDI, …

which form a free monoid with juxtaposition of strings. I have to admit, so far this is not very helpful for a practitioner – but I am pretty sure the next step has at least some potential to help in a complicated situation:

## Quotients

Quotients of sets by an equivalence relation are a very basic tool of modern math. For a monoid, it is not clear if a quotient of its underlying set will still be a monoid with the “same” operations.

Let us look at an example, where everything goes well. In the example from above, the counter should show the same integer if we decrement and then increment (or the other way around). So we could say that the two action sequences

• ID and
• DI

do really nothing and should be considered equivalent to the empty action sequence. So let’s say that any sequence of actions is equivalent to the same sequence with any occurence of “DI” or “ID” deleted. So for example we get:

IIDIIDD $\sim$ I

With this rule, we can reduce any sequence to an equivalent one that is a sequence of Is, a sequence of Ds or empty. So the quotient monoid can be identified with the integers (in two different ways, but that’s ok) and addition corresponds to juxtaposition of action sequences.

The point of this example and the moral of this post is, that we can take a syntactic description (the monoid of action sequences), which is easy to derive from the source code and look at a quotient of the action monoid by a reasonable relation to arrive at some algebraic structure which has a lot to do with the semantic of the app.

So the question remains, if this works just well for an example or if we have a general recipe.

Here is a problem in the general situation: Let $x,y,z\in M$ be elements of a monoid $M$ with operation “$\cdot$” and $\sim$ be an equivalence relation such that $x$ is identified with $y$. Then, denoting equivalence classes with $[\_]$ it is not clear if $[x] \cdot [y]$ should be defined to be $[x\cdot z]$ or $[y\cdot z]$.

Fortunately problems like that disappear for free monoids like our action monoid and equivalence relations constructed in a specific way. As you can see on wikipedia, it is always ok to take the equivalence relation generated by the same kind of identifications we made above: Pick some pairs of sequences which are known “to do the same” from a semantic point of view (like “ID” and “DI” did the same as the empty sequence) and declare sequences to be equivalent, if they arise by replacing sequences known to be the same.

So the approach is that general: It works for apps, where actions do not have parameters and can be fired in any order and for equivalence relations generated by defining finitely many action sequences to do the same. The “any order” is a real restriction, but this post also has a “Part 1” in the title…

## Configurable React backend in deployment

In my last post I explained how to make you React App configurable with the backend endpoint as an example. I did not make clear that the depicted approach is build-time configurability.

If you want deploy- or runtime-time configurability the most simple approach is to provide global variables in your index.html like so:

<!DOCTYPE html>
<html lang="en">
<script>
window.REACT_APP_BACKEND_API_BASE_URL= 'http://some.other.server:5000';
window.APPLICATION_CONFIGURATION = {
settingA: 'aValue',
anotherSetting: 'anotherValue'
};
</script>
<body>
<noscript>
You need to enable JavaScript to run this app.
</noscript>
<div id="root"></div>
</body>
</html>


We use (or activate) this configuration similar to the build-time approach with .env files:

// If we have a differing backend configured, replace the global fetch()
// instead of process.env.REACT_APP_BACKEND_API_BASE_URL
// we now use window.REACT_APP_BACKEND_API_BASE_URL
if (window.REACT_APP_BACKEND_API_BASE_URL !== undefined
&& window.REACT_APP_BACKEND_API_BASE_URL !== '') {
applyBaseUrlToFetch(window.REACT_APP_BACKEND_API_BASE_URL);
}


That way an automated process or a human administrator can deploy the same artifact to different servers with customized settings. This approach is briefly explained in the create-react-app documentation. In addition a server-side application could replace placeholders dynamically in the html file, e.g. with data from a configuration database.

I personally like this approach because it allows us to use the same build artifact for internal testing, staging systems and production at the clients site. It also allows the client to make some basic configuration themselves.

## Making the backend of your React App configurable

Nowadays, the frontend and backend of a web application usually are separate parts – oftentimes implemented using different technologies – communicating with each other using HTTP or websockets. For simplicity and smaller deployments they are hostet on the same web server. There are several reasons to deploy them on different servers like load distribution, security, different environments running the same frontend with differing backends and so on.

To allow separate deployments without changing the frontend code per deployment we need to make the backend transparently configurable. Fortunately, this is relatively easy for frontend written in React and set up with create-react-app. To make this fully transparent for your frontend code we need to

1. Make the backend URL configurable
2. Replace the fetch() function to use the configured backend
3. Activate the setup at the start of our app

# Configuring a React App

Create-react-app provides a configuration mechanism with custom environment variables using .env-files. We can simply provide different env-files for our environments where we can configure different aspects of our application. In our use case this is the backend URL.

// The base url of the backend API. Add path prefix if the API does not run at the server root.
REACT_APP_BACKEND_API_BASE_URL=http://some.other.server:5000


Inside our React App we can reference the configured values using {process.env.REACT_APP_BACKEND_API_BASE_URL}.

# Making the use of our configured backend transparent

In a modern JavaScript app the main mean to communicate with the backend is the fetch()-API. To make the use of our configured backend transparent we can replace the global fetch()-function with our version like so:

// remember the original fetch-function to delegate to
const originalFetch = global.fetch;

export const applyBaseUrlToFetch = (baseUrl) =&gt; {
// replace the global fetch() with our version where we prefix the given URL with a baseUrl
global.fetch = (url, options) =&gt; {
const finalUrl = baseUrl + url;
return originalFetch(finalUrl, options);
};
};


That way all of our fetch() calls are re-routed to the configured backend.

# Activating our fetch()-customization

Now that we have all the pieces of our infrastructure in place we need to activate the changes to fetch on application startup. So we add code like below to our index.js:

// If we have a differing backend configured, replace the global fetch()
if (process.env.REACT_APP_BACKEND_API_BASE_URL !== undefined &amp;&amp; process.env.REACT_APP_BACKEND_API_BASE_URL !== '') {
applyBaseUrlToFetch(process.env.REACT_APP_BACKEND_API_BASE_URL);
}


Now all our calls to a relative URL will be prefixed with a configurable base and that way different backends can be used with the same application code.

# Caveats

The above approach works nicely if you have exactly one backend for your app and do not fetch from other sources. If you do, you may want to expose the original fetch function as something like fetchExternal() to be able to explicitly fetch from other sources.

In addition, if frontend and backend reside on different servers/sites using differring DNS-names you will have to configure CORS for your backends or your browser will refuse to make the requests!

## Client-side web development: Drink the Kool-Aid or be cautious?

Client side web development is a fast-changing world. JavaScript libraries and frameworks come and go monthly. A couple of years ago jQuery was a huge thing, then AngularJS, and nowadays people use React or Vue.js with a state container like Redux. And so do we for new projects. Unfortunately, these modern client-side frameworks are based on the npm ecosystem, which is notoriously known for its dependency bloat. Even if you only have a couple of direct dependencies the package manager lock file will list hundreds of indirect dependencies. Experience has shown that lots of dependencies will result in a maintenance burden as time passes, especially when you have to do major version updates. Also, as mentioned above, frameworks come and then go out of fashion, and the maintainers of a framework move on to their next big thing pet project, leaving you and your project sitting on a barely or no longer maintained base, and frameworks can’t be easily replaced, because they tend to permeate every aspect of your application.

With this frustrating experience in mind we recently did an experiment for a new medium sized web project. We avoided frameworks and the npm ecosystem and only used JavaScript libraries with no or very few indirect dependencies, which really were necessary. Browsers have become better at being compatible to web standards, at least regarding the basics. Libraries like jQuery and poly-fills that paper over the incompatibilities can mostly be avoided — an interesting resource is the website You Might Not Need jQuery.

We still organised our views as components, and they are communicating via a very simple event dispatcher. Some things had to be done by foot, but not too much. It works, although the result is not as pure as it would have been with declarative views as facilitated by React and a functional state container like Redux. We’re still fans of the React+Redux approach and we’re using it happily (at least for now) for other projects, but we’re also skeptical regarding the long term costs, especially from relying on the npm ecosystem. Which approach will result in less maintenance burden? We don’t know yet. Time will tell.

## Analysing a React web app using SonarQube

Many developers especially from the Java world may know the code analysis platform SonarQube (formerly SONAR). While its focus was mostly integration all the great analysis tools for Java the modular architecture allows plugging tools for other languages to provide linter results and code coverage under the same web interface.

We are a polyglot bunch and are using more and more React in addition to our Java, C++, .Net and “what not” projects. Of course we would like the same quality overview for these JavaScript projects as we are used to in other ecosystems. So I tried SonarQube for react.

# The start

Using SonarQube to analyse a JavaScript project is as easy as for the other languages: Just provide a sonar-project.properties file specifying the sources and some paths for analysis results and there you go. It may look similar to the following for a create-react-app:

sonar.projectKey=myproject:webclient
sonar.projectName=Webclient for my cool project
sonar.projectVersion=0.3.0

#sonar.language=js
sonar.sources=src
sonar.exclusions=src/tests/**
sonar.tests=src/tests
sonar.sourceEncoding=UTF-8

#sonar.test.inclusions=src/tests/**/*.test.js
sonar.coverage.exclusions=src/tests/**

sonar.junit.reportPaths=test-results/test-report.junit.xml
sonar.javascript.lcov.reportPaths=coverage/lcov.info


For the coverage you need to add some settings to your package.json, too:

{ ...
"devDependencies": {
"enzyme": "^3.3.0",
"eslint": "^4.19.1",
"eslint-plugin-react": "^7.7.0",
"jest-junit": "^3.6.0"
},
"jest": {
"collectCoverageFrom": [
"src/**/*.{js,jsx}",
"!**/node_modules/**",
"!build/**"
],
"coverageReporters": [
"lcov",
"text"
]
},
"jest-junit": {
"output": "test-results/test-report.junit.xml"
},
...
}


This is all nice but the set of built-in rules for JavaScript is a bit thin and does not fit React apps nicely.

# ESLint to the recue

But you can make SonarQube use ESLint and thus become more useful.

First you have to install the ESLint Plugin for SonarQube from github.

Second you have to setup ESLint to your liking using eslint --init in your project. That results in a eslintrc.js similar to this:

module.exports = {
'env': {
'browser': true,
'commonjs': true,
'es6': true
},
'extends': 'eslint:recommended',
'parserOptions': {
'ecmaFeatures': {
'jsx': true
},
'sourceType': 'module'
},
'plugins': [
'react'
],
'rules': {
'indent': [
'error',
2
],
'linebreak-style': [
'error',
'unix'
],
'quotes': [
'error',
'single'
],
'semi': [
'error',
'always'
]
}
};



Lastly enable the ESLint ruleset for your project in sonarqube and look at the results. You may need to tune one thing or another but you will get some useful static analysis helping you to improve your code quality further.

## Internationalization of a React application with react-intl

For the internationalization of a React application I have recently used the seemingly popular react-intl package by Yahoo.

The basic usage is simple. To resolve a message use the FormattedMessage tag in the render method of a React component:

import {FormattedMessage} from "react-intl";

class Greeting extends React.Component {
render() {
return (
<div>
<FormattedMessage id="greeting.message"
defaultMessage={"Hello, world!"}/>
</div>
);
}
}


### Injecting the “intl” property

If you have a text in your application that can’t be simply resolved with a FormattedMessage tag, because you need it as a string variable in your code, you have to inject the intl property into your React component and then resolve the message via the formatMessage method on the intl property.

To inject this property you have to wrap the component class via the injectIntl() function and then re-assign the wrapped class to the original class identifier:

import {intlShape, injectIntl} from "react-intl";

class SearchField extends React.Component {
render() {
const intl = this.props.intl;
const placeholder = intl.formatMessage({
id: "search.field.placeholder",
defaultMessage: "Search"
});
return (<input type="search" name="query"
placeholder={placeholder}/>);
}
}
SearchField.propTypes = {
intl: intlShape.isRequired
};
SearchField = injectIntl(SearchField);


### Preserving references to components

In one of the components I had captured a reference to a child component with the React ref attribute:

ref={(component) => this.searchInput = component}

After wrapping the parent component class via injectIntl() as described above in order to internationalize it, the internal reference stopped working. It took me a while to figure out how to fix it, since it’s not directly mentioned in the documentation. You have to pass the “withRef: true” option to the injectIntl() call:

SearchForm = injectIntl(SearchForm, {withRef: true});

Here’s a complete example:

import {intlShape, injectIntl} from "react-intl";

class SearchForm extends React.Component {
render() {
const intl = this.props.intl;
const placeholder = intl.formatMessage({
id: "search.field.placeholder",
defaultMessage: "Search"
});
return (
<form>
<input type="search" name="query"
placeholder={placeholder}
ref={(c) => this.searchInput = c}/>
</form>
);
}
}
SearchForm.propTypes = {
intl: intlShape.isRequired
};
SearchForm = injectIntl(SearchForm,
{withRef: true});


### Conclusion

Although react-intl appears to be one of the more mature internationalization packages for React, the overall experience isn’t too great. Unfortunately, you have to litter the code of your components with dependency injection boilerplate code, and the documentation is lacking.