February 26, 2019

Watch me live code the Alert component example in this article and talk through my motivations as I refactor.

An anti pattern I've seen on many React projects is having multiple boolean flags on your component's state that control different aspects of the same component. All too often these flags can contradict each other and lead to very subtle bugs that are difficult to catch until it's too late. Let's look at one example of such an anti-pattern and how TypeScript's rich type system allows us to express our code in a way that makes it easier for our team members to understand and gives us confidence that our code won't break unexpectedly.

An Alert Component

A common need on web apps is to display an alert on the page confirming an action was sucessful or showing an error message. The simplest way to start is to track this in state using a boolean flag. Let's consider an example of showing an alert following user actions that save data to a server.

1 2 3 import React from 'react' 4 5 function UsernameForm ( ) { 6 const [ isShowing , setIsShowing ] = useState ( false ) 7 const [ isError , setIsError ] = useState ( false ) 8 const [ alertMessage , setAlertMessage ] = useState ( '' ) 9 const [ username , setUsername ] = useState ( '' ) 10 11 async function saveUsername ( ) { 12 const response = await saveUsernameViaApiCall ( ) 13 14 if ( response . errorMessage ) { 15 setAlertMessage ( response . errorMessage ) 16 setIsError ( true ) 17 } else { 18 setAlertMessage ( 'Your username has been updated.' ) 19 } 20 setIsShowing ( true ) 21 } catch ( e ) { 22 setIsError ( true ) 23 } 24 } 25 26 return ( 27 < section > 28 < h1 > Change Your Username < / h1 > 29 { isShowing ? 30 < Alert status = { isError ? 'error' : 'success' } > 31 { alertMessage } 32 < / Alert > 33 : null } 34 < form onSubmit = { saveUsername } > 35 < div > 36 < label htmlFor = "username" > Username < / label > 37 < input 38 type = "text" 39 id = "username" 40 name = "username" 41 onChange = { e => { 42 setUsername ( e . currentTarget . value ) 43 } } 44 / > 45 < / div > 46 < / form > 47 < button > Save < / button > 48 < / section > 49 ) 50 }

Note that the filename above has the `.tsx` extension. This extension allows us to write React components with TypeScript. See the React & Webpack section of the TypeScript Handbook for more information.

In the example above we have a form allowing a user to change their username. Upon submission of the form we make an API call to update the username and once that's complete we set the isShowing state to true . We're assuming here that our API returns errors with an "errorMessage" property, and if we get a value for errorMessage we use that as the message to display in the alert. Above the form we display an alert to inform the user that their username was updated successfully, or the error message if the save was not successful.

Dismissing the Alert

Then the feedback starts coming in. Product learns that users want to be able to dismiss this alert. Of course, our product manager wants to see how many users actually click the "close" button to dismiss it, therefore we need to track the number of times it's clicked. There are a number of ways to accomplish this (which are outside the scope of this post) but after careful consideration our team decides that we should track dismissed as another state of the alert, that way we can use a background script in a worker to periodically communicate this property to our analytics tracking tool. So we add another value to be tracked in our state.

1 function UsernameForm ( ) { 2 const [ isShowing , setIsShowing ] = useState ( false ) 3 const [ isError , setIsError ] = useState ( false ) 4 const [ alertMessage , setAlertMessage ] = useState ( '' ) 5 const [ isDimissed , setIsDismissed ] = useState ( false ) 6 7 const closeAlert = ( ) => { 8 setIsShowing ( false ) 9 setIsDimissed ( true ) 10 } 11 12 async function saveUsername ( ) { 13 try { 14 const response = await saveUsernameViaApiCall ( ) 15 16 if ( response . errorMessage ) { 17 setAlertMessage ( response . errorMessage ) 18 setIsError ( true ) 19 } else { 20 setAlertMessage ( 'Your username has been updated.' ) 21 } 22 setIsShowing ( true ) 23 } catch ( e ) { 24 setIsError ( true ) 25 } 26 } 27 28 return ( 29 < section > 30 < h1 > Change Your Username < / h1 > 31 { isShowing && ! isDismissed ? ( 32 < Alert 33 status = { isError ? 'error' : 'success' } 34 onClose = { closeAlert } 35 message = { alertMessage } 36 / > 37 ) : null } 38 < form onSubmit = { saveUsername } > 39 < div > 40 < label htmlFor = "username" > Username < / label > 41 < input 42 type = "text" 43 id = "username" 44 name = "username" 45 onChange = { e => { 46 setUsername ( e . currentTarget . value ) 47 } } 48 / > 49 < / div > 50 < / form > 51 < button > Save < / button > 52 < / section > 53 ) 54 }

Here we've added a callback to our Alert component to notify us when the close button is clicked. Then we call a closeAlert function that sets both isShowing and isDimsissed to make the alert go away.

Defining Complexity

This is a relatively simple example, but hopefully it's clear how it's already starting to grow in complexity. Every time we display an alert we have to remember to set 3 state values and when it's closed we have to set two of them. However, all of these values are tightly coupled: ie. anytime isError is true , isShowing is also true ; isShowing and isDimsissed are mutually exclusive, they should never be true at the same time.

Looking at the code it's very easy to accidentally introduce a very subtle bug. Thanks to the compound condition for showing the alert ( isShowing && !isDismissed ), if we don't reset isShowing to false when closing the alert, it will still go away like we want it to. It's not too far-fetched, therefore, to imagine someone doing some refactoring on this component and removing the setIsShowing(false) line, since technically it's not needed. Months later imagine that another developer is fixing a bug and introduces some code to reset isDismissed before showing the alert again. Because isShowing was never reset to false the alert is now both showing and dismissed at the same time. Contradictory states like this make for great physics experiments, but are a few of the thousand paper cuts that can make large codebases very difficult and expensive to maintain.

How do you avoid this situation? We started out with something very simple, but as our requirements got more complex we brought that complexity into our code. While at first glance this solution may not seem complex keep in mind that some synonyms of simple are "clear" and "understandable". The way that this solution hides such an important relationship between these values makes it not simple. The way to avoid it is to refactor the moment you notice the complexity starting to creep in.

We've now arrived at the big question: how do we model this state in such a way that we make unexpected states impossible and still meet the requirements? What if we consolidate the 3 boolean flags into a single string called alertStatus with possible values of "not_shown" , "success" , "error" and "dismissed" ?

1 function UsernameForm ( ) { 2 const [ alertStatus , setAlertStatus ] = useState ( 'not_shown' ) 3 const [ alertMessage , setAlertMessage ] = useState ( '' ) 4 5 async function saveUsername ( ) { 6 try { 7 const response = await saveUsernameViaApiCall ( ) 8 9 if ( response . errorMessage ) { 10 setAlertMessage ( response . errorMessage ) 11 setAlertStatus ( 'error' ) 12 } else { 13 setAlertMessage ( 'Your username has been updated.' ) 14 setAlertStatus ( 'success' ) 15 } 16 } catch ( e ) { 17 setAlertMessage ( 18 'An unknown error occurred trying to update your username. Please try again later.' , 19 ) , 20 setAlertStatus ( 'error' ) 21 } 22 } 23 24 return ( 25 < section > 26 < h1 > Change Your Username < / h1 > 27 { alertStatus !== 'not_shown' && alertStatus !== 'dismissed' ? ( 28 < Alert 29 status = { alertStatus } 30 onClose = { ( ) => { 31 setAlertStatus ( 'dismissed' ) 32 } } 33 message = { alertMessage } 34 / > 35 ) : null } 36 < form onSubmit = { saveUsername } > 37 < div > 38 < label htmlFor = "username" > Username < / label > 39 < input 40 type = "text" 41 id = "username" 42 name = "username" 43 onChange = { e => { 44 setUsername ( e . currentTarget . value ) 45 } } 46 / > 47 < / div > 48 < / form > 49 < button > Save < / button > 50 < / section > 51 ) 52 }

By consolidating these flags we made it impossible for the alert to be in two different states at once. We've also cut the number of values needed to show an alert down by a third, making it much less likely that someone will forget to set all the necessary values before rendering with the alert.

We Still Have a Problem

We may have eliminated the contradictory states from our code, but there's still a problem. Since we're using strings for our alertStatus it's very easy to set it to an invalid value; there's nothing stopping someone from calling setAlertStatus("cat") and causing the alert to display wrong. Thanks to the dynamic nature of JavaScript without implementing extra conditions around the Alert component, there is no absolute way to prevent this. You could use React's prop-types library to emit a warning in the browser when it receives an invalid value for status . However, in my experience prop type warnings are often ignored; you can configure a test suite to fail on invalid prop type warnings, or make sure everyone has eslint integration in their editor and use the React eslint plugin to validate prop types where possible.

A Brief Foray into TypeScript

If you want a more foolproof way to help your team members (and, very likely your future self) use your code correctly, TypeScript provides a few features that when used together provide a great defense against invalid states. If you are already familiar with type aliases, string literal types and union types feel free to skip to the next section. Otherwise read on for an introduction to these concepts before continuing.

Type Aliases

Type aliases create a new name for a type. – TypeScript Handbook: Advanced Types

For example, you might rename the string type to ID to show that a specific string represents a unique identifier for a record.

1 type ID = string

String Literal Types

String literal types allow you to specify the exact value a string must have. – TypeScript Handbook: Advanced Types

1 const message = 'Hello world'

In the above example it may look like message is going to have the type string , but if you look at it in an editor that has good TypeScript support, it actually has the type "Hello world" . This is an example of a string literal type. In this case the type is inferred because of the initial value of this string, but since string literal types are types, you can use type aliases to make them reusable.

1 type Message = 'Hello world' 2 3 const message : Message = 'blah'

View on TypeScript Playground

Using the link above you can see that assigning message with a literal value other than "Hello World" gives a compile error, thus allowing you to limit strings to a specific possible value.

You may be wondering at this point how these features will help us with our alert example. We have seen how to enforce a single string value using a type, but our example has four mutually exclusive values. What we need is a way to enforce one of a number of values. And that is where union types come in.

Introducing Union Types

A union type describes a value that can be one of several types. We use the vertical bar (|) to separate each type, so number | string | boolean is the type of a value that can be a number, a string, or a boolean. – TypeScript Handbook: Advanced Types

There has already been a lot of explanation around union types in JavaScript. If you're interested in discussion of the theory behind union types I recommend this overview of union type implementations in various languages. Let's look a quick example of using union types and then we'll get back to our regularly scheduled topic.

Imagine a function called padLeft that pads a string with either a particular character or a number of spaces.

1 padLeft ( 'example' , 5 ) 2 padLeft ( 'example' , '?' )

We have two different type signatures for this function, one that takes a string and a number and another that takes two string s. You could also say that it has a single type signature: it takes a string and a parameter that is either a string OR a number . That second parameter represents a union type.

1 function padLeft ( value : string , padding : string | number ) { 2 3 }

You can use any valid TypeScript types as parts of a union type: records, arrays, functions, strings, numbers, even string literal types – the very thing we need for our alert!

Back to Our Example

Getting back to our alert example, we now know that we can combine type aliases, string literal types & union types to ensure our component never gets in an invalid state.

1 2 3 export type AlertStatus = 'not_shown' | 'success' | 'error' | 'dismissed' 4 5 function Alert ( { status , message } : { status : AlertStatus ; message : string } ) { 6 return < div className = { `alert alert-- ${ status } ` } > { message } < / div > 7 } 8 9 export default Alert

Here we create a basic Alert compnonent that takes a prop called status of type AlertStatus ; it can have four possible values and by separating them with | we create a union type that can only ever be one of the four values. We use the status to determine a CSS class for the alert. We've also changed the interface slightly to make the TypeScript easier to undertand: it also takes a message prop of type string and displays it inside the div . Note that we're also exporting the AlertStatus type; this makes it usable elsewhere.

1 2 import Alert , { AlertStatus } from './Alert' 3 4 function UsernameForm ( ) { 5 const [ alertStatus , setAlertStatus ] = useState ( 'not_shown' as AlertStatus ) 6 const [ alertMessage , setAlertMessage ] = useState ( '' ) 7 8 async function saveUsername ( ) { 9 try { 10 const response = await saveUsernameViaApiCall ( ) 11 12 if ( response . errorMessage ) { 13 setAlertMessage ( response . errorMessage ) 14 setAlertStatus ( 'error' ) 15 } else { 16 setAlertMessage ( 'Your username has been updated.' ) 17 setAlertStatus ( 'success' ) 18 } 19 } catch ( e ) { 20 setAlertMessage ( 21 'An unknown error occurred trying to update your username. Please try again later.' , 22 ) , 23 setAlertStatus ( 'error' ) 24 } 25 } 26 27 return ( 28 < section > 29 < h1 > Change Your Username < / h1 > 30 { alertStatus !== 'not_shown' && alertStatus !== 'dismissed' ? ( 31 < Alert 32 status = { alertStatus } 33 onClose = { ( ) => { 34 setAlertStatus ( 'dismissed' ) 35 } } 36 message = { alertMessage } 37 / > 38 ) : null } 39 < form onSubmit = { saveUsername } > 40 < div > 41 < label htmlFor = "username" > Username < / label > 42 < input 43 type = "text" 44 id = "username" 45 name = "username" 46 onChange = { e => { 47 setUsername ( e . currentTarget . value ) 48 } } 49 / > 50 < / div > 51 < / form > 52 < button > Save < / button > 53 < / section > 54 ) 55 }

Now we're using the AlertStatus type in our UsernameForm component. The only change we needed to make is to add a type assertion. This provides a hint to the TypeScript compiler that we expect useState to deal with AlertStatus types.

Now if we try to call setAlertStatus with a value that it doesn't explicitly support (ie. "warning" ) we'll get a compiler error when trying to build the code. However, the code sent to the browser will be relatively the same as what we wrote (save for standard Babel transformations) – TypeScript will remove the type declarations and assertions and we're left the confidence that our code will work as expected.

Wrapping Up

In this post we've looked at a way to simplify a common example of complexity in React state using TypeScript's union types. I'm including some links at the end that provide options for runtime union types if you can't (or don't want to) use TypeScript (I highly recommend trying it if you haven't; it takes a little getting used to but types allow for an expressiveness & level of safety in your code you simply can't get otherwise). The example in this article was built easily illustrate the problem quickly, but if you are a large codebase that is challenging to maintain you will likely find this anti-pattern in many places. The key to fighting complexity is to refactor as soon as you see it coming. I hope that this article helps prime your intuition so you notice when your state is becoming complex before it's too late.