The True Delight of React’s Error and Warning Messages

July 10, 2018 0 Comments

The True Delight of React’s Error and Warning Messages



Photo by <a href=Štefan Štefančík on Unsplash">Photo by Štefan Štefančík on Unsplash


There is this fascinating idea that hasn’t left me for years now, that software developers, with all their prejudices, biases, and humanness in general, shape the world around us.

Think about it. Software, just like music, is an intangible good. It’s not wine or chocolate where raw materials can limit the product quantity then the number of people who enjoy it. There exists thousands of software pieces that are written by individuals on a Saturday night and are used by millions of people everywhere.

It’s fair to conclude that those individuals have an impact, small or big, on people’s lives. And even if we consider corporations, the number of people who produce software is a microscopic number in relation to the number of said software users. For instance, Google has ~18K engineers worldwide, while around 1.17 billion people use Google, that’s a ratio of 0.000015%. We can see the huge dependence on software developers’ opinions and ideas to shape the world. There is obviously a developer society and a sub-culture. And if a meme or an idea spreads in this society, it would inevitably shape the technology used by everyone.

Now imagine, in a sci-fi kind of way, that there is an imaginary software technology that solves a world problem. Say it can cure cancer or completely eliminate poverty. But, this technology has the cost of ruining the life of the software developer who uses it. Would that technology ever see the light? Hardly. This imaginary story is an extreme example of the conflict of interest between the software developer and the end user. If good software meant the developer being uncomfortable, we will not see good software. And a big part of software developers’ lives is error messages. They’re key to the experience, and paying attention to them goes a long long way. They are directly important to the developer, and indirectly important to everyone of us.


Of all the libraries and packages I’ve used in the last ~10 years, I haven’t seen a library where errors and warnings are as beautiful. Developers, including me, usually try to detect errors that break their own code and report them to the user along with the data they have on hand (scope?). Maybe mention the most common mistake that can cause the error along with an automatically-generated stack trace and that’s it.

TypeError: undefined is not an object 

But with React, the library works overtime trying to guess for you what went wrong. The error guessing effort is quite obvious and it’s tremendously helpful; it saves you a lot of time debugging and tries its best to save your day.

To help you enjoy the experience like I do writing React apps, I’ll make this article a ride. I’ll show React snippets with mistakes in them, you may try to find the issue or just look at the warning/error below and decide whether it would be helpful for you. This is not to list every message React logs. It’s a very small sample for inspiration.

Let’s start!

1. A simple one

class MyComponent extends React.Component { componentWillMount() { console.log('Component mounted!') } 

What’s wrong with this component?

Here is the message:

Warning: MyComponent(…): No render method found on the returned component instance: you may have forgotten to define render. 

Beautiful, isn’t it? The component name and a correct suggestion. Super easy to fix.

2. A trickier one

class MyComponent extends React.Component { componentDidUnmount() { console.log('Unmounted!'); } render() { return <h1>Hi!</h1> } 

What’s the issue here?

Here is the message:

Warning: MyComponent has a method called componentDidUnmount(). But there is no such lifecycle method. Did you mean componentWillUnmount()? 

React went out of its way here. It expected you to make this mistake and waited for you there. Silence wouldn’t break React in this case, it would only break your app. Quite sympathetic of the library.

3. A little obvious one

class GreetingComponent extends React.Component { constructor() { super(); this.state = {name: 'Omar'}; } render() { this.setState({name: 'Not Omar'}); return <div>Hi {}!</div> } 

What’s the issue?

Here is the warning:

Warning: Cannot update during an existing state transition (such as within render or another component’s constructor). Render methods should be a pure function of props and state; constructor side-effects are an anti-pattern, but can be moved to componentWillMount. 

4. Not quite obvious

class MyComponent extends React.Component { constructor() { super(); this.setState({name: 'John'}); } render() { return <h1>Hi {}!</h1> } 

What’s the issue?

Here is the message:

Warning: setState(…): Can only update a mounted or mounting component. This usually means you called setState() on an unmounted component. This is a no-op. Please check the code for the MyComponent component. 

5. Pure elegance

const type = true; ReactDOM.render( <input type={type} />, document.getElementById("root")

What’s wrong with type here?

Here is the warning:

Warning: Received true for non-boolean attribute type. If this is expected, cast the value to a string. in input 

And that’s another example of an error that doesn’t affect React, rather affects your app.

6. The beginner’s rescuer

class greetingComponent extends React.Component { render() { return <h1>Hi!</h1> } 
} ReactDOM.render( <greetingComponent />, document.getElementById("root")

The two warnings:

Warning: <greetingComponent /> is using uppercase HTML. Always use lowercase HTML tags in React. Warning: The tag <greetingComponent> is unrecognized in this browser. If you meant to render a React component, start its name with an uppercase letter. 

I definitely fell for this at least once.

7. OK I fixed it, but it still doesn’t work

class GreetingComponent extends React.Component { render() { return <h1>Hi!</h1> } 
} ReactDOM.render( GreetingComponent, document.getElementById("root")

What’s wrong now?

The message:

Warning: Functions are not valid as a React child. This may happen if you return a Component instead of <Component /> from render. Or maybe you meant to call this function rather than return it. 

Yup, it should be:

ReactDOM.render(<GreetingComponent />, document.getElementById("root")); 

8. A very common mistake in the first couple days

class GreetingComponent extends React.Component { render() { return <h1 class="bold">Hello!</h1> }
} ReactDOM.render( <GreetingComponent />, document.getElementById("root")

What’s up here?

The message:

Warning: Invalid DOM property class. Did you mean className? in h1 (created by GreetingComponent) in GreetingComponent 

Enough data to land you exactly at your mistake

9. Why don’t you come back? 🎼

class GreetingComponent extends React.Component { render() { <h1>Hello!</h1> }
} ReactDOM.render( <GreetingComponent />, document.getElementById("root")

You said goodbye
I was trying to hide what I felt inside
Until you passed me by
You said you’d return

Will you ever return? 🎵

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