# Errors and Known Issues for Learning React

**URL:** <https://forum.kirupa.com/t/errors-and-known-issues-for-learning-react/638529>\
**Category:** programming\
**Created:** [July 23, 2018, 5:49pm UTC](https://forum.kirupa.com/t/errors-and-known-issues-for-learning-react/638529 "2018-07-23T17:49:58Z")\
**Posts on this page:** 5\
**Page:** 1

<div class="post-metadata">

**Author:** ![kirupa](https://yyz1.discourse-cdn.com/flex011/user_avatar/forum.kirupa.com/kirupa/32/11616_2.png) [@kirupa](https://forum.kirupa.com/u/kirupa)\
**Post date:** [July 23, 2018, 5:49pm UTC](https://forum.kirupa.com/t/errors-and-known-issues-for-learning-react/638529/1 "2018-07-23T17:49:58Z")

</div>

A great combination of editors and reviewers help ensure that the book's content is error-free. Sometimes, errors do slip through, and this page calls them out. Do note that we fix these errors in subsequent printings, so you may not run into all of the errors listed below.

* * *
This is a companion discussion topic for the original entry at [https://www.kirupa.com/react/errata.htm](https://www.kirupa.com/react/errata.htm)

---

<div class="post-metadata">

**Author:** ![jamesnotjim](https://yyz1.discourse-cdn.com/flex011/user_avatar/forum.kirupa.com/jamesnotjim/32/9271_2.png) [@jamesnotjim](https://forum.kirupa.com/u/jamesnotjim)\
**Post date:** [August 29, 2018, 8:19pm UTC](https://forum.kirupa.com/t/errors-and-known-issues-for-learning-react/638529/2 "2018-08-29T20:19:14Z")

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Minor error in the web edition, Chapter 11, in the “See the Lifecycle Methods in Action” section, the URL to the example is rendered with a space. Here’s the original:  
`https://www.kirupa.com/react/ lifecycle_example.htm`  
And here’s how it should be:  
`https://www.kirupa.com/react/lifecycle_example.htm`

---

<div class="post-metadata">

**Author:** ![kirupa](https://yyz1.discourse-cdn.com/flex011/user_avatar/forum.kirupa.com/kirupa/32/11616_2.png) [@kirupa](https://forum.kirupa.com/u/kirupa)\
**Post date:** [August 30, 2018, 4:58pm UTC](https://forum.kirupa.com/t/errors-and-known-issues-for-learning-react/638529/3 "2018-08-30T16:58:58Z")

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> [@jamesnotjim](#):
>
> the URL to the example is rendered with a space. Here’s the original:  
> `https://www.kirupa.com/react/ lifecycle_example.htm`  
> And here’s how it should be:  
> `https://www.kirupa.com/react/lifecycle_example.htm`

Thanks for pointing it out. I have updated it on the errata location: [Errors and Known Issues for Learning React](https://www.kirupa.com/react/errata.htm) 🙂

---

<div class="post-metadata">

**Author:** ![Deut](https://avatars.discourse-cdn.com/v4/letter/d/49beb7/32.png) [@Deut](https://forum.kirupa.com/u/Deut)\
**Post date:** [June 13, 2019, 12:51pm UTC](https://forum.kirupa.com/t/errors-and-known-issues-for-learning-react/638529/4 "2019-06-13T12:51:03Z")

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Minor error in the printed version of the book.  
The example on page 263 contains a semicolon to much

> ```
> return {
> countValue: state.count;
> } 
> 
> ```

@kirupa, thanks for the great book!

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<div class="post-metadata">

**Author:** ![Fallin](https://yyz1.discourse-cdn.com/flex011/user_avatar/forum.kirupa.com/fallin/32/14208_2.png) [@Fallin](https://forum.kirupa.com/u/Fallin)\
**Post date:** [April 14, 2021, 1:32pm UTC](https://forum.kirupa.com/t/errors-and-known-issues-for-learning-react/638529/6 "2021-04-14T13:32:48Z")

</div>

i have problem in chapter 9

my code:

`\<! DOCTYPE html\>

```
<meta charset="utf-8">

<title>От данных к интерфейсу</title>

<script src="https://unpkg.com/react@16/umd/react.development.js"></script>
<script src="https://unpkg.com/react-dom@16/umd/react-dom.development.js"></script>
<script src="https://unpkg.com/babel-standalone@6.15.0/babel.min.js"></script>

<style>
	#container {
		padding: 50px;
		background-color: #FFF;
	}
</style>

```

```
<script type="text/babel">
	var destination = document.querySelector("#container");
	var colors = ["#393E41", "#E94F37", "#1C89BF", "#A1D363", "#85FFC7", "#297373", "#FF8552", "#A40E4C"];

	var renderData = [];

	for (var i = 0; i < colors.length; i++) {
		var color = colors[i];
		renderData.push(<Circle key={i + color} bgColor={color} />);
	}

		

	class Circle extends React.Component {

		render() {
			var circleStyle = {
				padding: 10,
				margin: 20,
				display: "inline-block",
				backgroundColor: this.props.bgColor,
				borderRadius: "50%",
				width: 100,
				height: 100,
			};

			return (
				<div style={circleStyle}>
				</div>
			);
		}

	}

	ReactDOM.render(
		<div>
			{renderData}
		</div>,
		destination
	);
</script>

```

`

err code:  
react.development.js:401 Warning: React.createElement: type is invalid – expected a string (for built-in components) or a class/function (for composite components) but got: undefined. You likely forgot to export your component from the file it’s defined in, or you might have mixed up default and named imports

I am learning react from your 9th edition book