Uncaught ReferenceError: React is not defined

Question

I am trying to make ReactJS work with rails using this tutorial. I am getting this error:

---

Uncaught ReferenceError: React is not defined

But I can access the React object in browser console
I also added public/dist/turbo-react.min.js as described here and also added //= require components line in application.js as described in this answer to no luck. Additionally,
var React = require('react') gives the error:
Uncaught ReferenceError: require is not defined

Can anyone suggest me on how to resolve this?

[EDIT 1]
Source code for reference:
This is my comments.js.jsx file:

var Comment = React.createClass({
  render: function () {
    return (
      <div className="comment">
        <h2 className="commentAuthor">
          {this.props.author}
        </h2>
          {this.props.comment}
      </div>
      );
  }
});

var ready = function () {
  React.renderComponent(
    <Comment author="Richard" comment="This is a comment "/>,
    document.getElementById('comments')
  );
};

$(document).ready(ready);

And this is my index.html.erb:

<div id="comments"></div>

Answer 1

I was able to reproduce this error when I was using webpack to build my javascript with the following chunk in my webpack.config.json:

externals: {
    'react': 'React'
},

This above configuration tells webpack to not resolve require('react') by loading an npm module, but instead to expect a global variable (i.e. on the window object) called React. The solution is to either remove this piece of configuration (so React will be bundled with your javascript) or load the React framework externally before this file is executed (so that window.React exists).

Answer 2

I got this error because I was using

import ReactDOM from 'react-dom'

without importing react, once I changed it to below:

import React from 'react';
import ReactDOM from 'react-dom';

The error was solved :)

Answer 3

If you are using Webpack, you can have it load React when needed without having to explicitly require it in your code.

Add to webpack.config.js:

plugins: [
   new webpack.ProvidePlugin({
      "React": "react",
   }),
],

See http://webpack.github.io/docs/shimming-modules.html#plugin-provideplugin

Answer 4

Possible reasons are 1. you didn't load React.JS into your page, 2. you loaded it after the above script into your page. Solution is load the JS file before the above shown script.

P.S

Possible solutions.

1. If you mention react in externals section inside webpack configuration, then you must load react js files directly into your html before bundle.js
2. Make sure you have the line import React from 'react';

Answer 5

If you are using Babel and React 17, you might need to add "runtime": "automatic" to config.

 {
     "presets": ["@babel/preset-env", ["@babel/preset-react", {
        "runtime": "automatic"
     }]]
 }

Answer 6

Try to add:

import React from 'react'
import { render } from 'react-dom'
window.React = React

before the render() function.

This sometimes prevents error to pop-up returning:

React is not defined

Adding React to the window will solve these problems.

Answer 7

Adding to Santosh :

You can load React by

import React from 'react'

Answer 8

import React, { Component, PropTypes } from 'react';

This may also work!

Answer 9

I got this error because in my code I misspelled a component definition with lowercase react.createClass instead of uppercase React.createClass.

Answer 10

I was facing the same issue. I resolved it by importing React and ReactDOM like as follows:

import React from 'react';
import ReactDOM from 'react-dom';

Answer 11

The displayed error

after that import react

Finally import react-dom

if error is react is not define,please add ==>import React from 'react';

if error is reactDOM is not define,please add ==>import ReactDOM from 'react-dom';

Answer 12

If you're using TypeScript, make sure that your tsconfig.json has "jsx": "react" within "compilerOptions".

Answer 13

Sometimes the order of import can cause this error, if after you have followed all the steps above and still finds it hard on you, try making sure the react import comes first.

import React from 'react'
import { render } from 'react-dom'

before any other important you need to make.

Answer 14

To whom using CRA and ended up to this question, can use customize-cra and react-app-rewired packages to override babel presets in CRA. To do this, create a config-overrides.js in the root and paste this code snippet in it.

const { override, addBabelPreset } = require('customize-cra');

module.exports = override(
    addBabelPreset('@emotion/babel-preset-css-prop'),
    addBabelPreset([
        '@babel/preset-react',
        {
            runtime: 'automatic',
            importSource: '@emotion/react',
        },
    ]),
);

And update scripts in package.json with the ones below.

    "start": "react-scripts start",
    "build": "react-scripts build",
    "test": "react-scripts test",

    "start": "react-app-rewired start",
    "build": "react-app-rewired build",
    "test": "react-app-rewired test --env=jsdom",