How to declare a global variable in React?

Why don't you try using Context?

You can declare a global context variable in any of the parent components and this variable will be accessible across the component tree by this.context.varname. You only have to specify childContextTypes and getChildContext in the parent component and thereafter you can use/modify this from any component by just specifying contextTypes in the child component.

However, please take a note of this as mentioned in docs:

Just as global variables are best avoided when writing clear code, you should avoid using context in most cases. In particular, think twice before using it to "save typing" and using it instead of passing explicit props.

Beyond React

You might not be aware that an import is global already. If you export an object (singleton) it is then globally accessible as an import statement and it can also be modified globally.

If you want to initialize something globally but ensure its only modified once, you can use this singleton approach that initially has modifiable properties but then you can use Object.freeze after its first use to ensure its immutable in your init scenario.

const myInitObject = {}
export default myInitObject

then in your init method referencing it:

import myInitObject from './myInitObject'
myInitObject.someProp = 'i am about to get cold'
Object.freeze(myInitObject)

The myInitObject will still be global as it can be referenced anywhere as an import but will remain frozen and throw if anyone attempts to modify it.

Example of react state using singleton

https://codesandbox.io/s/adoring-architecture-ru3vt (see UserContext.tsx)

If using react-create-app

(what I was looking for actually) In this scenario you can also initialize global objects cleanly when referencing environment variables.

Creating a .env file at the root of your project with prefixed REACT_APP_ variables inside does quite nicely. You can reference within your JS and JSX process.env.REACT_APP_SOME_VAR as you need AND it's immutable by design.

This avoids having to set window.myVar = %REACT_APP_MY_VAR% in HTML.

See more useful details about this from Facebook directly:

https://facebook.github.io/create-react-app/docs/adding-custom-environment-variables

Is not recommended but.... you can use componentWillMount from your app class to add your global variables trough it... a bit like so:

componentWillMount: function () {
    window.MyVars = {
        ajax: require('../helpers/ajax.jsx'),
        utils: require('../helpers/utils.jsx')
    };
}

still consider this a hack... but it will get your job done

btw componentWillMount executes once before rendering, see more here: https://reactjs.org/docs/react-component.html#mounting-componentwillmount