React create constants file

Expanding on Monad's answer, for situations where you don't want to type myConstClass all the time:

fileWithConstants.js:

export const ACTION_INVALID = "This action is invalid!"
export const CONSTANT_NUMBER_1 = 'hello I am a constant';
export const CONSTANT_NUMBER_2 = 'hello I am also a constant';

fileThatUsesConstants.js:

import { ACTION_INVALID } from 'path/to/fileWithConstants';

const errorMsg = ACTION_INVALID;

(Also, if Monad's way works better for you, I believe the convention is for 'MyConstClass' to start with a capital letter, since it's acting like a class in code.)

You can simply create an object for your constants:

const myConstClass = {
    ACTION_INVALID: "This action is invalid!"
}

And then use it.

If you are bundling, you can export this object and then import for each component file.

I'm not entirely sure I got your question but if I did it should be quite simple:

From my understanding you just want to create a file with constants and use it in another file.

fileWithConstants.js:

export const ACTION_INVALID = "This action is invalid!"
export const CONSTANT_NUMBER_1 = 'hello I am a constant';
export const CONSTANT_NUMBER_2 = 'hello I am also a constant';

fileThatUsesConstants.js:

import * as myConstClass from 'path/to/fileWithConstants';

const errorMsg = myConstClass.ACTION_INVALID;

If you are using react you should have either webpack or packager (for react-native) so you should have babel which can translate your use of export and import to older js.