Utils.js with latest Javascript ES6 syntax

Create the Utils.js file like this with multiple functions, etc

const someCommonValues = ['common', 'values'];
    
export const doSomethingWithInput = (theInput) => {
   //Do something with the input
   return theInput;
};
    
export const justAnAlert = () => {
   alert('hello');
};

Then in your components that you want to use the util functions, import the specific functions that are needed. You don't have to import everything

import {doSomethingWithInput, justAnAlert} from './path/to/Utils.js'

And then use these functions within the component like this:

justAnAlert();
<p>{doSomethingWithInput('hello')}</p>

Just instantiate the class and export it at the top level of one of your modules. For example:

./First.jsx:

// class UtilityClass {/* ...*/}

export const utilityClass = new UtilityClass();

export function ReactComponent () {
  const doSomething = () => {
    return utilityClass.doingSomething();
  };
}

Then, in every other module where you want to use it (in other components, etc.), just import and use it:

./Second.jsx:

import {utilityClass} from './First';

export function SecondReactComponent () {
  const doSomething = () => {
    return utilityClass.doingSomething();
  };
}