Multiple CSS conditional classes using React

Others have already provided some more "flexible" solutions, so I'll just add the quick and dirty one:

<li className={`list-group-item ${todo.completed ? " strike-through" : ""} ${todo.priority === 1 ? "alert alert-info" : (todo.priority === 2 ? "alert alert-warning" : "alert alert-danger")}`} />

For readability's sake:

const completed = todo.completed ? " strike-through" : "";
const priority = todo.priority === 1 ? "alert alert-info" : (todo.priority === 2 ? "alert alert-warning" : "alert alert-danger");
...
<li className={`list-group-item ${completed} ${priority}`} />

I think this is what you're looking for:

import clsx from "clsx";

className={
  clsx(classes.firstClass,  
  { [classes.coach]: user.id === "F6E2080B-E61F-416D-8841-3E0249DF2715" })
}

You could keep on adding classes in the same vein, or you could use a library like classnames to make it a bit easier to compose classes.

Example

import React from 'react';
import classNames from 'classnames';

class App extends React.Component {
  // ...

  render() {
    // ...

    const className = classNames({
      'list-group-item': true,
      'strike-through': todo.completed,
      'alert alert-info': todo.priority === 1,
      'alert alert-warning': todo.priority === 2,
      'alert alert-danger': todo.priority === 3,
    });
    return <li className={className}> ... </li>;
  }
}

I recommend the classnames package. It's a widely-used package and serves a straightforward purpose. Any key whose value is truthy will make it into the final class name:

import cx from 'classnames';

<li className={cx('list-group-item', {
   'alert alert-danger': value === 3,
   'alert alert-info': value === 1,
   'alert alert-warning': value === 2,
   'strike-through': todo.completed,
})} />