When you use redux, you might have an actions keys file, a common error is having two const export the same thing. This will test for that, and maybe make your coverage 100%!

action-keys.test.js

import * as actionKeys from './action-keys';

describe('action keys', () => {
    it('should have a unique string for each key', () => {
        let uniq = Object.keys(actionKeys)
            .map(objKey => {
                return { count: 1, name: actionKeys[objKey] };
            })
            .reduce((a, b) => {
                a[b.name] = (a[b.name] || 0) + b.count;
                return a;
            }, {});

        const duplicates = Object.keys(uniq).filter(a => uniq[a] > 1);

        expect(duplicates).toHaveLength(0);
    });
});