// a.js
let a;

export const setA = () => {
a = 5;
};

export const getA = () => {
if(!a) { throw new Error() }
return a;
}

// a.spec.js

describe('a', () => {
it('test setA', () => {
setA();
});

it('should throw a is not set', () => {
expect(() => getA()).toThrow(Error);
})
});