We're going to describe the isRealString function. Then we can add our arrow function (=>), and inside there, we can go ahead and provide our individual test cases, it, and I'm going to copy it directly, should reject non-string values:

describe('isRealString', () => {
  it('should reject non-string values')
});

This is going to be a synchronous test, so there's no reason to add the done argument. Inside here, we're going to pass in a non-string value. I'll make a variable called response, which will store the return result from isRealString. We're going to call it passing in some sort of non-string value. Anything would work, I'm going to use a number, 98:

describe('isRealString', () => {
  it('should reject non-string values', () => {
    var res = isRealString(98);

Now down below we can use expect to assert that the response variable equals false, which should be the case. We're expecting response toBe(false):

describe('isRealString', () => {
  it('should reject non-string values', () => {
    var res = isRealString(98);
    expect(res).toBe(false);
  });
});