How to test for a passed function?

doc is an instance of pdfkit document...

import PDFDocument from 'pdfkit'
const doc = new PDFDocument()

...which gets passed to my function:

export const outputTitle = (doc, title) => {
  if (!title) return null

  doc
    .fontSize(15)
    .font('Helvetica-Bold')
    .text(title, 380, 160)
}

Now I need to write unit tests for this functions using jest.

describe('outputTitle()', () => {
  const doc = jest.fn()

  test('should return null if parameter title is missing', () => {
    // SETUP
    const title = undefined
    // EXECUTE
    const result = outputTitle(doc, title)
    // VERIFY
    expect(result).toBeNull()
  })

  test('should call doc()', () => {
    // ???
  })
})

But how do I test the second part, which is the case of passing a title value? I think my mock of doc is wrong.