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Use the spyOn utility to track method calls with Bun’s test runner. 
import { test, expect, spyOn } from "bun:test";

const leo = {
  name: "Leonardo",
  sayHi(thing: string) {
    console.log(`Sup I'm ${this.name} and I like ${thing}`);
  },
};

const spy = spyOn(leo, "sayHi");
Once the spy is created, it can be used to write expect assertions relating to method calls. 
import { test, expect, spyOn } from "bun:test";

const leo = {
  name: "Leonardo",
  sayHi(thing: string) {
    console.log(`Sup I'm ${this.name} and I like ${thing}`);
  },
};

const spy = spyOn(leo, "sayHi");

test("turtles", () => { 
  expect(spy).toHaveBeenCalledTimes(0); 
  leo.sayHi("pizza"); 
  expect(spy).toHaveBeenCalledTimes(1); 
  expect(spy.mock.calls).toEqual([["pizza"]]); 
});
See Docs > Test Runner > Mocks for complete documentation on mocking with the Bun test runner.