Create mocks with the mock function from bun:test.
import { test, expect, mock } from "bun:test";
const random = mock(() => Math.random());
The mock function can accept arguments.
import { test, expect, mock } from "bun:test";
const random = mock((multiplier: number) => multiplier * Math.random());
The result of mock() is a new function that's been decorated with some additional properties.
import { mock } from "bun:test";
const random = mock((multiplier: number) => multiplier * Math.random());
random(2);
random(10);
random.mock.calls;
// [[ 2 ], [ 10 ]]
random.mock.results;
// [
// { type: "return", value: 0.6533907460954099 },
// { type: "return", value: 0.6452713933037312 }
// ]
These extra properties make it possible to write expect assertions about usage of the mock function, including how many times it was called, the arguments, and the return values.
import { test, expect, mock } from "bun:test";
const random = mock((multiplier: number) => multiplier * Math.random());
test("random", async () => {
const a = random(1);
const b = random(2);
const c = random(3);
expect(random).toHaveBeenCalled();
expect(random).toHaveBeenCalledTimes(3);
expect(random.mock.args).toEqual([[1], [2], [3]]);
expect(random.mock.results[0]).toEqual({ type: "return", value: a });
});
See Docs > Test Runner > Mocks for complete documentation on mocking with the Bun test runner.