Guides Test Runner

Mock functions in `bun test`

Create mocks with the mock function from bun:test.

test.ts

import { test, expect, mock } from "bun:test";

const random = mock(() => Math.random());

The mock function can accept arguments.

test.ts

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.

test.ts

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.

test.ts

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.calls).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.

Mock system time

Re-run tests

Mock functions in `bun test`

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