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`bun:test` lets you change what time it is in your tests. This was introduced in Bun v0.6.13.
This works with any of the following:
- `Date.now`
- `new Date()`
- `new Intl.DateTimeFormat().format()`
Timers are not impacted yet, but may be in a future release of Bun.
## `setSystemTime`
To change the system time, use `setSystemTime`:
```ts
import { setSystemTime, beforeAll, test, expect } from "bun:test";
beforeAll(() => {
setSystemTime(new Date("2020-01-01T00:00:00.000Z"));
});
test("it is 2020", () => {
expect(new Date().getFullYear()).toBe(2020);
});
```
To support existing tests that use Jest's `useFakeTimers` and `useRealTimers`, you can use `useFakeTimers` and `useRealTimers`:
```ts
test("just like in jest", () => {
jest.useFakeTimers();
jest.setSystemTime(new Date("2020-01-01T00:00:00.000Z"));
expect(new Date().getFullYear()).toBe(2020);
jest.useRealTimers();
expect(new Date().getFullYear()).toBeGreaterThan(2020);
});
test("unlike in jest", () => {
const OriginalDate = Date;
jest.useFakeTimers();
if (typeof Bun === "undefined") {
// In Jest, the Date constructor changes
// That can cause all sorts of bugs because suddenly Date !== Date before the test.
expect(Date).not.toBe(OriginalDate);
expect(Date.now).not.toBe(OriginalDate.now);
} else {
// In bun:test, Date constructor does not change when you useFakeTimers
expect(Date).toBe(OriginalDate);
expect(Date.now).toBe(OriginalDate.now);
}
});
```
{% callout %}
**Timers** — Note that we have not implemented builtin support for mocking timers yet, but this is on the roadmap.
{% /callout %}
### Reset the system time
To reset the system time, pass no arguments to `setSystemTime`:
```ts
import { setSystemTime, beforeAll } from "bun:test";
test("it was 2020, for a moment.", () => {
// Set it to something!
setSystemTime(new Date("2020-01-01T00:00:00.000Z"));
expect(new Date().getFullYear()).toBe(2020);
// reset it!
setSystemTime();
expect(new Date().getFullYear()).toBeGreaterThan(2020);
});
```
## Set the time zone
To change the time zone, either pass the `$TZ` environment variable to `bun test`.
```sh
TZ=America/Los_Angeles bun test
```
Or set `process.env.TZ` at runtime:
```ts
import { test, expect } from "bun:test";
test("Welcome to California!", () => {
process.env.TZ = "America/Los_Angeles";
expect(new Date().getTimezoneOffset()).toBe(420);
expect(new Intl.DateTimeFormat().resolvedOptions().timeZone).toBe(
"America/Los_Angeles",
);
});
test("Welcome to New York!", () => {
// Unlike in Jest, you can set the timezone multiple times at runtime and it will work.
process.env.TZ = "America/New_York";
expect(new Date().getTimezoneOffset()).toBe(240);
expect(new Intl.DateTimeFormat().resolvedOptions().timeZone).toBe(
"America/New_York",
);
});
```
|
