# bun:test/valid-expect

> Require `expect` to get a value, end in a called matcher, and be awaited when it is async

`expect(value)` doesn't check anything by itself. The check happens when a matcher such as `.toBe()` is called, so an `expect` with no matcher, or with a matcher that isn't called, passes no matter what the value is.

`.resolves` and `.rejects` make the matcher return a promise. If the test doesn't await or return it, the test can end first, and a failed assertion doesn't fail the test.

This rule is on by default in files that import from `"bun:test"`.

{/* Generated by src/codegen/lint-rule-docs.ts. Do not edit this section. */}

## Configure

On by default as an error. To turn it off:

```toml bunfig.toml
[lint.rules]
"bun:test/valid-expect" = "off" # or "warn"
```

Every option, set to its default:

```toml bunfig.toml
[lint.rules]
"bun:test/valid-expect" = ["error", {
  alwaysAwait = false,
  asyncMatchers = ["toResolve", "toReject"],
  minArgs = 1,
  maxArgs = 1,
}]
```

{/* End of the generated section. */}

## Examples

Reported:

{/* bad {"count": 4} */}
```ts
import { expect, test } from "bun:test";

test("adds the tax", () => {
  expect(total(100)); // no matcher
  expect(total(100)).toBe; // the matcher is not called
  expect().toBe(108); // no value
  expect(total(100)).never.toBe(0); // `never` is not a modifier
});
```

{/* bad */}
```ts
import { expect, test } from "bun:test";

test("loads the user", () => {
  expect(loadUser(1)).resolves.toEqual({ id: 1 });
});
```

Not reported:

{/* good */}
```ts
import { expect, test } from "bun:test";

test("adds the tax", () => {
  expect(total(100)).toBe(108);
  expect(total(100)).not.toBe(0);
});

test("loads the user", async () => {
  await expect(loadUser(1)).resolves.toEqual({ id: 1 });
});

test("rejects an unknown id", () => {
  return expect(loadUser(-1)).rejects.toThrow("not found");
});
```

A second argument is the message to print on failure, and it doesn't count toward `maxArgs`. `expect().pass()` and `expect().fail()` don't need a value:

{/* good */}
```ts
import { expect, test } from "bun:test";

test("adds the tax", () => {
  expect(total(100), "after tax").toBe(108);
});

test("never calls back", () => {
  subscribe(() => expect().fail("called back"));
});
```

A missing matcher or a missing `await` is only reported when the `expect` is its own statement. If you keep it in a variable or pass it to a function, it can get its matcher or its `await` somewhere else:

{/* good */}
```ts
import { expect, test } from "bun:test";

test("adds the tax", () => {
  const result = expect(total(100));
  result.toBe(108);
});

test("loads both users", async () => {
  const first = expect(loadUser(1)).resolves.toEqual({ id: 1 });
  const second = expect(loadUser(2)).resolves.toEqual({ id: 2 });
  await Promise.all([first, second]);
});
```

## Options

| Option | Default | What it does |
| --- | --- | --- |
| `alwaysAwait` | `false` | Report every async assertion that isn't awaited where it's written, even one that's returned or kept in a variable. |
| `asyncMatchers` | `["toResolve", "toReject"]` | Matchers that return a promise, so they must be awaited or returned. Your list replaces the default. |
| `minArgs` | `1` | The fewest arguments `expect` can take. |
| `maxArgs` | `1` | The most arguments `expect` can take. |

{/* bad {"options": [{"alwaysAwait": true}]} */}
```ts
import { expect, test } from "bun:test";

test("rejects an unknown id", () => {
  return expect(loadUser(-1)).rejects.toThrow("not found");
});
```

Use `asyncMatchers` for matchers that you add with `expect.extend()` and that return a promise:

{/* bad {"options": [{"asyncMatchers": ["toResolveWith"]}]} */}
```ts
import { expect, test } from "bun:test";

test("loads the user", () => {
  expect(loadUser(1)).toResolveWith({ id: 1 });
});
```