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doc: add guides on writing tests involving promises

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Mention `common.crashOnUnhandledRejection()` and wrapping the
handlers in `common.mustCall()` or `common.mustNotCall()`.

PR-URL: https://github.com/nodejs/node/pull/20988
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
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Joyee Cheung 2018-05-28 00:59:28 +08:00 committed by Anatoli Papirovski
parent 529b173cf3
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doc/guides/writing-tests.md
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@ -223,6 +223,37 @@ countdown.dec();
countdown.dec(); // The countdown callback will be invoked now.
```
#### Testing promises
When writing tests involving promises, either make sure that the
`onFulfilled` or the `onRejected` handler is wrapped in
`common.mustCall()` or `common.mustNotCall()` accordingly, or
call `common.crashOnUnhandledRejection()` in the top level of the
test to make sure that unhandled rejections would result in a test
failure. For example:
```javascript
const common = require('../common');
const assert = require('assert');
const fs = require('fs').promises;
// Use `common.crashOnUnhandledRejection()` to make sure unhandled rejections
// will fail the test.
common.crashOnUnhandledRejection();
// Or, wrap the `onRejected` handler in `common.mustNotCall()`.
fs.writeFile('test-file', 'test').catch(common.mustNotCall());
// Or, wrap the `onFulfilled` handler in `common.mustCall()`.
// If there are assertions in the `onFulfilled` handler, wrap
// the next `onRejected` handler in `common.mustNotCall()`
// to handle potential failures.
fs.readFile('test-file').then(
common.mustCall(
(content) => assert.strictEqual(content.toString(), 'test2')
))
.catch(common.mustNotCall());
```
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