103 lines
2.0 KiB
Plaintext
103 lines
2.0 KiB
Plaintext
---
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title: Use snapshot testing in `bun test`
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sidebarTitle: Snapshot testing
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mode: center
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---
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Bun's test runner supports Jest-style snapshot testing with `.toMatchSnapshot()`.
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```ts snap.test.ts icon="/icons/typescript.svg"
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import { test, expect } from "bun:test";
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test("snapshot", () => {
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expect({ foo: "bar" }).toMatchSnapshot();
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});
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```
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---
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The first time this test runs, Bun evaluates the value passed into `expect()` and writes it to a `__snapshots__` directory alongside the test file. (Note the `snapshots: +1 added` line in the output.)
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```sh terminal icon="terminal"
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bun test test/snap
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```
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```txt
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test/snap.test.ts:
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✓ snapshot [1.48ms]
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1 pass
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0 fail
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snapshots: +1 added
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1 expect() calls
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Ran 1 test across 1 file. [82.00ms]
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```
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---
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The `__snapshots__` directory contains a `.snap` file for each test file in the directory.
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```txt File Tree icon="folder-tree"
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test
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├── __snapshots__
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│ └── snap.test.ts.snap
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└── snap.test.ts
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```
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---
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The `snap.test.ts.snap` file is a JavaScript file that exports a serialized version of the value passed into `expect()`. Bun pretty-prints the `{foo: "bar"}` object in Jest's snapshot format, which is not strict JSON (note the trailing comma).
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```js snap.test.ts.snap icon="file-code"
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// Bun Snapshot v1, https://bun.sh/docs/test/snapshots
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exports[`snapshot 1`] = `
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{
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"foo": "bar",
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}
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`;
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```
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---
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On later runs, Bun reads the snapshot file and compares it to the value passed into `expect()`. If they differ, the test fails.
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```sh terminal icon="terminal"
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bun test
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```
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```txt
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bun test v1.3.3 (9c68abdb)
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test/snap.test.ts:
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✓ snapshot [1.05ms]
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1 pass
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0 fail
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1 snapshots, 1 expect() calls
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Ran 1 test across 1 file. [101.00ms]
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```
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---
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To update snapshots, use the `--update-snapshots` flag.
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```sh terminal icon="terminal"
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bun test --update-snapshots
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```
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```txt
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bun test v1.3.3 (9c68abdb)
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test/snap.test.ts:
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✓ snapshot [0.86ms]
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1 pass
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0 fail
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snapshots: +1 added # the snapshot was regenerated
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1 expect() calls
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Ran 1 test across 1 file. [102.00ms]
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```
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---
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See [Snapshots](/test/snapshots).
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