Jest is useful tool for snapshot testing. Snapshot testing is used in many library.
For example, prettier use snapshot testings.

Snapshot Testing · Jest

I want to write snapshots testing without Jest.
In such a case, I often use following template code.

Write snapshot testing

For example, following code work on Mocha.

Input: json
Ouput: json

snapshot-test.js:

const fs = require("fs");
const path = require("path");
const assert = require("assert");
const fixturesDir = path.join(__dirname, "snapshots");
// TODO: your transform function
const transform = require("../transform");

describe("Snapshot testing", () => {
  fs.readdirSync(fixturesDir)
    .map(caseName => {
      const normalizedTestName = caseName.replace(/-/g, " ");
      it(`Test ${normalizedTestName}`, function() {
        const fixtureDir = path.join(fixturesDir, caseName);
        const actualFilePath = path.join(fixtureDir, "input.json");
        const actualContent = JSON.parse(fs.readFileSync(actualFilePath, "utf-8"));
        const actual = transform(actualContent);
        const expectedFilePath = path.join(fixtureDir, "output.json");
        // Usage: update snapshots
        // UPDATE_SNAPSHOT=1 npm test
        if (!fs.existsSync(expectedFilePath) || process.env.UPDATE_SNAPSHOT) {
          fs.writeFileSync(expectedFilePath, JSON.stringify(actual, null, 4));
          this.skip(); // skip when updating snapshots - Mocha's function
          return;
        }
        // compare input and output
        const expected = JSON.parse(fs.readFileSync(expectedFilePath, "utf-8"));
        assert.deepStrictEqual(
          actual,
          expected,
          `
${fixtureDir}
${JSON.stringify(actual)}
`
        );
      });
    });
});

Setup snapshot input file

Create each test case directory to fixturesDir and put input.json as input and output.json as output into the test case directory.

├── snapshot-test.js
└── snapshots
    ├── TEST_CASE_NAME_1
    │   ├── input.json
    │   └── output.json
    └── TEST_CASE_NAME_2
        ├── input.json
        └── output.json

Create output.json(snapshot) from input.json

Snapshot testing create output file(snapshot) from input file(input.json).
It is advantage to use snapshot testing.

Run the test code with UPDATE_SNAPSHOT=1 enviroment variable and then create output.json into each test case directory.

UPDATE_SNAPSHOT=1 npm test

If You have checked the snapshot file(output.json) and it is valid, commit the snapshot file.

Next, You can check the snapshot via Running npm test:

npm test

Conclustion

I have used the above snapshot testing pattern.
Snapshot testing is easy to increase test case by putting input case .

Examples:

sentence-splitter/fixtures-test.ts at master · azu/sentence-splitter
- Use parsed JSON as snapshot
ecmascript-proposals-json/snapshot-test.js at master · azu/ecmascript-proposals-json
- Use scrapted HTML as snapshot
textlint/parsing-test.js at master · textlint/textlint
- Use parsed Markdown AST as snapshot
comment-to-assert/snapshot-test.ts at master · azu/comment-to-assert
- Use transformed JS code as snapshot

Write Snapshots testing without Jest

Write snapshot testing

Setup snapshot input file

Create output.json(snapshot) from input.json

Conclustion