This package combines the power of ESLint's RuleTester and the simplicity of snapshot testing. Snapshots are stored in a diff-friendly custom format, making it easy to review changes.

npm install --save-dev eslint-snapshot-rule-tester

Requirements

• ESLint v8.4.0 or above
• Node.js v18.x or above
• Mocha v6.0.0 or above OR Jest v28.0.0 or above

Write tests like you usually would with ESLint's RuleTester, but use SnapshotRuleTester instead and omit errors and output for invalid test cases.

Example:

// file: tests/rules/no-strings.test.js
import { SnapshotRuleTester } from "eslint-snapshot-rule-tester"
import rule from "../path/to/rules/no-strings"

const tester = new SnapshotRuleTester()

tester.run("no-strings", rule, {
    valid: [
        String.raw`1 + 2 + 3`
    ],
    invalid: [
        `const a = ["foo", "bar"]${"\n"}const b = a.join("\\n")`,
    ],
})

A snapshot file will be created in a __snapshots__ directory next to the test file:

Test: no-strings >> invalid
Code:
  1 | const a = ["foo", "bar"]
    |            ^~~~~  ^~~~~
    |            [1]    [2]
  2 | const b = a.join("\n")
    |                  ^~~~ [3]

Output:
  1 | const a = [42, 42]
  2 | const b = a.join(42)

[1] Strings are not allowed. Use a number instead.
[2] Strings are not allowed. Use a number instead.
[3] Strings are not allowed. Use a number instead.
---

For the full example, see the test file here and snapshot file here.

To update snapshots, run the tests with the --update or -u flag:

mocha tests/**/*.js --update
jest -u