feat: read snapshot configuration from jest config in package.json

package.json

@@ -35,6 +35,7 @@
     "bin/jest.js",
     "src/dark.cjs",
     "src/jest.js",
+    "src/jest.config.js",
     "src/jest.fn.js",
     "src/jest.mock.js",
     "src/jest.snapshot.js",

src/jest.config.js

@@ -0,0 +1,41 @@
+import assert from 'node:assert/strict'
+import { readFile } from 'node:fs/promises'
+import path from 'node:path'
+
+const files = process.argv.slice(1)
+const baseDir = files.length === 1 ? path.dirname(path.resolve(files[0])) : undefined
+
+async function getJestConfig(dir) {
+  if (!dir) return
+
+  try {
+    const pkg = JSON.parse(await readFile(path.resolve(dir, 'package.json'), 'utf8'))
+    if (pkg.jest) return pkg.jest
+  } catch {}
+
+  const parent = path.dirname(dir)
+  return parent === dir ? undefined : getJestConfig(parent)
+}
+
+const normalizeJestConfig = (config) => ({
+  testEnvironment: 'node',
+  snapshotSerializers: [],
+  ...config,
+  snapshotFormat: {
+    // jest-snapshot defaults
+    indent: 2,
+    escapeRegex: true,
+    printFunctionName: false,
+    // defaults from https://jestjs.io/docs/configuration#snapshotformat-object
+    escapeString: false,
+    printBasicPrototype: false,
+    // user config
+    ...config?.snapshotFormat,
+    // not overridable per doc
+    compareKeys: undefined,
+  },
+})
+
+export const config = normalizeJestConfig(await getJestConfig(baseDir))
+
+assert.equal(config.testEnvironment, 'node', 'Only "node" testEnvironment is supported')

src/jest.mock.js

@@ -7,10 +7,13 @@ import { jestfn } from './jest.fn.js'
 
 const files = process.argv.slice(1)
 const baseUrl = files.length === 1 && existsSync(files[0]) ? normalize(files[0]) : undefined
-const require = createRequire(baseUrl || import.meta.url)
 const mapMocks = new Map()
 const mapActual = new Map()
 
+const require = createRequire(baseUrl || import.meta.url)
+
+export const relativeRequire = require
+
 export function resolveModule(name) {
   assert(baseUrl || /^[@a-zA-Z]/u.test(name), 'Mocking relative paths is not possible')
   return require.resolve(name)

src/jest.snapshot.js

@@ -4,16 +4,27 @@ import { expect } from 'expect'
 import { format } from 'pretty-format'
 import assert from 'node:assert/strict'
 import { basename, dirname, join } from 'node:path'
+import { config } from './jest.config.js'
+import { relativeRequire } from './jest.mock.js'
 
+const { snapshotFormat, snapshotSerializers } = config
 const plugins = []
-const opts = { indent: 2, escapeRegex: true, printFunctionName: false, printBasicPrototype: false }
-const serialize = (val) => format(val, { ...opts, plugins }).replaceAll(/\r\n|\r/gu, '\n')
+const serialize = (val) => format(val, { ...snapshotFormat, plugins }).replaceAll(/\r\n|\r/gu, '\n')
 
+let serializersAreSetup = false
 let snapshotsAreJest = false
 
+function maybeSetupSerializers() {
+  if (serializersAreSetup) return
+  // empty require and serializers should not let this fail, non-empty serializers and empty require should
+  if (snapshotSerializers.length > 0) plugins.push(...snapshotSerializers.map(relativeRequire))
+  serializersAreSetup = true
+}
+
 // We want to setup snapshots to behave like jest only when first used from jest API
 function maybeSetupJestSnapshots() {
   if (snapshotsAreJest) return
+  maybeSetupSerializers()
   const require = createRequire(import.meta.url)
   const { snapshot } = require('node:test') // attempt to load them, and we need to do that synchronously
   assert(snapshot, 'snapshots require Node.js >=22.3.0')
@@ -68,6 +79,7 @@ const throws = (fn, check) =>
 const snapInline = (obj, inline) => {
   assert(inline !== undefined, 'Inline Snapshots generation is not supported')
   assert(typeof inline === 'string')
+  maybeSetupSerializers()
   getAssert().strictEqual(serialize(obj).trim(), inline.trim())
 }