add jest

jest.config.js

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+module.exports = {
+  roots: ['<rootDir>'],
+  moduleFileExtensions: ['ts', 'tsx', 'js', 'json', 'jsx'],
+  testPathIgnorePatterns: ['<rootDir>[/\\\\](node_modules|.next)[/\\\\]'],
+  transformIgnorePatterns: ['[/\\\\]node_modules[/\\\\].+\\.(ts|tsx)$'],
+  transform: {
+    '^.+\\.(ts|tsx)$': 'babel-jest',
+  },
+  watchPlugins: [
+    'jest-watch-typeahead/filename',
+    'jest-watch-typeahead/testname',
+  ],
+  moduleNameMapper: {
+    '\\.(css|less|sass|scss)$': 'identity-obj-proxy',
+    '\\.(gif|ttf|eot|svg|png)$': '<rootDir>/test/__mocks__/fileMock.js',
+  },
+}

lib/id.test.ts

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+import { getId } from './id'
+
+const defaultDoc = {
+  address: 'Max Mustermann\nMusterstrasse\n12345 Musterstadt',
+  body: 'Inhalt des Briefs',
+  closing: 'Mit freundlichen Grüßen',
+  customer: '',
+  date: new Date('2021-01-01').toLocaleDateString('de-DE'),
+  enclosing: '',
+  invoice: '',
+  myRef: '',
+  opening: 'Sehr geehrte Damen und Herren',
+  ps: '',
+  signature: '',
+  specialMail: '',
+  subject: 'Betreffzeile',
+  letterOption: 'brief-fam',
+  yourMail: '',
+  yourRef: '',
+  yourRefName: '',
+}
+
+describe('getId', () => {
+  it('generates id for default doc', () => {
+    const id = getId(defaultDoc)
+    expect(id).toEqual('836fbb708bb6807db39ba06332b559d44063e8ba')
+  })
+})