Easily create property matchers for use with jest's snapshots.

If you are using jest's snapshots for testing, you'll most likely run into problem that not all data you are using to compare against snapshots could actually be generated in a deterministic way (so that they stay always the same, each time you run the tests). Typically it's auto-generated ids or dates that causes these issues. Jest addresses this with property matchers but sometimes it can be quite tedious to write.

jest-property-matchers package can help you with constructing matcher. Especially in cases where the matchers are not really consised or if there are too many properties to match.

Typescript declarations are included.

npm install jest-property-matchers --save-dev

See how to use jest-property-matchers in this example of a jest test:

import generateMatcher from 'jest-property-matchers'

describe('User', () => {

  test('Should create a new user in database', async () => {

    const user = await db.createUser({
      name: 'John',
    })

    // Generate your property matchers here
    const matcher = generateMatcher({
      $number: 'id', // created user will have auto-generated id
      $dateTime: ['createdAt', 'updatedAt'], // also timestamps will be always different
    })

    expect(user).toMatchSnapshot(matcher)
  })

})

To use this library in environment not supporting ES import, you can require it like this:

const generateMatcher = require('jest-property-matchers').default

Generates and returns property matcher object according to specification provided in matcherDefs argument.

matchersDefs - Object containing one or more of specialized keys that defines types of the property matcher to be used. Values of these keys are property names in the object being matched against jest snapshot or array of property names if given matcher should be applied on more thatn one property.

These are the currently supported keys:

• $number - matches any js Number
• $numericString - matches any numeric string (no minus sign allowed), useful for BIGINT database values that needs to be represented as strings in JS
• $dateTime - matches js Date
• $date - matches js Date (same as $dateTime)
• $stringDateTime - matches ISO 8601 string representation of a dateTime (such as "2019-12-08T12:24:22.591Z")
• $stringDate - matches date in a "YYYY-MM-DD" format
• $string - any string

Any other properties passed inside matchersDefs argument will be propagated intact to the returned matcher so you can pass any additional matchers that are not supported by jest-property-matchers

Definitions can also be nested. See examples bellow:

generateMatcher({
    $number: 'id',
    $dateTime: ['createdAt', 'updatedAt'],
})

// above call is equivalent to this jest matcher:
{
  id: expect.any(Number),
  createdAt: expect.any(Date),
  updatedAt: expect.any(Date),
}

Matchers definitions can also be nested in sub-objects. With more complex matchers it also starts to show how much simpler it is to use jest-property-matchers to generate property matcher.

generateMatcher({
    $numericString: ['id', 'orderId'],
    $stringDateTime: ['createdAt', 'updatedAt'],
    address: {
        $number: 'id',
    }    
})

// above call is equivalent to this jest matcher:
{
  id: expect.stringMatching(/[0-9]+/u),
  orderId: expect.stringMatching(/[0-9]+/u),
  createdAt: expect.stringMatching(/^\d{4}-[01]\d-[0-3]\dT[0-2]\d:[0-5]\d:[0-5]\d\.\d+([+-][0-2]\d:[0-5]\d|Z)$/u),
  updatedAt: expect.stringMatching(/^\d{4}-[01]\d-[0-3]\dT[0-2]\d:[0-5]\d:[0-5]\d\.\d+([+-][0-2]\d:[0-5]\d|Z)$/u),
  address: {
      id: expect.any(Number),
  },
}