snap-shot-it

Smarter snapshot utility for Mocha and BDD test runners + data-driven testing!

Why

This tool makes snapshot testing for Mocha (and other BDD) frameworks quick and painless. This module spies on global it function, which allows it to accurately get test information (beating static code parsing done in snap-shot); it should work in transpiled code.

This package uses snap-shot-compare to display object and text difference intelligently.

This function also includes data-driven testing mode, similar to sazerac, see Data-driven testing section below.

Install

Requires Node version 4 or above.

npm install --save-dev @smpx/snap-shot-it

Use

Example from spec.js

const snapshot = require('@smpx/snap-shot-it')
describe('example', () => {
  it('works', () => {
    snapshot(add(10, 20))
    snapshot('a text message')
    return Promise.resolve(42).then(snapshot)
  })
})

Run Mocha tests, then open the created snapshots/spec.js file

exports['example works 1'] = 30

exports['example works 2'] = "a text message"

exports['example works 3'] = 42

Suppose you change the resolved value from 42 to 80

const snapshot = require('@smpx/snap-shot-it')
describe('example', () => {
  it('works', () => {
    snapshot(add(10, 20))
    snapshot('a text message')
    return Promise.resolve(80).then(snapshot)
  })
})

The test will fail

1) example works:
   Error: 42 !== 80

The error message should intelligently handle numbers, objects, arrays, multi-line text, etc.

Config

This is read from package.json. See below for format fo config.

{
  "snap-shot-it": {
    "useRelativePath": true,
    "extension": ".js.snap"
  }
}

This is how it is used along with env vars to send to snap-shot-core

const config = require(process.cwd() + '/package.json')['snap-shot-it'] || {}

const EXTENSION = config.extension || '.js'

const opts = {
  show: Boolean(process.env.SHOW),
  dryRun: Boolean(process.env.DRY),
  update: Boolean(process.env.UPDATE),
  ci: Boolean(process.env.CI),
  useRelativePath: Boolean(config.useRelativePath),
}

Advanced use

You can see the saves snapshot values by running with environment variable

SNAPSHOT_SHOW=1 npm test

You can see snapshot values without writing them into the snapshot file

SNAPSHOT_DRY=1 npm test

You can update snapshot values

SNAPSHOT_UPDATE=1 npm test

By default, the snapshots are saved sorted alphabetically. You can skip sorting using an environment variable

SNAPSHOT_SKIP_SORTING=1 npm test

Named snapshots: Named-demo

Renaming tests might lead to confusion and pruning snapshots. You can name the snapshots yourself

const value = 42
snapshot('my name', value)

The snapshots will be saved as

exports['my name'] = 42

Note you should make sure that the name is unique per spec file.

Chunked snapshots: Chunked-demo

If you have lots of tests in a single file and you want to break down the snapshots into chunks. You can specifiy a chunk parameter.

// File: specs.test.js
const someIdentifier = 'type1'
snapshot({chunk: someIdentifier}, value)

The snapshots will then be saved in file specs.type1.test.js in the __snapshots__ dir

You can use named snapshots along with chunked snapshots:

// File: specs.test.js
const someIdentifier = 'type1'
snapshot({title: 'my name', chunk: someIdentifier}, value)

const otherIdentifier = 'type2'
snapshot({title: 'my name', chunk: otherIdentifier}, value)

The snapshots will then be saved as:

// File: __snapshots__/specs.type1.test.js
exports['my name [type1]'] = 42

// File: __snapshots__/specs.type2.test.js
exports['my name [type2]'] = 42

The chunk is automatically added to the title so need to do that.

Pruning

If the test run is successful and executed all tests (there was no .only) then snapshots without a test are pruned.

Data-driven testing

Writing multiple input / output pairs for a function under test quickly becomes tedious. Luckily, you can test a function by providing multiple inputs and a single snapshot of function's behavior will be saved.

// checks if n is prime
const isPrime = n => ...
it('tests prime', () => {
  snapshot(isPrime, 1, 2, 3, 4, 5, 6, 7, 8, 9)
})

The saved snapshot file will have clear mapping between given input and produced result

// snapshot file
exports['tests prime 1'] = {
  "name": "isPrime",
  "behavior": [
    {
      "given": 1,
      "expect": false
    },
    {
      "given": 2,
      "expect": true
    },
    {
      "given": 3,
      "expect": true
    },
    {
      "given": 4,
      "expect": false
    },
    {
      "given": 5,
      "expect": true
    },
    ...
  ]
}

You can also test functions that expect multiple arguments by providing arrays of inputs.

const add = (a, b) => a + b
it('checks behavior of binary function add', () => {
  snapshot(add, [1, 2], [2, 2], [-5, 5], [10, 11])
})

Again, the snapshot file gives clear picture of the add behavior

// snapshot file
exports['checks behavior of binary function add 1'] = {
  "name": "add",
  "behavior": [
    {
      "given": [
        1,
        2
      ],
      "expect": 3
    },
    {
      "given": [
        2,
        2
      ],
      "expect": 4
    },
    {
      "given": [
        -5,
        5
      ],
      "expect": 0
    },
    {
      "given": [
        10,
        11
      ],
      "expect": 21
    }
  ]
}

See src/data-driven-spec.js for more examples.

Debugging

Run with environment variable DEBUG=snap-shot-it ... to see log messages. Because under the hood it uses snap-shot-core you might want to show messages from both libraries with DEBUG=snap-shot* ...

Examples

TypeScript

An example using ts-mocha is shown in folder ts-demo

CoffeeScript

CoffeeScript example is in coffee-demo folder. Watch mode is working properly.

Inspiration

Came during WorkBar Cambridge Happy Hour on the terrace as I was thinking about difficulty of adding CoffeeScript / TypeScript support to snap-shot project. Got the idea of overriding global.it when loading snap-shot because a day before I wrote repeat-it which overrides it and it is very simple repeat/src/index.js.

Related projects

This NPM module is part of my experiments with snapshot testing. There are lots of other ones, blog posts and slides on this topic.

• snap-shot-core implements loading and saving snapshots
• snap-shot is an alternative to this package that tries to determine spec name using stack trace and static source code parsing. Hard to do for transpiled code!
• schema-shot is "schema by example" snapshot testing
• subset-shot where new value can be a superset of the saved snapshot
• Blog post Picking snapshot library
• Slides Snapshot testing the hard way

Original Author and License:

Small print

Author: Gleb Bahmutov <[email protected]> © 2017

• @bahmutov
• glebbahmutov.com
• blog

License: MIT - do anything with the code, but don't blame me if it does not work.

Support: if you find any problems with this module, email / tweet / open issue on Github

MIT License

Copyright (c) 2017 Gleb Bahmutov <[email protected]>

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.