snap-shot-it
Smarter snapshot utility for Mocha and BDD test runners
Last updated 4 months ago by bahmutov .
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snap-shot-it

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

NPM

Build status semantic-release js-standard-style

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 snap-shot-it

Use

Example from spec.js

const snapshot = require('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('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.

Returned value

The returned value includes saved value (after any transformations) and saved snapshot name. Usually it is spec name + index, or could be exact name

const out = snapshot('my name', 42)
// {value: 42, key: 'my name'}

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

You can use the following aliases: SNAPSHOT_UPDATE=1, SNAPSHOTS_UPDATE=1 or SNAP_SHOT_UPDATE=1.

Sorted snapshots

If you want to sort saved snapshots alphabetically inside each snapshot file, run with

SNAPSHOT_SORT=1 npm test

You can also set the config option in the package.json file

{
  "config": {
    "snap-shot-it": {
      "sortSnapshots": true
    }
  }
}

Hopefully sorting snapshots would help when updating them.

Named snapshots

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.

Shared snapshot name

If you do want to share a named snapshot value from several places or tests in the same spec file, you need to pass an option when calling snapshot. The the first snapshot is saved, and the next ones will just compare against the value.

snapshot('my shared snapshot', value, { allowSharedSnapshot : true })
// some time later
snapshot('my shared snapshot', value, { allowSharedSnapshot : true })

Pruning

If the test run is successful and executed all tests (there was no .only) then snapshots without a test are pruned. You can skip pruning by running with environment variable

SNAPSHOT_SKIP_PRUNING=1 npm test

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* ...

Data callbacks

You can pass your own NPM modules as pre-compare, compare and store functions using package.json. For example, to use both local and 3rd party NPM modules

{
  "config": {
    "snap-shot-it": {
      "pre-compare": "./pre-compare",
      "compare": "snap-shot-compare",
      "store": "./store"
    }
  }
}

Each NPM module in this case should export a definition of a function that matches the expected core function

Nested snapshots

By default, all snapshots are stored in the same folder __snapshots__, which can lead to name clashes. You can set an option in your package.json file to create a nested folder structure inside __snapshots__ folder that mimics the spec structure. Use config > snap-shot-it object in the package.json file.

{
  "config": {
    "snap-shot-it": {
      "useRelativePath": true
    }
  }
}

input spec files

specs/
  spec.js
  subfolder/
    spec2.js

result should be

__snapshots__/
  specs/
    spec.js
    subfolder/
      spec2.js

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.

Small print

Author: Gleb Bahmutov <gleb.bahmutov@gmail.com> © 2017

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 <gleb.bahmutov@gmail.com>

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.

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