snap-shot-core

snap-shot-core

Save / load named snapshots, useful for tests

This is the snapshot loading and saving utility, used by snap-shot and schema-shot projects. Can be used to save snapshots from any testing project.

npm install --save-dev snap-shot-core

const snapShot = require('snap-shot-core')
const what // my object
const out = snapShot({
  what,
  file: __filename,    // aliases: file, __filename
  specName: 'my test', // or whatever name you want to give,
  store, // optional function to preprocess the value before storing
  compare: compareFn, // optional function that compares values
  raiser: raiseErrorFn, // optional
  ext: '.test' // default value is '.snapshot'
})

Store function

Sometimes you want to store not the value itself, but something derived, like the object's schema (check out schema-shot). You can pass a function store that transforms the object before saving. For example if we are only interested in the type of value, we can do the following (paired with the right compare function).

const store = x => typeof x
// expected - previously saved "type of" value
// value - current original value
const compare = ({expected, value}) => ({
  valid: typeof value === expected,
  message: 'check the type'
})
snapShot({
  what,
  store,
  compare
})

Compare function

The comparator function needs to compare two values and return an object. Here is an example

const compareFn = ({expected, value}) => {
  const e = JSON.stringify(expected)
  const v = JSON.stringify(value)
  if (e === v) {
    return {
      valid: true
    }
  }
  return {
    valid: false,
    message: `${e} !== ${v}`
  }
}

The above function will be used by default or you can pass your own function that expects ({expectd, value}) and returns either {valid: true} or {valid: false, message: 'to throw as error'}

Raise function

Default function will compare current and loaded values using compare function and if the values are different will throw an error. You can provide your own function to fail a test differently. Your function will be called with these parameters

raiser({
  value,    // current value
  expected, // loaded value
  specName, // the name of the test
  compare   // compare function
})

Default raiser function just throws an Error with good message.

Returned value

The snapShotCore function returns the expected value. If this is the first time, it will be store(what) value. Otherwise it will be the loaded expected value.

Options

You can pass several options to control the behavior. I usually grab them from the environment variables.

• show - log snapshot value when saving new one
• dryRun - only show the new snapshot value, but do not save it
• update - override snapshot value with the new one if there is difference
• ci - the tests are running on CI, which should disallow saving snapshots

const opts = {
  show: Boolean(process.env.SHOW),
  dryRun: Boolean(process.env.DRY),
  update: Boolean(process.env.UPDATE),
  ci: Boolean(process.env.CI)
}
snapShot({
  what,
  file: __filename,
  specName: 'my test',
  compare: compareFn,
  ext: '.test',
  opts
})

If opts.ci is not set, it will use is-ci to determine if running on CI or not.

Debugging

Run the code with DEBUG=snap-shot-core option to see more log messages.

Testing in watch mode

In case you execute your tests in watch mode and you notice the snapshots are always new-created for the same set of tests, then you need to restore the counters per file.

tape example:

//foo.test.js
const test = require('tape');
const snapShot = require('snap-shot-core')

test.onFinish(snapShot.restore)

test('one test', function (t) {
    t.plan(1)
    snapShot({
        what: 1,
        file: __filename,
        specName: 'one test'
    })
})

Small print

Author: Gleb Bahmutov <[e-mail]> © 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 <[e-mail]>

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.