no-large-snapshots.md

Disallow large snapshots (no-large-snapshots)

When using Jest's snapshot capability one should be mindful of the size of created snapshots. As a general best practice snapshots should be limited in size in order to be more manageable and reviewable. A stored snapshot is only as good as its review and as such keeping it short, sweet, and readable is important to allow for thorough reviews.

Usage

Because Jest snapshots are written with back-ticks (` `) which are only valid with ES2015 onwards you should set parserOptions in your config to at least allow ES2015 in order to use this rule:

module.exports = {
  parserOptions: {
    ecmaVersion: 2015,
  },
};

In order to check external snapshots, you must also have eslint check files with the .snap extension by either passing --ext snap on the command line or by explicitly specifying .snap in overrides.

Rule details

This rule looks at all Jest inline and external snapshots (files with .snap extension) and validates that each stored snapshot within those files does not exceed 50 lines (by default, this is configurable as explained in Options section below).

Example of incorrect code for this rule:

exports[`a large snapshot 1`] = `
line 1
line 2
line 3
line 4
line 5
line 6
line 7
line 8
line 9
line 10
line 11
line 12
line 13
line 14
line 15
line 16
line 17
line 18
line 19
line 20
line 21
line 22
line 23
line 24
line 25
line 26
line 27
line 28
line 29
line 30
line 31
line 32
line 33
line 34
line 35
line 36
line 37
line 38
line 39
line 40
line 41
line 42
line 43
line 44
line 45
line 46
line 47
line 48
line 49
line 50
line 51
`;

Example of correct code for this rule:

exports[`a more manageable and readable snapshot 1`] = `
line 1
line 2
line 3
line 4
`;

Options

This rule has options for modifying the max number of lines allowed for a snapshot:

In an eslintrc file:

{
  "rules": {
    "jest/no-large-snapshots": ["warn", { "maxSize": 12, "inlineMaxSize": 6 }]
  }
}

Max number of lines allowed could be defined by snapshot type (Inline and External). Use inlineMaxSize for Inline Snapshots size and maxSize for External Snapshots. If only maxSize is provided on options, the value of maxSize will be used for both snapshot types (Inline and External).

Since eslint-disable comments are not preserved by Jest when updating snapshots, you can use the allowedSnapshots option to have specific snapshots allowed regardless of their size.

This option takes a map, with the key being the absolute filepath to a snapshot file, and the value an array of values made up of strings and regular expressions to compare to the names of the snapshots in the .snap file when checking if the snapshots size should be allowed.

Note that regular expressions can only be passed in via .eslintrc.js as instances of RegExp.

In an .eslintrc.js file:

module.exports = {
  rules: {
    'jest/no-large-snapshots': [
      'error',
      {
        allowedSnapshots: {
          '/path/to/file.js.snap': ['snapshot name 1', /a big snapshot \d+/],
        },
      },
    ],
  },
};

Since absolute paths are typically not very portable, you can use the builtin path.resolve function to expand relative paths into absolutes like so:

const path = require('path');

module.exports = {
  rules: {
    'jest/no-large-snapshots': [
      'error',
      {
        allowedSnapshots: {
          [path.resolve('test/__snapshots__/get.js.snap')]: ['full request'],
          [path.resolve('test/__snapshots__/put.js.snap')]: ['full request'],
        },
      },
    ],
  },
};