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0.9.0 • Public • Published


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Applies waves of mutations provided by other tools, such as linters or codemods.

There are many linters out there and most include ways to --fix rule failures automatically. This is great but hard to do for a couple of reasons:

  • Overlapping mutations - The possibility of mutations applying to overlapping sets of characters requires logic to handle applying one, then re-running linting, and so on.
  • Code bloat verses duplication - Most linters either provide hooks to apply fixes themselves (which can result in code bloat) or have an external project (which duplicates logic for finding rules).

automutate proposes that linters only propose how to fix rules, via a standardized JSON format.

Having a standardized source-agnostic project to apply mutations brings a couple of benefits:

  • Reduced overhead - Projects no longer need to do this work themselves.
  • Standardized base - Ramp-up time to switch between projects using automutate is reduced with common code.

In general, detecting rule failures is a separate concern from fixing them. Linters need to run quickly over a read-only set of files, often during built processes, while fixers typically run slowly and modify files on user request.

How it works

The main automutate algorithm is started in autoMutator.ts and mostly applied in mutationsApplier.ts:

while mutationsWave = getMutationsWave():
    for (file, fileMutations) of groupMutationsByFile(mutationsWave):
        for mutation of getNonOverlappingMutationsInReverse(fileMutations):
            applyMutation(file, mutation)
  1. getMutationsWave calls to an external tool, such as a linter, to receive a wave of suggested mutations.
  2. groupMutationsByFile organizes the suggested mutations by file.
  3. getNonOverlappingMutationsInReverse removes overlapping mutations that would conflict with each other, and sorts the remainder in reverse order so that later mutations don't interfere with character positions of earlier mutations.
  4. applyMutation modifies files on disk using the remaining mutations.


A single mutation contains a unique type identifier, a range of character position(s) to apply to, and optionally other logic.

The following basic text manipulations are provided out of the box:

  • multiple - Container for multiple mutations. This indicates to automutate that these must be applied all at once or not at all, which guarantees consistency with the built-in mutation overlap detection.
  • text-delete - Deletes a range of characters.
  • text-insert - Inserts a string at a point.
  • text-replace - Replaces characters matching a string or regular expression within a range.
  • text-swap - Swaps a range of characters with a new string.

For example:

  "ugly-file.txt": [
      "range": {
        "begin": 7,
        "end": 14
      "type": "text-delete"
      "insertion": "inconceivable!",
      "range": {
        "begin": 21
      "type": "text-insert"

Linter-specific utilities may define their own mutations. For example, a language's linter may define a node-rename mutation rather than use a multiple mutation containing text-swap mutations.

See Mutators for more on custom mutators.

Project Onboarding

See Onboarding.

automutate requires NodeJS >= 14.

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  • joshuakgoldberg