Mustachr 🥸

Mustachr is a lightweight, open-source CLI and TypeScript library for extracting and injecting values in structured files using Mustache-style templates.

It lets you sanitize sensitive data by replacing it with template tokens, or inject dynamic values into templated files. mustachr is especially useful for test fixture sanitization, API mocking, and configuration management.

Installation

npm install --save-dev mustachr

Or globally for CLI use:

npm install -g mustachr

Concepts

Mustachr supports two primary operations:

• Extraction: Replaces matching values in a file with {{ TOKEN }} placeholders
• Injection: Replaces {{ TOKEN }} placeholders with actual values

Example

Input file (example.txt)

{
  "apiKey": "SuperSecret123",
  "user": "admin"
}

Extractions file (mustachr.extractions.ts)

import { defineExtractions } from '@adameisfeld/mustachr';

export default defineExtractions([
  {
    type: 'string',
    property: 'API_KEY',
    search: 'SuperSecret123',
    replace: '{{ property }}',
  },
]);

After Extraction

{
  "apiKey": "{{ API_KEY }}",
  "user": "admin"
}

Injections file (mustachr.injections.ts)

import { defineInjections } from '@adameisfeld/mustachr';

export default defineInjections({
  API_KEY: 'Mocked1234'
});

After Injection

{
  "apiKey": "Mocked1234",
  "user": "admin"
}

Configuration 🛠️

Mustachr uses two configuration files:

• Extractions File: Defines how values should be matched and replaced (for sanitization)
• Injections File: Defines values to replace {{ tokens }} (for hydration)

These can be written as .ts, .js, or .json files and exported using defineExtractions() or defineInjections():

// mustachr.extractions.ts
import { defineExtractions } from '@adameisfeld/mustachr';

export default defineExtractions([ /* extractions here */ ]);

// mustachr.injections.ts
import { defineInjections } from '@adameisfeld/mustachr';

export default defineInjections({ /* injections here */ });

The CLI will automatically pick up these files if named:

• mustachr.extractions.ts|js|json
• mustachr.injections.ts|js|json

Or you can specify them manually via --extractions and --injections.

Extraction Types

mustachr supports three types of extractions: string, regex, and env.

string 🔤 Extraction

Matches exact string values and replaces them with tokens.

{
  type: 'string',
  property: 'API_KEY',
  search: 'SuperSecret123',
  replace: '{{ property }}'
}

Replaces "SuperSecret123" with {{ API_KEY }}

regex 🧪 Extraction

Matches patterns using regular expressions.

{
  type: 'regex',
  property: 'TOKEN',
  search: 'token=[a-zA-Z0-9]+',
  replace: 'token={{ property }}'
}

Replaces "token=abcd1234" with "token={{ TOKEN }}"

env 📦 Extraction

Reads values from a .env file and replaces matching values in the target file.

{
  type: 'env',
  path: './.env'
}

If .env contains:

API_KEY=SuperSecret123
DB_PASSWORD=Hunter2

Then any matching strings in the input file are replaced with {{ API_KEY }} and {{ DB_PASSWORD }} respectively.

CLI Usage

You can use mustachr from the command line to extract or inject values in any file:

mustachr extract <file> [options]
mustachr inject <file> [options]

Extract Example

mustachr extract ./example.txt --extractions ./mustachr.extractions.ts --out ./output.txt

Inject Example

mustachr inject ./output.txt --injections ./mustachr.injections.ts --out ./hydrated.txt

If --extractions or --injections are omitted, mustachr will look for default files like mustachr.extractions.ts in the current directory.

Programmatic API

You can use mustachr directly in code:

import { extract, inject, defineExtractions, defineInjections } from '@adameisfeld/mustachr';

const sanitized = await extract({
  input: 'key=SuperSecret123',
  extractions: defineExtractions([
    {
      type: 'string',
      property: 'API_KEY',
      search: 'SuperSecret123',
      replace: '{{ property }}',
    },
  ]),
});

const hydrated = inject({
  input: sanitized,
  injections: defineInjections({ API_KEY: 'Mocked1234' }),
});

Testing

Mustachr is fully covered with unit tests using Vitest:

npm run test

Use Cases

• Redacting secrets in HAR files or other structured test artifacts
• Injecting dynamic data into mock API responses
• Generating reproducible test fixtures
• Cleaning up sensitive config before sharing

License

MIT © 2025