How does Prettier's caching mechanism work and how to use cache to improve performance?

Prettier Caching Mechanism Explained

Prettier 2.0+ introduced a built-in caching mechanism that can significantly improve performance for repeated formatting, especially in large projects.

Caching Mechanism Principle

Prettier's cache is based on hash values of file content and configuration:

1. First Formatting: Parse and format the file, store result in cache
2. Subsequent Formatting: Compare hash values of file content and configuration
3. Cache Hit: If hash values are same, use cached result directly
4. Cache Miss: Reformat file and update cache

Using Cache

Basic Usage:

bash
# Enable cache formatting
prettier --write --cache "**/*.js"

# Specify cache directory
prettier --write --cache --cache-location .prettier-cache "**/*.js"

Cache Strategy:

bash
# Content-based cache (default)
prettier --write --cache --cache-strategy content "**/*.js"

# Metadata-based cache
prettier --write --cache --cache-strategy metadata "**/*.js"

Cache Configuration Options

1. cache-location Specify cache file storage location:

bash
# Default location
prettier --write --cache "**/*.js"

# Custom location
prettier --write --cache --cache-location .my-cache "**/*.js"

# Use relative path
prettier --write --cache --cache-location node_modules/.cache/prettier "**/*.js"

2. cache-strategy Choose cache strategy:

• content: Based on file content (default, more accurate but slightly slower)
• metadata: Based on file metadata (faster but potentially less accurate)

3. Clear Cache

bash
# Clear cache and reformat
prettier --write --cache --cache-strategy content "**/*.js"

# Manually delete cache directory
rm -rf .prettier-cache

Cache Best Practices

1. CI/CD Environment

yaml
# GitHub Actions
- name: Format with cache
  run: |
    prettier --write --cache "**/*.js"
    # Cache directory will be automatically saved and restored

2. Local Development

bash
# Configure in package.json
{
  "scripts": {
    "format": "prettier --write --cache \"src/**/*.js\"",
    "format:check": "prettier --check --cache \"src/**/*.js\""
  }
}

3. Git Hooks

json
// .husky/pre-commit
{
  "lint-staged": {
    "*.{js,jsx,ts,tsx}": [
      "prettier --write --cache"
    ]
  }
}

Performance Comparison

Without Cache:

bash
# First format 1000 files
time prettier --write "**/*.js"
# Actual execution: ~5s

With Cache:

bash
# First format 1000 files
time prettier --write --cache "**/*.js"
# Actual execution: ~5s

# Second format (no changes)
time prettier --write --cache "**/*.js"
# Actual execution: ~0.5s (10x improvement)

Cache Considerations

1. Configuration Changes

• Modifying Prettier configuration automatically invalidates cache
• Ensure configuration files are in version control
• Team members use same configuration

2. File Changes

• File content changes automatically update cache
• Deleting files cleans corresponding cache
• Renaming files regenerates cache

3. Cache Management

• Regularly clean cache directory
• Cache .prettier-cache directory in CI
• Monitor cache size

Troubleshooting

Cache Not Working:

bash
# Check cache directory
ls -la .prettier-cache

# Clear cache and reformat
rm -rf .prettier-cache
prettier --write --cache "**/*.js"

Cache Too Large:

bash
# Check cache size
du -sh .prettier-cache

# Clean old cache
find .prettier-cache -type f -mtime +7 -delete

By properly using Prettier's caching mechanism, you can significantly improve formatting performance, especially in large projects.