Wraps a transform and provides caching.

Caching transform results can greatly improve performance. nyc saw dramatic performance increases when we implemented caching.

$ npm install caching-transform

const cachingTransform = require('caching-transform');

const transform = cachingTransform({
	cacheDir: '/path/to/cache/directory',
	salt: 'hash-salt',
	transform: (input, metadata, hash) => {
		// ... Expensive operations ...
		return transformedResult;
	}
});

transform('some input for transpilation')
// => fetch from the cache,
//    or run the transform and save to the cache if not found there

Returns a transform callback that takes two arguments:

• input a string to be transformed
• metadata an arbitrary data object

Both arguments are passed to the wrapped transform. Results are cached in the cache directory using an sha256 hash of input and an optional salt value. If a cache entry already exist for input, the wrapped transform function will never be called.

Type: string Buffer
Default: ''

A value that uniquely identifies your transform:

const pkg = require('my-transform/package.json');
const salt = pkg.name + ':' + pkg.version;

Including the version in the salt ensures existing cache entries will be automatically invalidated when you bump the version of your transform. If your transform relies on additional dependencies, and the transform output might change as those dependencies update, then your salt should incorporate the versions of those dependencies as well.

Type: Function(input: string|Buffer, metadata: *, hash: string): string|Buffer

• input: The value to be transformed. It is passed through from the wrapper.
• metadata: An arbitrary data object passed through from the wrapper. A typical value might be a string filename.
• hash: The salted hash of input. Useful if you intend to create additional cache entries beyond the transform result (i.e. nyc also creates cache entries for source-map data). This value is not available if the cache is disabled, if you still need it, the default can be computed via hasha([input, salt]).

The transform function will return a string (or Buffer if encoding === 'buffer') containing the result of transforming input.

Type: Function(cacheDir: string): transformFunction

If the transform function is expensive to create, and it is reasonable to expect that it may never be called during the life of the process, you may supply a factory function that will be used to create the transform function the first time it is needed.

A typical usage would be to prevent eagerly requireing expensive dependencies like Babel:

function factory() {
	// Using the factory function, you can avoid loading Babel until you are sure it is needed.
	const babel = require('babel-core');

	return (code, metadata) => {
		return babel.transform(code, {filename: metadata.filename, plugins: [/* ... */]});
	};
}

Required unless caching is disabled
Type: string

The directory where cached transform results will be stored. The directory is automatically created with mkdirp. You can set options.createCacheDir = false if you are certain the directory already exists.

Type: string
Default: ''

An extension that will be appended to the salted hash to create the filename inside your cache directory. It is not required, but recommended if you know the file type. Appending the extension allows you to easily inspect the contents of the cache directory with your file browser.

Type: Function(input: string|Buffer, additionalData: *)
Default: Always transform

A function that examines input and metadata to determine whether the transform should be applied. Returning false means the transform will not be applied and input will be returned unmodified.

Type: boolean
Default: false

If true, the cache is ignored and the transform is used every time regardless of cache contents.

Type: Function(input: string|Buffer, metadata: *): string|Buffer|Array[string|Buffer]

Provide additional data that should be included in the hash.

One potential use is including the metadata in the hash by coercing it to a hashable string or buffer:

function hashData(input, metadata) {
	return JSON.stringify(metadata);
}

(Note that metadata is not taken into account otherwise.)

Type: Function(metadata: *): string

Provide a filename to prefix the cache entry. The return value may not contain any path separators.

function filenamePrefix(metadata) {
	return path.parse(metadata.filename || '').name + '-';
}

Type: Function(input: string|Buffer, metadata: *, hash: string)

Function that is called after input is hashed.

Type: string
Default: 'utf8'

The encoding to use when writing to / reading from the filesystem. If set it to buffer, then buffers will be returned from the cache instead of strings.

MIT © James Talmage