@isaacs/cached

Cache the results of a function based on a path argument, and clear the cache whenever the mtime of the path changes.

USAGE

import { cachedMtime } from '@isaacs/cached'
import { writeFile, readFile } from 'node:fs/promises'

const readFileCached = cachedMtime(readFile)

// first time, actually reads the file
const results = await readFileCached('file.txt')
// second time, just serve from cache
const cachedResults = await readFileCached('file.txt')

// file changes!
await writeFile('file.txt', 'new contents')
const cacheBusted = await readFileCached('file.txt')

The cached method is the same thing, but without the mtime stuff, so you'd have to expire the cache yourself if you want to do that.

import { cached } from '@isaacs/cached'
import { writeFile, readFile } from 'node:fs/promises'

const readFileCached = cached(readFile)

await writeFile('file.txt', 'some contents')

// first time, actually reads the file
const results = await readFileCached('file.txt')
// second time, just serve from cache
const cachedResults = await readFileCached('file.txt')

// file changes! but this is ignored, because we're using
// cached(), not cachedMtime()
await writeFile('file.txt', 'new contents')
const cacheNotBusted = await readFileCached('file.txt')
assert.equal(cacheNotBusted, 'some contents')

You can supply your own cache to the method if you'd like to use an LRU or something. It must implement get, set, and has, at minimum.

import { cached } from '@isaacs/cached'
const map1 = new Map([
  [1, 11],
  [2, 22],
])
const map2 = new Map([
  [3, 13],
  [4, 14],
])
const myCache = {
  get: key => map1.get(key) || map2.get(key),
  set: (key, value) => map1.set(key),
  has: key => map1.has(key) || map2.has(key),
  delete: key => map1.delete(key) || map2.delete(key),
}
const cachedFunction = cached(key => key * 100)
assert.equal(cachedFunction(1), 11)
assert.equal(cachedFunction(4), 14)
assert.equal(cachedFunction(5), 500)
assert.equal(map1.get(5), 500) // cached now

If you want to interact with the cache (and in the case of cachedMtime, the mtime cache) then that's hanging on the returned function. You can use this to pre-seed cached values, clear the cache in a targetted way, etc.

import { cachedMtime } from '@isaacs/cached'
import { writeFile, readFile } from 'node:fs/promises'

const readFileCached = cachedMtime(readFile)
await readFileCached('file.txt')
console.log(readFileCached.cache) // Map { 'file.txt' => 'some contents' }
console.log(readFileCached.mtimeCache) // Map { 'file.txt' => 853283471948 }

You can set the minimum time between stat calls, and optionally provide your own cache map objects for the return values and for the mtime values.

import { cachedMtime } from '@isaacs/cached'
import { readFileSync } from 'node:fs'
const mtimes = new Map<string, number>
const contents = new Map<string, string>
const cached = cachedMtime(
  (path: string) => readFileSync(path, 'utf8'),
  1000, // check for mtime changes at most 1ce per second
  contents,
  mtimes
)

See the typedocs for more information.