Module resolution: tsc unexpectedly rewrites the import path when exports field contains multiple entries resolving to the same file

Demo Repo

https://github.com/sodatea/ts-exports-rewrite

Which of the following problems are you reporting?

Something else more complicated which I'll explain in more detail

Demonstrate the defect described above with a code sample.

In the reproduction repo, the parse.ts file indirectly depends on the lru-cache module's type by importing the cache.ts file:

import { createCache } from './cache'
export const parseCache = createCache<{}>()

After compilation, in parse.d.ts, we would normally expect a import('lru-cache').LRUCache expression.
However, it actually becomes import("lru-cache/min").LRUCache.
On the other hand, the compiled cache.d.ts file still imports from lru-cache, not lru-cache/min.

This seems to be related to lru-cache@10.0.1's exports field: https://unpkg.com/browse/lru-cache@10.0.1/package.json#L34
It contains 2 entries, ./min and ., both pointing to the same file. TypeScript picks the first entry.

If I modify the exports field to make . appear before ./min, then the compiled declaration file would use import("lru-cache") as expected.

While this isn't a very severe bug and has easy workarounds, this is quite unexpected.
Because lru-cache/min is an entry only available in exports field, this compilation result breaks the compatibility with older build tools. And it is quite hard to debug, because no documentation ever mentioned that the order of keys in the exports field would make a difference.

Run tsc --showConfig and paste its output here

{
    "compilerOptions": {
        "outDir": "./temp",
        "target": "esnext",
        "module": "esnext",
        "moduleResolution": "bundler",
        "esModuleInterop": true,
        "declaration": true,
        "emitDeclarationOnly": true,
        "types": []
    },
    "files": [
        "./cache.ts",
        "./parse.ts"
    ],
    "include": [
        "cache.ts",
        "parse.ts"
    ],
    "exclude": [
        "temp"
    ]
}

Run tsc --traceResolution and paste its output here

======== Resolving module 'lru-cache' from '/Users/haoqun/Reproductions/ts-exports-rewrite/cache.ts'. ========
Explicitly specified module resolution kind: 'Bundler'.
Resolving in CJS mode with conditions 'import', 'types'.
Found 'package.json' at '/Users/haoqun/Reproductions/ts-exports-rewrite/package.json'.
Loading module 'lru-cache' from 'node_modules' folder, target file types: TypeScript, JavaScript, Declaration, JSON.
Searching all ancestor node_modules directories for preferred extensions: TypeScript, Declaration.
Found 'package.json' at '/Users/haoqun/Reproductions/ts-exports-rewrite/node_modules/lru-cache/package.json'.
Entering conditional exports.
Matched 'exports' condition 'import'.
Entering conditional exports.
Matched 'exports' condition 'types'.
Using 'exports' subpath '.' with target './dist/mjs/index.d.ts'.
File '/Users/haoqun/Reproductions/ts-exports-rewrite/node_modules/lru-cache/dist/mjs/index.d.ts' exists - use it as a name resolution result.
Resolved under condition 'types'.
Exiting conditional exports.
Resolved under condition 'import'.
Exiting conditional exports.
Resolving real path for '/Users/haoqun/Reproductions/ts-exports-rewrite/node_modules/lru-cache/dist/mjs/index.d.ts', result '/Users/haoqun/Reproductions/ts-exports-rewrite/node_modules/lru-cache/dist/mjs/index.d.ts'.
======== Module name 'lru-cache' was successfully resolved to '/Users/haoqun/Reproductions/ts-exports-rewrite/node_modules/lru-cache/dist/mjs/index.d.ts' with Package ID 'lru-cache/dist/mjs/index.d.ts@10.0.1'. ========
======== Resolving module './cache' from '/Users/haoqun/Reproductions/ts-exports-rewrite/parse.ts'. ========
Explicitly specified module resolution kind: 'Bundler'.
Resolving in CJS mode with conditions 'import', 'types'.
Loading module as file / folder, candidate module location '/Users/haoqun/Reproductions/ts-exports-rewrite/cache', target file types: TypeScript, JavaScript, Declaration, JSON.
File '/Users/haoqun/Reproductions/ts-exports-rewrite/cache.ts' exists - use it as a name resolution result.
======== Module name './cache' was successfully resolved to '/Users/haoqun/Reproductions/ts-exports-rewrite/cache.ts'. ========

Paste the package.json of the importing module, if it exists

{
  "name": "ts-exports-rewrite",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "build": "tsc"
  },
  "keywords": [],
  "author": "",
  "license": "ISC",
  "dependencies": {
    "lru-cache": "10.0.1",
    "typescript": "^5.2.2"
  }
}

Paste the package.json of the target module, if it exists

{
  "name": "lru-cache",
  "description": "A cache object that deletes the least-recently-used items.",
  "version": "10.0.1",
  "author": "Isaac Z. Schlueter <i@izs.me>",
  "keywords": [
    "mru",
    "lru",
    "cache"
  ],
  "sideEffects": false,
  "scripts": {
    "build": "npm run prepare",
    "preprepare": "rm -rf dist",
    "prepare": "tsc -p tsconfig.json && tsc -p tsconfig-esm.json",
    "postprepare": "bash fixup.sh",
    "pretest": "npm run prepare",
    "presnap": "npm run prepare",
    "test": "c8 tap",
    "snap": "c8 tap",
    "preversion": "npm test",
    "postversion": "npm publish",
    "prepublishOnly": "git push origin --follow-tags",
    "format": "prettier --write .",
    "typedoc": "typedoc --tsconfig tsconfig-esm.json ./src/*.ts",
    "benchmark-results-typedoc": "bash scripts/benchmark-results-typedoc.sh",
    "prebenchmark": "npm run prepare",
    "benchmark": "make -C benchmark",
    "preprofile": "npm run prepare",
    "profile": "make -C benchmark profile"
  },
  "main": "./dist/cjs/index.js",
  "module": "./dist/mjs/index.js",
  "exports": {
    "./min": {
      "import": {
        "types": "./dist/mjs/index.d.ts",
        "default": "./dist/mjs/index.min.js"
      },
      "require": {
        "types": "./dist/cjs/index.d.ts",
        "default": "./dist/cjs/index.min.js"
      }
    },
    ".": {
      "import": {
        "types": "./dist/mjs/index.d.ts",
        "default": "./dist/mjs/index.js"
      },
      "require": {
        "types": "./dist/cjs/index.d.ts",
        "default": "./dist/cjs/index.js"
      }
    }
  },
  "repository": "git://github.com/isaacs/node-lru-cache.git",
  "devDependencies": {
    "@size-limit/preset-small-lib": "^7.0.8",
    "@types/node": "^20.2.5",
    "@types/tap": "^15.0.6",
    "benchmark": "^2.1.4",
    "c8": "^7.11.2",
    "clock-mock": "^1.0.6",
    "esbuild": "^0.17.11",
    "eslint-config-prettier": "^8.5.0",
    "marked": "^4.2.12",
    "mkdirp": "^2.1.5",
    "prettier": "^2.6.2",
    "size-limit": "^7.0.8",
    "tap": "^16.3.4",
    "ts-node": "^10.9.1",
    "tslib": "^2.4.0",
    "typedoc": "^0.24.6",
    "typescript": "^5.0.4"
  },
  "license": "ISC",
  "files": [
    "dist"
  ],
  "engines": {
    "node": "14 || >=16.14"
  },
  "prettier": {
    "semi": false,
    "printWidth": 70,
    "tabWidth": 2,
    "useTabs": false,
    "singleQuote": true,
    "jsxSingleQuote": false,
    "bracketSameLine": true,
    "arrowParens": "avoid",
    "endOfLine": "lf"
  },
  "tap": {
    "coverage": false,
    "node-arg": [
      "--expose-gc",
      "-r",
      "ts-node/register"
    ],
    "ts": false
  },
  "size-limit": [
    {
      "path": "./dist/mjs/index.js"
    }
  ]
}

Any other comments can go here

Have a nice day!

Start with the linked ts-exports-rewrite reproduction, especially parse.ts and cache.ts, and run its tsc build with declaration output enabled. Compare parse.d.ts and cache.d.ts, then trace module resolution for lru-cache and its exports entries. Done means the declaration output remains compatible with the expected package entry when multiple exports resolve to the same file.