Module path maps are not resolved in emitted code

[mika-fischer]

TypeScript Version: 2.0.2

Code

tsconfig.json

{
    "compilerOptions": {
        "target": "ES6",
        "module": "commonjs",
        "baseUrl": ".",
        "paths": {
            "foo/*": ["*"]
        }
    }
}

app.ts

import {Foo} from 'foo/utils';
console.log(Foo);

utils.ts

export const Foo = 'Foo';

Expected behavior:

% ./node_modules/.bin/tsc && node app.js
Foo

Actual behavior:

% ./node_modules/.bin/tsc && node app.js
module.js:457
    throw err;
    ^

Error: Cannot find module 'foo/utils'
    at Function.Module._resolveFilename (module.js:455:15)
    at Function.Module._load (module.js:403:25)
    at Module.require (module.js:483:17)
    at require (internal/module.js:20:19)
    at Object.<anonymous> (/home/mfischer/src/videmo/tsc-test/app.js:2:17)
    at Module._compile (module.js:556:32)
    at Object.Module._extensions..js (module.js:565:10)
    at Module.load (module.js:473:32)
    at tryModuleLoad (module.js:432:12)
    at Function.Module._load (module.js:424:3)

app.js

"use strict";
const utils_1 = require('foo/utils');
console.log(utils_1.Foo);

Typescript is finding the right module, but in the emitted code, the module path is left as-is instead of applying the path aliases from tsconfig.json. Obviously node has no idea where to find the module. I would have expected typescript to resolve the module path and replace it with something that node can resolve.

If this behavior is intended, then how can the path maps be used to solve the relative-import-hell in conjunction with node?

[mhegazy]

Do you use some other bundling tool like browserify or webpack on the generated output? or do you expect this to run directly on node?

[kitsonk]

If this behavior is intended, then how can the path maps be used to solve the relative-import-hell in conjunction with node?

Well and to add context, "paths" is designed for use with loaders that allow remapping, unlike the Node.js require(). The intended behaviour is to allow TypeScript to resolve type information for various module IDs used by various loaders, not to rewrite module IDs. Basically it doesn't do what you thought it did. Nor should it in my opinion, it should only have the capability to mirror the resolution strategies of loaders.

[mika-fischer]

@mhegazy I expected it to work directly with node. It's for a backend application. Is @kitsonk correct in stating that this is working as intended and typescript will not rewrite module paths?

[vladima]

yes, this was the intent - to mitigate the mismatch between runtime and design time experience when module (as it is written by user) can be resolved in runtime but failed to be found by the compiler. At this point compiler always assumes that module id provided by the user is correct and never tries to change it.

[mhegazy]

similar response #9910 (comment)

[mika-fischer]

All right, thanks. It might be useful to document this better in order to prevent more people from being confused. I now use https://www.npmjs.com/package/module-alias to make it work with node.

[papercuptech]

Appreciating TS's position, here's a simple solution to the 90% use case for those of us using node, but wanting the convenience of using baseUrl relative require() calls without any fuss.

This solution hooks node's require() call, and resolves requests using the dirname of "main" to mimic baseUrl. It therefore assumes the baseUrl compiler option was also set to the same directory where the source "main.ts" was located.

To use, paste this tiny chunk of code at the top of your "main.ts".

import * as path from 'path'
import * as fs from 'fs'
(function() {
  const CH_PERIOD = 46
  const baseUrl = path.dirname(process['mainModule'].filename)
  const existsCache = {d:0}; delete existsCache.d
  const moduleProto = Object.getPrototypeOf(module)
  const origRequire = moduleProto.require
  moduleProto.require = function(request) {
    let existsPath = existsCache[request]
    if(existsPath === undefined) {
      existsPath = ''
      if(!path.isAbsolute(request) && request.charCodeAt(0) !== CH_PERIOD) {
        const ext = path.extname(request)
        const basedRequest = path.join(baseUrl, ext ? request : request + '.js')
        if(fs.existsSync(basedRequest)) existsPath = basedRequest
        else {
          const basedIndexRequest = path.join(baseUrl, request, 'index.js')
          existsPath = fs.existsSync(basedIndexRequest) ? basedIndexRequest : ''
        }
      }
      existsCache[request] = existsPath
    }
    return origRequire.call(this, existsPath || request)
  }
})()

[Parziphal]

If you're going to use the module-alias package that mika-fischer mentioned, note that the paths you register to the package shouldn't end in /, and the paths are relative to the path where package.json is (it may be obvious but it's good to get it clear),

So if you have this in your tsconfig file:

"outDir": "./dist",
"baseUrl": ".",
"paths": {
  "foo/*": ["./src"]
}

You have to register this in your package.json:

"_moduleAliases": {
  "foo": "dist"
}

[jbgraug]

Well and to add context, "paths" is designed for use with loaders that allow remapping, unlike the Node.js require(). The intended behaviour is to allow TypeScript to resolve type information for various module IDs used by various loaders, not to rewrite module IDs. Basically it doesn't do what you thought it did. Nor should it in my opinion, it should only have the capability to mirror the resolution strategies of loaders.

Got here after wasting some time trying to set it up in a big project.
If this behaviour is not going to change the least you could do is to update the documentation before closing this issue.
The official documentation https://www.typescriptlang.org/docs/handbook/module-resolution.html#path-mapping%20docs does not say a thing about having to use "loaders that allow remapping" whatsoever.

[duffman]

install and run "tspath" in your project folder... https://www.npmjs.com/package/tspath

[mattyclarkson]

@ef-carbon/tspm

[0ct0d4n]

Also you can try "momothepug/tsmodule-alias" to alias path resolution:

https://www.npmjs.com/package/@momothepug/tsmodule-alias

[BorntraegerMarc]

Only https://www.npmjs.com/package/module-alias worked for me...