Unable to find module in `node_modules` only when targeting ES6, ES5 works fine

[benlesh]

TypeScript Version: 1.8.10 and higher

Code

Actual code that causes the problem can be found on this issue: ReactiveX/rxjs#1637

# clone the repo
git clone git@github.com:ReactiveX/rxjs.git

# get PR branch
git fetch master pull/1637/head:pr-1637
git checkout pr-1637

# install dependencies (runs build, too, which will fail, but that's okay)
npm i

# try building the CJS version (success)
npm run build_cjs

# try building the ES6 version (failure)
npm run build_es6

The exact scripts being run are:

tsc typings/main/ambient/es6-shim/index.d.ts ./dist/cjs/src/Rx.ts ./dist/cjs/src/Rx.KitchenSink.ts ./dist/cjs/src/Rx.DOM.ts ./dist/cjs/src/add/observable/of.ts                             -m commonjs --sourceMap --outDir ./dist/cjs --target ES5 -d --diagnostics --pretty --noImplicitAny --suppressImplicitAnyIndexErrors
tsc                                          ./dist/es6/src/Rx.ts ./dist/es6/src/Rx.KitchenSink.ts ./dist/es6/src/Rx.DOM.ts ./dist/es6/src/add/observable/of.ts                             -m es2015   --sourceMap --outDir ./dist/es6 --target ES6 -d --diagnostics --pretty --noImplicitAny --suppressImplicitAnyIndexErrors

Expected behavior:

Both build successfully.

Actual behavior:

Only the CJS build succeeds, the ES6 build fails:

11 import * as observableSymbol from 'symbol-observable';
                                     ~~~~~~~~~~~~~~~~~~~

dist/es6/src/Observable.ts(11,35): error TS2307: Cannot find module 'symbol-observable'.


16 import * as observableSymbol from 'symbol-observable';
                                     ~~~~~~~~~~~~~~~~~~~

dist/es6/src/observable/FromObservable.ts(16,35): error TS2307: Cannot find module 'symbol-observable'.


11 import * as observableSymbol from 'symbol-observable';
                                     ~~~~~~~~~~~~~~~~~~~

dist/es6/src/util/subscribeToResult.ts(11,35): error TS2307: Cannot find module 'symbol-observable'.


142 import * as observable from 'symbol-observable';
                                ~~~~~~~~~~~~~~~~~~~

dist/es6/src/Rx.ts(142,29): error TS2307: Cannot find module 'symbol-observable'.

[benlesh]

cc @kwonoj @jayphelps for visibility

[kwonoj]

I noticed specifying moduleResolution (--moduleResolution node) makes es15 build works, but not sure why CJS doesn't affected. (on 1.8.10)

[benlesh]

I noticed specifying moduleResolution (--moduleResolution node) makes es15 build works, but not sure why CJS doesn't affected. (on 1.8.10)

Is that the answer then? Perhaps targeting commonjs forces that behavior?

[kwonoj]

Perhaps targeting commonjs forces that behavior?

: maybe? at least I could apply that explicitly on build script to ensure for immediate resolution.

[benlesh]

"there's a CLI flag for that"™

[benlesh]

Closing for now, I guess.

[RyanCavanaugh]

This is the expected behavior. The ES6 module spec does not say "Go look in node_modules for a matching folder", so --module es6 doesn't do that either. Since you're loading modules from node_modules, commonjs is the correct module target for your project.

[jayphelps]

Yes, that's indeed the behavior of their code:

https://github.com/Microsoft/TypeScript/blob/81ebab4c391bef98299321e0cce7090edff36c35/src/compiler/program.ts#L297-L302

if (moduleResolution === undefined) {
    moduleResolution = getEmitModuleKind(compilerOptions) === ModuleKind.CommonJS ? ModuleResolutionKind.NodeJs : ModuleResolutionKind.Classic;
    if (traceEnabled) {
        trace(host, Diagnostics.Module_resolution_kind_is_not_specified_using_0, ModuleResolutionKind[moduleResolution]);
    }
}