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Issue of esModuleInterop flag

This demo shows why esModuleInterop / allowSyntheticDefaultImports flag is kind of "contagious":

  • pkg-foo depends on lodash.merge, and enables esModuleInterop flag to make it work.
  • pkg-bar depends on pkg-foo, then it also have to enable esModuleInterop / allowSyntheticDefaultImports / skipLibCheck flag to make things work.
  • If other projects are depending on pkg-foo or pkg-bar, they all have to enable esModuleInterop / allowSyntheticDefaultImports / skipLibCheck flag.

Therefore, as a suggestion, it's better not to enable esModuleInterop / allowSyntheticDefaultImports flag in a library project, so that it will not "spread" the flag to other projects that depend on it.

However, in a private project, it's still OK to enable them to make importing easier.

Steps to repro

# Initialize monorepo
npm install
# build pkg-foo with tsc
npm run build-pkg-foo
# build pkg-bar with tsc
npm run build-pkg-bar

Then error occurs:

../pkg-foo/lib/index.d.ts:2:8 - error TS1259: Module '"..../esmoduleinterop-issue/node_modules/@types/lodash.merge/index"' can only be default-imported using the 'allowSyntheticDefaultImports' flag

2 import merge from 'lodash.merge';
         ~~~~~

  ../../node_modules/@types/lodash.merge/index.d.ts:10:1
    10 export = merge;
       ~~~~~~~~~~~~~~~
    This module is declared with using 'export =', and can only be used with a default import when using the 'allowSyntheticDefaultImports' flag.