esModuleInterop does not implicitly enable allowSyntheticDefaultImports

[ffxsam]

TypeScript Version: 2.9.x and 3.x

Search Terms:
esmoduleinterop
esmoduleinterop allowsyntheticdefaultimports

Expected behavior:
Enabling esModuleInterop should also implicitly enable allowSyntheticDefaultImports, according to https://www.typescriptlang.org/docs/handbook/compiler-options.html

Actual behavior:
This is not the case, as per this issue: vuejs/vue-cli#2062

[DanielRosenwasser]

What are your thoughts here @weswigham?

[weswigham]

It does but only if targeting a module < es6, since that's all we control the emit behavior for. It's a weak justification, though - I can see changing it.

[ffxsam]

Hey @DanielRosenwasser, I saw your other comment about the docs being wrong. So this is a toss-up. Whatever you guys think makes the most sense. I only stumbled upon this because the Vue CLI tool was only setting esModuleInterop, expecting the synthetic import option to also be enabled (which it wasn't). And this was causing webpack issues.

[kellycampbell]

The case of this I just ran into with both 3.0.1 and 2.9.2 has "target": "es5" and "esModuleInterop": "true" in the tsconfig. The import that was failing until I also enabled allowSyntheticDefaultImports was:

import React from 'react';

[RyanCavanaugh]

Would appreciate a PR for this!

[NMinhNguyen]

@RyanCavanaugh Per the contributing guide,

If you've submitted a PR for a bug, please post a comment in the bug to avoid duplication of effort.

I've submitted PR #26866.

@DanielRosenwasser I have submitted microsoft/TypeScript-Handbook#816 reverting microsoft/TypeScript-Handbook@8e974c8 for when/if my PR lands.