Use import in nodejs will cause error

[zhmushan]

❔ Question

Use import in nodejs will cause error.

🔦 Context

When I use parcel to package a node application written in typescript, I can only use require to reference third-party dependencies.

💻 Code Sample

import * as koa from 'koa'
const app = new koa() // error

const koa = require('koa')
const app = new koa() // success

🌍 Your Environment

Software	Version(s)
Parcel	1.8.1
Node	8.11.1
Yarn	1.6.0
Operating System	windows 10

[DeMoorJasper]

Perhaps this helps #202

[zhmushan]

Thank you.
But I don't think this is an error caused by tsconfig. When I use import * as koa from 'koa', parcel changes the variable koa to {default: Application}. The correct one should be {Application}.
At the same time, my tsconfig file did not make any configuration for the path

[DeMoorJasper]

you could use this as a workaround.

import {default as koa} from 'koa'

@fathyb Any idea why this doesn't work?

[zhmushan]

What I mean is that koa has no default export, but parcel thinks it has a default export, and set newObj.default = obj in output file

[mohsen1]

Related to #1378

Parcel needs to understand allowSyntheticDefaultImports config of TypeScript.

[zhmushan]

@mohsen1 you are right, thanks

[fathyb]

Solution (replace express with koa, allowSyntheticDefaultImports has nothing to do with the runtime) :

For a definitive fix set the compiler option esModuleInterop to true and use import express instead of import * as express.

The problem :

The ES6 specification defines the notion of "namespace object". A namespace object is namespaceObject in this statement : import * as namespaceObject from 'a-module'. The typeof this object is object, you are not supposed to be able to call it.

You were using import * as express until now because express is a CommonJS module, for Node.js, it is exported using module.exports. However it is illegal in the ES6 spec, and TypeScript now warns you.

The solution :

Setting esModuleInterop to true will make TypeScript wrap your import calls to check if the module is an ES6 module or a CommonJS. If it's a CommonJS module and you are using import default from 'module' TypeScript will find out and return the correct CommonJS module.

From the TypeScript release note :

Note: The new behavior is added under a flag to avoid unwarranted
breaks to existing code bases. We highly recommend applying it both to
new and existing projects. For existing projects, namespace imports
(import * as express from "express"; express();) will need to be
converted to default imports (import express from "express";
express();).