I've noticed that
React can be imported like this:
import * as React from 'react';
...or like this:
import React from 'react';
The first imports everything in the
react module (see: Import an entire module's contents)
The second imports only the
default module export (see: Importing defaults)
It seems like the two approaches are different and fundamentally incompatible.
Why do they both work?
Please reference the source code and explain the mechanism...I'm interested in understanding how this works.
This is not a duplicate of What is the difference between import * as react from 'react' vs import react from 'react'
That question was answered with general ES6 module information.
I am asking about the mechanism that makes the
react module work like this. It seems to be related to "hacky" export mechanism in the source here but it's not clear how that enables both importing the entire module and just the default export into
React and having both of those approaches work with transpiling JSX, etc.
Indeed ES import statements
import default and
import * are not the same thing, the fact that they behave the same in this case is a combination of how React authors chose to publish the library and compatibility layers in TypeScript (using
esModuleInterop) or Babel and your bundler to make them "just work". It probably shouldn't work according to ES6 spec, but today we are still working in an era where JS modules are a mess, so tools like Babel, TypeScript, Webpack, etc try to normalize behavior.
React is not an ES6 library. If you look at the source code you see this in
const React = require('./src/React'); // TODO: decide on the top-level export form. // This is hacky but makes it work with both Rollup and Jest. module.exports = React.default || React;
(Note the comment, even in React source code they struggle with ES6 default export compatibility.)
module.exports = syntax is CommonJS (NodeJS). A browser would not understand this. This is why we use bundlers like Webpack, Rollup, or Parcel. They understand all kinds of module syntax and produce bundles that should work in the browser.
But even though React is not an ES library, both TypeScript and Babel let you import it as if it is (using
import syntax, rather than
require(), etc), but there are differences between CJS and ES that have to be resolved. One of them is the fact that
export = can give you things that ES has no spec-compliant way to import, like a function or a class as the module. To work around these incompatibilities Babel has for awhile allowed you to import CJS modules as if they were exporting something by default, or import as a namespace. TypeScript for awhile didn't do this, but more recently added that as an option under
esModuleInterop. So now both Babel and TypeScript can pretty consistently allow a CJS module to be imported using default or namespace ES imports.
With TypeScript it also depends on how the type-definitions for the library are actually defined. I won't get into that, but you can imagine situations where thanks to transpilers and bundlers a particular import works at runtime, but TypeScript doesn't compile without errors.
Another thing worth mentioning is that if you look at the built code for React there is a UMD module version as well as the CJS version. The UMD version includes some gnarly runtime code to try to make it work in any module environment, including the browser. It's mainly for use if you want to just include React at runtime (ie you don't use a bundler). Example.
Confusing? Yeah, I think so. :)
You most likely have
"allowSyntheticDefaultImports": true, set in your
tsconfig.json, which essentially shuts the compiler up about default imports it thinks are invalid. Typescript added
esModuleInterop which does essentially what babel does for module loading.
This allows you to use ES6 default imports even when the source code you're importing doesn't export anything as default
Typescript is strict (follows the rules) when it comes to this, which is why they require you to
import * as React from 'react'. Or requires you to tell it to allow synthetic default imports in its base config.