遇到错误：如何从打字稿中正确使用已翻译并键入的打字稿npm模块？

Question

I'm still giving a shot at typescript. I've written a trivial "hello world" typescript module and published it to npm . Really trivial, just does a default export:

export default function hello(target: string = 'World'): void
{
    console.log(`Hello, ${target} :-(`)
}

It is consumed perfectly well by node.js 0.10 -> 6. The module also has a proper "typings" property in package.json, pointing to an existing .d.ts file generated by tsc as explained in the official documentation :

export default function hello(target?: string): void;

However, I can't get it consumed by typescript code, neither in typescript 1.8 nor 2 :

import hello = require('hello-world-emo')

hello()
hello('Offirmo')

Transpiling with tsc or executing with ts-node both give the same error message:

TSError: ⨯ Unable to compile TypeScript
hello.ts (3,1): Cannot invoke an expression whose type lacks a call signature. (2349)

However, the generated .js works. It just seems a typing issue. Using other import formats like import hello from 'hello-world-emo' doesn't work either.

What is typescript complaining ? What did I miss ?

In case you want to inspect tsconfig , the module is here and I'm consuming it from here

Answer 1

The problem lies not in typescript but rather in ES6/CommonJS interop : there is no perfect matching between ES6 and CommonJS exports, especially for the ES6 "export default" , cf. this article and this open issue on webpack .

Here the module code is written in ES6 using ES6 modules. The build procedure is:

1. transpile typescript to ES6 with tsc (typescript compiler)
2. transpile the ES6 code and bundle it with rollup +babel for:
   • ES6@node4 / CommonJS (1st class citizen)
   • ES5@node0.10 / CommonJS (for still widely used legacy)
   • ES5 / umd (for browsers)

Node 4 (stable) being the 1st class citizen, the corresponding bundle (ES6@node4 / CommonJS) is exposed via the npm "main" entry. It has been converted to CommonJS as best as rollup could do, making it consumable by node 4 but no longer by typescript .

Another option would have been to transpile to ES5/CommonJs with tsc , which adds special annotations for linking back CommonJS to ES6 modules . But then it's 1) no longer bundled 2) more roughly transpiled (not using ES6 features included in node 4) and 3) more hardly consumable by node

A compromise solution : avoid the CommonJS / ES6 modules interop problem by not using export default but only named exports. This mitigation technique only downside is for node 4 :

• ES6: import hello from '...' becomes import { hello } from '...' OK
• node >=6: const hello = require('...') becomes const { hello } = require('...') OK
• node <=4: var hello = require('...') becomes var hello = require('...').hello less elegant but acceptable