npm链接会导致找不到模块问题吗？

Question

I'm trying to learn the concept of how to use TypeScript modules from plain JavaScript projects, and it seems to me that I can only use a npm linked module, but not a module that npm link to others. Let me explain with an example:

$ cat index1.js 
const { add, multiply, divide } = require('module-a')

const newfunc = (a, b) =>
       divide(multiply(add(a, b), 6), 2);

const result = newfunc(1, 2)
console.log(result);

$ node index1.js
9

The module-a is a TypeScript module that I npm linked to from my JavaScript project. And it works fine. Now:

$ diff -wU 1 index1.js index2.js
--- index1.js   2019-01-01 16:25:50.000000000 -0500
+++ index2.js   2019-01-01 16:37:33.000000000 -0500
@@ -1,2 +1,3 @@
 const { add, multiply, divide } = require('module-a')
+const { myfunc } = require('module-b')

@@ -7 +8,3 @@
 console.log(result);
+
+console.log(myfunc(1, 2));

$ node index2.js
internal/modules/cjs/loader.js:605
    throw err;
    ^

Error: Cannot find module 'module-b'
    at Function.Module._resolveFilename (internal/modules/cjs/loader.js:603:15)
    at Function.Module._load (internal/modules/cjs/loader.js:529:25)
    at Module.require (internal/modules/cjs/loader.js:657:17)
    at require (internal/modules/cjs/helpers.js:22:18)
...

$ ls -l node_modules/
total 0
lrwxrwxrwx 1 tong tong 30 2019-01-01 16:17 module-a -> /usr/lib/node_modules/module-a
lrwxrwxrwx 1 tong tong 30 2019-01-01 16:28 module-b -> /usr/lib/node_modules/module-b

$ ls -l /usr/lib/node_modules/module-b
lrwxrwxrwx 1 root root 83 2019-01-01 16:32 /usr/lib/node_modules/module-b -> /paths/to/ts-modules-test/module-b

Ie, to me module-b looks nothing different than module-a . but why it is OK to require('module-a') but not to require('module-b') ?

Is it really because my module-b npm linked to module-a ?

The whole npm link setup from module-b to module-a , and all the code, can be found at this repo .

UPDATE. I don't have a project's package.json for either module-a or module-b , but why module-a works? Moreover, having created module-c/package.json , the problem remains the same:

$ find . 
.
./node_modules
./node_modules/module-a
./node_modules/module-b
./index1.js
./index2.js

$ npm init --force --yes
Wrote to /paths/to/ts-modules-test/module-c/package.json:

{
  "name": "module-c",
  "version": "1.0.0",
  "description": "",
  "main": "index1.js",
  "dependencies": {
    "module-a": "^1.0.0",
    "module-b": "^1.0.0"
  },
  "devDependencies": {},
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "keywords": [],
  "author": "",
  "license": "ISC"
}

$ node index2.js
internal/modules/cjs/loader.js:605
    throw err;
    ^

Error: Cannot find module 'module-b'

Answer 1

When nodejs requires a folder, it will try to find a main file.

By default the main file is index.js . Since you are using TypeScript, you do not have index.js , but instead you have index.ts .

To define a main file, you will need to define it in package.json . I can see you have done so in module-a .

{
   "main": "build/index.js"
}

This means that at some point during installation you have compiled the ts to js and the output from the compiler was placed in folder build .

Looking in module-b , your compiler for this module is also pointing to build folder, but the package.json "main" property has value "index.js". I assume that if you point it to build/index.js like you did in module-a it will work.