What's the mean of "If X begins with '/'. set Y to be the filesystem root"?

[JianmingXia]

folder: /Users/ryoma/web/Test/NodeJS/NodeTest
files:

• a.js: console.log(module)
• b.js: require('/a')

In link https://nodejs.org/api/modules.html#modules_all_together, I see:

require(X) from module at path Y
1. If X is a core module,
   a. return the core module
   b. STOP
2. If X begins with '/'
   a. set Y to be the filesystem root
...

So, when I run node b.js, I get like below:

internal/modules/cjs/loader.js:985
  throw err;
  ^

Error: Cannot find module '/a'
Require stack:
- /Users/ryoma/web/Test/NodeJS/NodeTest/b.js
    at Function.Module._resolveFilename (internal/modules/cjs/loader.js:982:15)
    at Function.Module._load (internal/modules/cjs/loader.js:864:27)
    at Module.require (internal/modules/cjs/loader.js:1044:19)
    at require (internal/modules/cjs/helpers.js:77:18)
    at Object.<anonymous> (/Users/ryoma/web/Test/NodeJS/NodeTest/b.js:1:1)
    at Module._compile (internal/modules/cjs/loader.js:1158:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1178:10)
    at Module.load (internal/modules/cjs/loader.js:1002:32)
    at Function.Module._load (internal/modules/cjs/loader.js:901:14)
    at Function.executeUserEntryPoint [as runMain] (internal/modules/run_main.js:74:12) {
  code: 'MODULE_NOT_FOUND',
  requireStack: [ '/Users/ryoma/web/Test/NodeJS/NodeTest/b.js' ]
}

And how do i understand If X begins with '/'. set Y to be the filesystem root

[B3ns44d]

The simplest, is just to resolve the path into an absolute path, this will be the recommended approach for most if not all cases.

var path = require('path');

var basedir = '/another/dir';
var filename = 'foo'; // renamed from dirname

var filepath = path.join(basedir, 'node_modules', filename);
var imports = require(filepath);

If you really need to make require act as if it is in a different directory, you can push the base directory to module.paths

module.paths.unshift('/another/dir/node_modules');

var imports = require('foo');

module.paths.shift();

module.paths can also be modified externally via the environment variable NODE_PATH, tho that would be the least recommended
approach but this does apply it globally across all modules.

[aduh95]

If you really need to make require act as if it is in a different directory

module.createRequire is probably a better fit for this use case.