Node.js - What's the scope of the require()d modules?

Question

I am trying to organize a Node.js application developed with Express 4 and am confused about the scope of modules which are imported with require().

Imagine that I use require('./services/user') to import a service in a module such as routes/user.js:

var userService = require('./services/user');

Then I do the same require('./services/user') in another module routes/department.js.

My question is: is userService the same instance in user.js and department.js or each of them has it's own userService object? That is to say, once you've exported some element through module.exports = XXX if you require the same file, will you get always the same instance? Could you show me where in the Node.js docs that's specified?

Answer 1

If I understand your question correctly, you have theses files:

.
|_. app.js
|_. routes/
  |_. user.js
  |_. department.js
  |_. services/
    |_. user

And your code do this:

app.js call user.js
    user.js call user
app.js call department.js
    department.js

In that case, at the first time user is required, it is put on cache in require.cache.

Then the second time it is called, the caller get require.cache['./service/user'], where is stored your object.

As such, you do have the same object in both department.js and user.js.

Source:

• http://nodejs.org/docs/latest/api/modules.html#modules_caching
• http://nodejs.org/docs/latest/api/modules.html#modules_cycles (helped me understand)
• Understanding Node.js modules: multiple requires return the same object?
• Self modifying code in node.js, would cluster work? (about require.cache)

EDIT:

Other helpful links:

node.js require() cache - possible to invalidate?