Including JavaScript class definition from another file in Node.js

Another way in addition to the ones provided here for ES6

module.exports = class TEST{
    constructor(size) {
        this.map = new MAp();
        this.size = size;

    }

    get(key) {
        return this.map.get(key);
    }

    length() {
        return this.map.size;
    }    

}

and include the same as

var TEST= require('./TEST');
var test = new TEST(1);

Modify your class definition to read like this:

exports.User = function (socket) {
  ...
};

Then rename the file to user.js. Assuming it's in the root directory of your main script, you can include it like this:

var user = require('./user');
var someUser = new user.User();

That's the quick and dirty version. Read about CommonJS Modules if you'd like to learn more.

Using ES6, you can have user.js:

export default class User {
  constructor() {
    ...
  }
}

And then use it in server.js

const User = require('./user.js').default;
const user = new User();

You can simply do this:

user.js

class User {
  //...
}

module.exports = User // 👈 Export class

server.js

const User = require('./user.js')

let user = new User()

This is called CommonJS module.

ES Modules

Since Node.js version 14 it's possible to use ES Modules with CommonJS. Read more about it in the ESM documentation.

user.mjs (👈 extension is important)

export default class User {}

server.mjs

import User from './user.mjs'

let user = new User()