Load and execute external file in node.js

You simply run require('test2.js'), and then call a function on the exported object. From the documentation on modules:

Node has a simple module loading system. In Node, files and modules are in one-to-one correspondence. As an example, foo.js loads the module circle.js in the same directory.

The contents of foo.js:

var circle = require('./circle.js');
console.log( 'The area of a circle of radius 4 is ' + circle.area(4));

The contents of circle.js:

var PI = Math.PI;

exports.area = function (r) {
  return PI * r * r;
};

exports.circumference = function (r) {
  return 2 * PI * r;
};

The module circle.js has exported the functions area() and circumference(). To export an object, add to the special exports object.

Note that exports is a reference to module.exports making it suitable for augmentation only. If you are exporting a single item such as a constructor you will want to use module.exports directly instead.

function MyConstructor (opts) {
  //...
}

// BROKEN: Does not modify exports
exports = MyConstructor;

// exports the constructor properly
module.exports = MyConstructor;

Variables local to the module will be private. In this example the variable PI is private to circle.js.

The module system is implemented in the require("module") module.

There are different scenarios here - using modules, loading them "the right way" - it's the way to go when writing your own code.

What about "random" .js files, e.g. downloaded via web scraping? (If this is a good idea to execute them is beyond the scope of this answer...)

Well - you can just require them, if you're only interested in the side effects:

test2.js:

console.log('hello')

test1.js:

console.log('about to execute')
require('./test2.js')
console.log('done')

Note the ./ in require(). But, if you want to run it twice, this won't work:

test3.js:

console.log('about to execute twice?')
require('./test2.js')
require('./test2.js')
console.log('surprise')

This shows, that require works like a Python import - only executing the file if it hasn't been loaded yet. But - it's possible to circumvent it and force a reload: How to remove module after "require" in node.js?

test4.js:

console.log('about to execute twice!')
require('./test2.js')
delete require.cache[require.resolve('./test2.js')]
require('./test2.js')
console.log('NO surprise this time around')

The difference from a Python import is that you can't import anything unless it's exported. So you would have to change the required file and do something with module.exports.

If you're working with the node shell, there is an alternative:

test5.js:

console.log('the const below is private?')
const x = 5

And then:

$ node
> .load test5.js
console.log('the const below is private?')
const x = 5

the const below is private?
undefined
> x
5

Note that there are no quotes around filename in .load, and also no ./. This is somewhat verbose when used (echoing the loaded script). But it is at least some way of playing with the values the script creates.

Final warning: always be careful about what you're about to execute!