intro_node.md

intro_node.js

Intro Node

File Separation

Running A .js File With Node

You can say

$ node file.js

or

node some/path/to/your/file.js

Using Require

Let's make a file called hello_world.js with the following code,

hello_world.js

console.log("Hello world");

and let's also make an app.js.

app.js

var greeter = require("./hello_world.js");
// the require says go to the current working directory
//    the './' and find the hello_world.js file.

If we run app.js

$ node app.js

we should see hello world in our terminal. This is because node is finding the file hello_world.js in the current working directory, thus the ./ in the require call, and then running it.

If we create a folder called sub_folder and move hello_world.js into it

$ mkdir sub_folder
$ mv hello_world.js sub_folder/

When we run node app.js we get an error. This is because now hello_world.js is in ./sub_folder/hello_world.js, so we should change our require statement to reflect it.

app.js

var greeter = require("./sub_folder/hello_world.js");

If we make another_folder and move app.js into it we have tell node to go up a directory to find the right file.

$ mkdir another_folder
$ mv app.js another_folder/
$ node another_folder/app.js

So to fix this we update our require statement.

var greeter = require("../sub_folder/app.js");

The ../ just means go up a directory and find the sub_folder with the app.js file.

Modules And Exporting

Exporting A Single Method

Notice in the above we are doing a require for a file that basically logs hello world... Who cares?

First, let's checkout what the value of that greeter variable is.

another_folder/app.js

var greeter = require("../sub_folder/app.js");
console.log("The value of greeter is", greeter);

and run the app.js file

$ node another_folder/app.js

When we run this file we should see that greeter is {}. The file automatically returns an empty object without us doing anything to it.

Well, now we can update our hello_world.js to actually expose functionality after we require it.

sub_folder/hello_world.js

console.log("hello world!");

/*
   module.exports is a key we can use
    to expose functionality on the
    object that get required in another 
    file... just try the following
    and see.
*/

module.exports.sayHello = function () {
  return "Hello!!";
};

Now when we run our app.js

$ node another_folder/app.js

we see that it is an object with the sayHello: function key value pair.

Let's modify our app.js to have it use the sayHello method.

another_folder/app.js

var greeter = require("../sub_folder/app.js");

console.log("The value of greeter is", greeter);

// log the return of the greeter.sayHello method
console.log(greeter.sayHello());

Exporting A Specific Value/Function

The module.exports utility doesn't just work with exposing different methods. We can tell module.exports to just expose a function or some other value entirely.

Let's modify our hello_world.js to demonstrate this.

sub_folder/hello_world.js

console.log("hello world!");

// now we only expose a string "foobar"
module.exports = "foobar";

and we should probably modify our app.js to not try to use our now non-existent sayHello method.

another_folder/app.js

var greeter = require("../sub_folder/app.js");
console.log("The value of greeter is", greeter);

Running our app.js

$ node another_folder/app.js

We see that

The value of greeter is foobar

Similarly to how we exposed the foobar value, we can also expose just a function.

sub_folder/hello_world.js

console.log("hello world!");

/*
   module.exports is a key we can use
    to expose functionality on the
    object that get required in another 
    file... just try the following
    and see.
*/

module.exports = function () {
  return "Hello!!";
};

And running our app.js we see that

The value of greeter is function () {
  return "Hello!!";
}