vimwiki/lang/Node.js.wiki

= Node.js =

Node.js is an Asynchronus, event driven javascript runtime. On line the command
is `node`.

== Hello World ==

1) Create a file called `index.js`. This is always the entry point (similar to
   a main()).
2) Place code. It is interpreted the same as if it were in the browser. For
   example, 
   {{{
       console.log("hello world")
    }}}
   
== Events ==

Events are times when certain things happen. We can do things on those events
by using a callback. This is accessed via the process object. For example,

{{{
    process.on('exit', function() {
        Console.log("Goodbye!");
            });
  }}}
  
Will print goodbye to the console when the program terminates. process.on takes
two paramaters, the event name '`exit`', and the function (or lambda).

To create an event, we can use the Event Emitter, a node.js builtin. The
following example is an annotated example of exactly that.

{{{
//pull in the event emitter dependency
const { EventEmitter } = require('events');

//create the event emitter
const eventEmitter = new EventEmitter();

//create the callback 'lunch'
eventEmitter.on('lunch', function() {
    console.log("Lunch time");
        });
        
//trigger the callback function
eventEmitter.emit('lunch');
  }}}
  
As you can see, this is a similar concept to something like function pointers,
where a pointer is assigned then later called through the pointer.

== File system ==

The file system can be read in two modes, either in a block or non blocking
mode (similar to async calls in C#)

Below is an example of exactly that. The 'Sync' calls are ones that are done in
a blocking manner

{{{
//import read file 
const { readFile, readFileSync } = require('fs');

//read the file, blocking
const txt = readFileSync('./sample.txt', 'utf8');
console.log(txt);

//or read the file on a seperate thread
//the third argument is the file name, encoding, then the function (which gets
//passed both the error object, and the contents of the file)
readFile('./sample.txt', 'utf8', (err, txt) => {
    //stuff to do after we get the file
    console.log(txt)
});

console.log('this is called after the fact');
  }}}
  
== Packages ==

Node.js's package manager is npm. It can download packages for you to use in
your project. To start using npm in a project, run `npm init -y` to setup a
package.json file, which is how npm keeps track of files it needs. Now to
install a framework (for example, express), run `npm install express`, and
thats all that is needed

To include a package (which includes the several built in packages), use the
`require()` syntax, and pass it a string argument of the name of the module.
This system is how you can use several files on one project. To include a
module called 'my-module', you first create a my-module.js file, the add this
to the top of your code

{{{
const myModule = require('./my-module');

console.log(myModule);
  }}}
  
Then in the module file you must export code from it. In the module file, add
the following

{{{
    module.exports = {
        myvar : 'some value'
    }
  }}}
  
Now myModule.myvar will return 'some value'. You can use functions and the like
here to make easy to import functionality.