JSNAD - notes

About how to install and not Node.js

• Do not install Node with OS package managers.
  • It tends to lag the fast Node.js release cycle.
  • Binary and config files could not be standardized causing compatibility issues.
  • Installing global modules required sudo, this is not ideal since it grants root privileges to third party libraries.
• The recommend way to install Node is using a version package management, on our case nvm.

Javascript, key concepts

Closure scope

• key words: invisible object naming collision
• When naming collision occurs, the reference to the nearest closure scope overrides the outer one.
• Closure scopes apply in the same way to fat/flat arrow (lambda) functions.
• Closure scopes provide encapsulation.
• It provides a way to prototypal inheritance.
  • Actually will be better to explain this as a way to compose objects.
  • Remove the complexity related to prototypes, new and this.

function outerFn() {
  const foo = true;
  function print(foo) {  console.log(foo) }
  print(1) //prints 1
  foo = false
  print(2) //prints 2
}

outerFn();

Process & Operating System

• We can control and gather information about a process using the global process object.
• We can find out information about the Operating System of a running process using the core os module.

STDIO

• The process object exposes three streams:
  • process.stdin A Readable stream for process input.
  • process.stdout A Writable stream for process output.
  • process.stderr A Writable stream for process error output.
• In order to interface with process.stdin some input is needed.
• connect process.stdin to process.stdout
• process.stdin.pipe(process.stdout)
• process.stdin.pipe(uppercase).pipe(process.stdout)
• The process.stdin, process.stdout and process.stderr streams are unique they never finish, error or close.
• The process.stdin.isTTY property can be checked to determine whether our process is being piped to on the command line or whether input is directly connected to the terminal.
• console.log function prints to STDOUT and STDERR is a separate output device which also prints to the terminal.
• We can also use console.error to write to STDERR.

Exiting

• When a process has nothing left to do, it exits by itself.
• Some API's have active handles. An active handle is a reference that keeps the process open.
• To force a process to exit at any point we can call process.exit.
• When exiting a process an exit status code can already be set. Status codes are a large subject, and can mean different things on different platforms. The only exit code that has a uniform meaning across platforms is 0. An exit code of 0 means the process executed successfully. On Linux and macOS (or more specifically, Bash, Zsh, Sh, and other *nix shells) we can verify this with the command echo $? which prints a special variable called $?.
• Any non-zero code indicates failure.
• The exit code can also be set independently be assigning process.exitCode.
• The 'exit' event can also used to detect when a process is closing and perform any final actions, however no asynchronous work can be done in the event handler function because the process is exiting

Process Info

• The process object also contains information about the process.
  • The current working directory of the process
  • The platform on which the process is running
  • The Process ID
  • The environment variables that apply to the process.
• To get the environment variables we can use process.env
• Environment variables are key value pairs, when process.env is accessed, the host environment is dynamically queried and an object is built out of the key value pairs. This means process.env works more like a function.

Process Stats

• The process object has methods which allow us to query resource usage. We're going to look at the process.uptime(), process.cpuUsage and process.memoryUsage functions.
  • Process uptime is the amount of seconds (with 9 decimal places) that the process has been executing for.
  • The process.cpuUsage function returns an object with two properties: user and system. The user property represents time that the Node process spent using the CPU. The system property represents time that the kernel spent using the CPU due to activity triggered by the process.

System Info

• The os module can be used to get information about the Operating System.
• There are two ways to identify the Operating System with the os module:
  • The os.platform function which returns the same as process.platform property.
  • The os.type function which on non-Windows systems uses the uname command and on Windows it uses the ver command, and to get the Operating System identifier.
• The os.uptime function returns the amount of time the system has been running in seconds.
• The os.freemem and os.totalmem functions return available system memory and total system memory in bytes.

Commands

• node -v node --version
• npm - v npm --version
• node --help
• node --v8-options shows javascript V8 runtime engine options.
• node --check foo.js node -c foo.js
• node -p EXPRESSION node --print EXPRESSION
• node -e EXPRESSION node --eval EXPRESSION
• node -r ./foo-before.js ./foo-after.js
• node --require ./foo-before.js ./foo-after.js
• node --stack-trace-limit=N foo.js
• node --inspect foo.js
• node --inspect-brk foo.js
• npm help
• npm [COMMAND] -h
• npm init
• npm init -y
• npm ls
• npm ls --depth=[lvl]
• npm install --save-dev [PACKAGE]