04_REPL_Terminal.md

REPL Terminal

REPL (Read Eval Print Loop)

Refers to an environment where the system returns a value when the user enters a command, similar to the Windows command prompt or UNIX/LINUX Shell

Node.js comes with a REPL environment and can perform the following functions

• Read

  : Reads the user's input, converts it into JavaScript data structures, and stores it in memory

• Eval

  : Evaluates (processes) the data

• Print

  : Prints the result

• Loop

  : Repeats Read, Eval, Print until the user presses ctrl + c twice to exit

Node.js's REPL environment is useful for testing & debugging JavaScript code!

Starting REPL

REPL can be started by running node without any parameters in the Shell / Console

$ node
>

1. Using simple expressions

$ node
> 1 + 5
6
> 1 + ( 6 / 2 ) - 3
1
>

2. Using variables

• Like other scripts, you can store values in variables and print them later
• When using the var keyword
• The value of the variable is not printed when the command is entered,
• When the var keyword is not used
  • The value of the variable is printed
• You can print using console.log()

$ node
> x = 10
10
> var y = 5
undefined
> x + y
15
> console.log("Value is " + ( x + y ))
Value is 15
undefined

3. Using Multi-line Expressions

Run a do-while loop in REPL!

> var x = 0
undefined
> do {
... x++;
... console.log("x: " + x);
... } while(x<3);
x: 1
x: 2
x: 3
undefined
>

Underscore(_) Variable

The underscore variable refers to the most recent result!

$ node
> var x = 10;
undefined
> var y = 5;
undefined
> x + y;
15
> var sum = _
undefined
> console.log(sum)
15
undefined
>

REPL Commands

• ctrl + c

  : terminate the current command

• ctrl + c twice

  : terminate the Node REPL

• ctrl + d

  : terminate the Node REPL.

• Up/Down keys

  : see command history and modify previous commands

• Tab

  : list of current commands

• .help

  : list of all commands

• .break

  : exit from multiline expression

• .clear

  : exit from multiline expression

• .save [filename]

  : save the current Node REPL session to a file

• .load [filename]

  : load file content in current Node REPL session

Stopping REPL

As mentioned above, use ctrl + c twice to come out of Node.js REPL

$ node
>
(^C again to quit)
>