REPL Terminal

REPL stands for Read Eval Print Loop and it represents a computer environment like a window console or Unix/Linux shell where a command is entered and system responds with an output in interactive mode. Node.js or Node comes bundled with a REPL environment. It performs the following desired tasks.

REPL feature of Node is very useful in experimenting with Node.js codes and to debug JavaScript codes.

Starting REPL

REPL can be started by simply running node on shell/console without any argument as follows.

$ node

You will see the REPL Command prompt > where you can type any Node.js command:

$ node
>

Simple Expression

Let's try simple mathematics at Node.js REPL command prompt:

$ node
> 1 + 3
4
> 1 + ( 2 * 3 ) - 4
3
>

Use Variables

You can make use variables to store values and print later like any conventional script. If var keyword is not used then value is stored in the variable and printed. Whereas if var keyword is used then value is stored but not printed. You can print variables usind console.log().

$ node
> x = 10
10
> var y = 10
undefined
> x + y
20
> console.log("Hello World")
Hello World
undefined

Multiline Expression

Node REPL supports multiline expression similar to JavaScript. Let's check the following do-while loop in action:

$ node
> var x = 0
undefined
> do {
... x++;
... console.log("x: " + x);
... } while ( x < 5 );
x: 1
x: 2
x: 3
x: 4
x: 5
undefined
>

... comes automatically when you press enters after opening bracket. Node automatically checks the continuity of expressions.

Underscore Variable

You can use underscore _ to get the last result:

$ node
> var x = 10
undefined
> var y = 20
undefined
> x + y
30
> var sum = _
undefined
> console.log(sum)
30
undefined
>

REPL Commands

Stopping REPL

As mentioned above you will need to use ctrl + c twice command to come out of Node.js REPL.

$ node
>
(^C again to quit)
>