Johnny Five, how to use a REPL

Apr 29, 2021

Learn how to use the Johnny Five REPL to control hardware live from the terminal, requiring classes and chaining commands like lcd.clear().print().

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This post is part of the Johnny Five series. See the first post here.

When you run a program using Johnny Five, you can see that in the terminal, we have access to a REPL, a term that means Read-Evaluate-Print-Loop.

Terminal showing Johnny Five board connection and REPL initialization with Available, Connected, and Repl Initialized messages

In other words, we can write commands in here.

Let’s try by creating a repl.js file with this code:

const { Board } = require("johnny-five")
const board = new Board()

I am going to play with the LCD circuit made in the previous lesson.

Run the program with node repl.js:

Terminal showing node repl.js command execution with board connection messages and REPL prompt

Next, we’re going to write some commands in the REPL.

Start by requiring the LCD class:

const { LCD } = require("johnny-five")

Terminal REPL showing LCD class being imported from johnny-five with const LCD require command

Then initialize an lcd object from it:

const lcd = new LCD({ pins: [7, 8, 9, 10, 11, 12] })

Terminal REPL showing LCD object initialization with pins array configuration for pins 7 through 12

Now write to the LCD display:

lcd.print("Hello!")

You’ll see a big message coming back:

Terminal showing LCD object details returned after print command with board configuration and LCD properties

Because the command returns a reference to the LCD object. This is to let us chain commands together, like this:

lcd.clear().print("Hello!")

If you don’t run clear(), any new thing you write is going to be appended to the one already there.

To write to the second row, you call cursor(1) (the default row is 0:

lcd.clear().print("Hello from")
lcd.cursor(1, 0).print("Johnny-Five!")

Physical LCD display on breadboard showing Hello from on first row and Johnny-Five! on second row

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