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JavaScript Basics

This quick tour covers the core JavaScript you will use in mcujs. It is friendly for beginners, and still useful if you are coming from other languages.

REPL tip

If you are copy-pasting examples in the REPL, reset the session (.usbreset) between sections to avoid redeclared variables.

Variables and values​

Use variables to store simple data like strings, numbers, and booleans.

const name = 'mcujs';
let counter = 0;
const enabled = true;
const tempC = 21.5;

Use const for values that do not change, and let for values you update.

Arrays and objects​

Arrays hold ordered data, objects hold named fields.

const pins = [25, 2, 3];
const device = {
  name: 'pico',
  version: '0.1.0',
  ledPin: 25,
};

Arrays are ordered lists. Objects are key-value maps.

Functions​

Functions let you group logic so you can reuse it later.

function blink(pin, delayMs) {
  console.log('Blinking', pin, 'every', delayMs, 'ms');
}

blink(25, 500);

Functions let you reuse logic and organize your scripts.

Control flow​

Use if and loops to make decisions and repeat work.

const tempC = 22;

if (tempC > 30) {
  console.log('Too warm');
} else if (tempC < 10) {
  console.log('Too cold');
} else {
  console.log('All good');
}

for (let i = 0; i < 3; i += 1) {
  console.log('count', i);
}

Timers​

Timers schedule work in the future without blocking everything else.

let ticks = 0;

const id = setInterval(() => {
  ticks += 1;
  console.log('tick', ticks);

  if (ticks >= 5) {
    clearInterval(id);
  }
}, 500);

Here is a one-shot timer with setTimeout.

setTimeout(() => {
  console.log('done waiting');
}, 1000);

Timers are how you keep doing work without blocking the runtime.

Modules with require()​

Modules help you split code into smaller files.

// index.js
const math = require('./math');
console.log(math.add(2, 3));

// math.js
exports.add = (a, b) => a + b;

Files with fs​

The fs module lets you save and load small files on the board.

const fs = require('fs');

fs.writeFileSync('/note.txt', 'hello from mcujs');
const note = fs.readFileSync('/note.txt');
console.log(note);

A tiny mcujs script​

This is a complete LED blink program using GPIO.

const GPIO = require('gpio');

const LED = 25;
GPIO.init(LED, GPIO.OUTPUT);

let on = false;
setInterval(() => {
  on = !on;
  GPIO.set(LED, on);
}, 500);

Next steps​

Key terms​