Demo of parsing ev3dev button events from Linux input subsystem in JavaScript

ev3dev-button-demo.js

var fs = require('fs');

var EV_KEY = 0x01;
var buttonEventFile = '/dev/input/by-path/platform-gpio-keys.0-event';

// Hard-coded button values for EV3
var buttonNames = {
    103: 'up',
    108: 'down',
    105: 'left',
    106: 'right',
    28: 'enter',
    14: 'back'
}

var eventStream = fs.createReadStream(buttonEventFile);
eventStream.on('data', function (chunk) {
    var allEvents = parseInputEvents(chunk);

    // Filter events to get only key events
    var keyEvents = allEvents.filter(function (event) {
        return event.type == EV_KEY;
    });

    // Process the key events that correspond to known key codes
    for (var keyEventIndex in keyEvents) {
        var keyEvent = keyEvents[keyEventIndex];
        if (keyEvent.code in buttonNames) {
            // This will run every time one of the 6 buttons is pressed or released. keyEvent.value contains
            // a number indicating whether the button was pressed or released.
            console.log("Button " + buttonNames[keyEvent.code] + " is " + (keyEvent.value ? 'released' : 'pressed'));
        }
    }
});

function parseInputEvents(eventChunk) {
    // Parse all events received in this chunk
    var events = [];
    for (var i = 0; i < eventChunk.length; i += 16) {
        events.push({
            time: {
                sec: eventChunk.readUInt32LE(i + 0),
                usec: eventChunk.readUInt32LE(i + 4)
            },
            type: eventChunk.readUInt16LE(i + 8),
            code: eventChunk.readUInt16LE(i + 10),
            value: eventChunk.readUInt32LE(i + 12)
        });
    }

    return events;
}