iotDeviceListFunction.js

//index.js

var iothub = require('azure-iothub');
var config = require("./config");
var registry = iothub.Registry.fromConnectionString(config.connectionString);

module.exports = function (context, req) {
  // get devices registered in your iot hub
    const query = registry.createQuery("SELECT * FROM devices", 100);
    query.nextAsTwin(function (error, result) {
        if (!error) {
            context.log(JSON.stringify(result));
            context.res = {
                body: JSON.stringify(result),
                headers: {
                    'Content-Type': 'application/json'
                }
            };
        } else {
            context.res = {
                status: 400,
                body: error.message + ' : ' + error.responseBody
            };
        }
        context.done();
    });
};

// config.js

var config = {};
config.connectionString = 'IOT_HUB_CONNECTION_STRING';
module.exports = config;