Starting with JavaScript

Connect your device to your computer via the USB cable. Start mos by double-clicking the executable or by typing mos in your Linux or Mac terminal. This will start a simple Web UI. The drop-down list for the device address must list the corresponding serial device. If it does not, go back to the downloads page and install USB-to-Serial drivers.

Select serial device from the dropdown list, click on Connect button, choose your device architecture, flash the firmware and configure wifi:

This will flash the pre-built firmware called default (you can see the full list here). This particular firmware (we call it an app) has many capabilities built-in. One of which is an mJS library that adds JavaScript prototyping ability.

A firmware with JavaScript supports loads init.js file after boot, therefore if you needs to customise a default app, you need to modify that file and reboot the device. Let's do it.

Click on "Device files" tab, and click on init.js file in the list:

This default code does several things: sets up a timer to blink a built-in LED if your device has it, sets up a button press handler that sends an message to an MQTT server on a button press, etc.

Modify a blinking interval from 1 second to 100 millisecond, click on "Save and Reboot" button, and see notice the change in the blinking speed:

Note that the firmware has not been modified. What was modified is a file init.js on the filesystem, which gets loaded after firmware boot and JavaScript engine interprets it. This demonstrates the ability to modify the behavior of the built firmware using JavaScript.

You can play with the other JavaScript API - take a look at the api_*.js files present on a device to get an idea what API is available in the default app. If you need other API, for example some specific sensor, you will need to include appropriate library and rebuild the app, which is subject of the following sections. However, the default app provides quite broad functionality in terms of accessing hardware (e.g. PWM, GPIO, I2C, etc) and cloud integrations (e.g. AWS IoT).

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