I've got the new RaspberryPi Pico and I just want to control the pins by simply sending micropython commands to it over its REPL
Just for proof of concept to switch the onboard LED off and on
I think that way beyond my pay grade to be the 1st person to implement that in Snap! 
[edit
But I am making progress having enabled web-platform experimetal thing in my browser
const port = navigator.serial.requestPort();
You can't do this, the Serial API is asynchronous.
Here's how you would do it:
function encodeString(str) {
const buffer = new Uint8Array(str.length);
for (let i = 0; i < str.length; ++i) buffer[i] = str[i].charCodeAt(0);
return buffer;
}
async function writeString(writer, str) {
const buffer = encodeString(str);
await writer.write(buffer);
}
(async () => {
const port = await navigator.serial.requestPort();
await port.open({ baudRate: 9600 });
const writer = port.writable.getWriter();
await writeString(writer, 'from machine import Pin');
})();
edit 2/13/2022: added quotes for the quoted code snippet
This is way beyond my JavaScript knowledge
There was a typo, try running the code now.
No error but what I need to do is send 3 strings one after each other so I tried this but my LED didn't light
I believe you need to add a newline (\r\n), but I'm not sure.
Your right - my NodeRED solution adds \r to each string
Tried it with this but is didn't make any difference 
I think I'll need to replace the Pico with a Pi so I can see if any serial data is going out
function encodeString(str) {
...
return buffer;
Eureka - 1st block sets it up
2nd and third switch the LED on and off
Many thanks @programmer_user (and @dardoro for his crucial addition)
How did I not notice that? Thanks, I've edited my post.
You should probably encapsulate the functionality into a block that sends Python code to the Pi.
yep - that's the ultimate plan
Block to select and open the port
Block to close it
Then parameterised blocks to setup and read and write to the pins.
I've hit a bit of a problem (presumably due to the async code)
If I try to wrap the opening function in a custom block - my browser just starts eating up memory and have have to kill it using task manager
Any ideas on how to get around this?
Where's your code?
I tried creating a simple custom command block to simply wrap the call block reporter
In general I worry about using await in Snap! blocks. The reason it was designed to be asynchronous is that it might take a long time to work. While it is waiting nothing else can happen.
While more complex, I prefer passing in a command to the custom block (in this case the one that is opening a port) and calling it when the asynchronous function (navigator.serial.requestPort in this case) returns a value.