Signal.State.prototype.update to update values using callbacks

[Ocean-OS]

When using signals, often the value of a signal is not known, but a change needs to be made based on the current value. This pattern can be found in Svelte's stores, Solid's Signals, and React's useState, to name a few. This pattern leads to cleaner code with less clutter. Since using a callback in Signal.State.prototype.set is too ambiguous (setting the value to a function might be the preferred operation), it seems reasonable to add Signal.State.prototype.update. Here's some examples of how the update function would work:

let count = new Signal.State(0);
function increment() {
    count.update(c => c + 1);
}
function createCountdown(start, duration) {
    let countdown = new Signal.State(start);
    let interval = setInterval(()=>{
        countdown.update(c=> c - 1);
        if (countdown.get() === 0) clearInterval(interval);
    }, duration/start);
    return countdown;
}

function createTypewriter(text) {
    let typewriter = new Signal.State("");
    let chars = text.split('');

    function type(char) {
        return function() {
            typewriter.update(t => t + char);
        }
    };
    for (let index = 0; index < chars.length; index++) {
        setTimeout(type(chars[index]), 200 + index * 200);
    }
    return typewriter;
}

[NullVoxPopuli]

can you describe the situation in which the value can't be known? I don't understand

let known = count.get()

[Ocean-OS]

can you describe the situation in which the value can't be known? I don't understand

let known = count.get()

By known, I mean a case where the developer can't just do this:

count.set(42); //or any other constant value

...because the value could be acquired through user input, fetching, etc.
The update function is primarily a cleaner way to do:

let updater = c => c+1;
count.set(updater(count.get()));

[NullVoxPopuli]

...because the value could be acquired through user input, fetching, etc.

sorry I still don't understand this

is primarily a cleaner way to do

this I understand (and am ok with, tbh) -- but I'd like to know what " user input, fetching, etc." patterns need the updater.
I feel like I'm missing something :\

sorry im dense!

[Ocean-OS]

this I understand (and am ok with, tbh) -- but I'd like to know what " user input, fetching, etc." patterns need the updater. I feel like I'm missing something :\

sorry im dense!

I meant like something where you'd have to use count.get() instead of a constant value, apologies for the bad explanation.
Example:

count.set(42); //constant ("known") value
count.set(count.get() + 1); //not "known" value