The Signal Function: `$( )`

The $( ) function is the core constructor of SigPro. It defines how data is stored, computed, and persisted.

Function Signature

$(initialValue: any, key?: string): Signal
$(computation: Function): ComputedSignal

Parameter	Type	Required	Description
`initialValue`	`any`	Yes*	The starting value of your signal.
`computation`	`Function`	Yes*	A function that returns a value based on other signals.
`key`	`string`	No	A unique name to persist the signal in `localStorage`.

*Either an initial value or a computation function must be provided.

$(value) Creates a writable signal. It returns a function that acts as both getter and setter.

const count = $(0); 

count();    // Read (0)
count(10);  // Write (10)

$(value, key) Creates a writable signal that syncs with the browser's storage.

const theme = $("light", "app-theme"); 

theme("dark"); // Automatically calls localStorage.setItem("app-theme", '"dark"')

Note: On page load, SigPro will prioritize the value found in localStorage over the initialValue.

$(function) Creates a read-only signal that updates automatically when any signal used inside it changes.

const price = $(100);
const tax = $(0.21);

// This tracks both 'price' and 'tax' automatically
const total = $(() => price() * (1 + tax()));

When calling the setter, you can pass an updater function to access the current value safely.

const list = $(["A", "B"]);

// Adds "C" using the previous state
list(prev => [...prev, "C"]);