Alternate Svelte Store

gistfile1.txt

// REPL: https://svelte.dev/repl/2c56758fb2ca4de9bf842e3dfb61ed36

function customStore<T>(initialValue: T, mutationFunction: (value: T)=>T, validationFunction: (value: T)=>boolean) {

  let currentValue = initialValue
  let previousValue: T | null = null;

  const subscribers = new Set<(value: T, previousValue: T | null) => void>();

  const set = (value: T): void => {
    if (typeof window === 'undefined') return
    if (mutationFunction) value = mutationFunction(value);
    if (validationFunction && !validationFunction(value)) return console.log("validation statement failed... not updating the store")
    
    previousValue = currentValue;
    currentValue = value;

    broadcastValue(currentValue, previousValue)
  };

  const subscribe = (callback: (value: T, previousValue: T | null) => void) => {
    subscribers.add(callback);
    callback(currentValue, previousValue);

    return () => {
      subscribers.delete(callback);
    };
  };

  const update = (callback: (value: T, previousValue: T | null) => T): void => {
    const newValue = callback(structuredClone(currentValue), previousValue);
    set(newValue)
  };


  function broadcastValue(value: T, previousValue: T | null) {
    subscribers.forEach((callback) => {
      callback(value, previousValue);
    });
  }

  function get(): T {
    return currentValue
  }

  return {
    subscribe,
    update,
    set,
    get,
  }
}