Very simple state class with an update callback

simple-state.js

/** @template T */
class State {
  /** @typedef {function(T):void} Callback */

  /**
   * @param {T} initialValue
   */
  constructor (initialValue) {
    this.value = initialValue

    /** @type Callback[] */
    this.callbacks = []
  }

  /**
   * @param {T} newValue
   */
  update (newValue) {
    this.value = newValue

    this.callbacks.forEach(cb => {
      cb(this.value)
    })
  }

  /**
   * @param {Callback} cb
   * @returns {void}
   */
  onUpdate (cb) {
    this.callbacks.push(cb)
  }
}