setState function

/**
  * Update component state and schedule a re-render.
  * @this {import('./internal').Component}
  * @param {object | ((s: object, p: object) => object)} update A hash of state
  * properties to update with new values or a function that given the current
  * state and props returns a new partial state
  * @param {() => void} [callback] A function to be called once component state is
  * updated
  */
Component.prototype.setState = function(update, callback) {
  // only clone state when copying to nextState the first time.
  let s;
  if (this._nextState != null && this._nextState !== this.state) {
  s = this._nextState;
  } else {
  s = this._nextState = assign({}, this.state);
  }

  if (typeof update == 'function') {
  // Some libraries like `immer` mark the current state as readonly,
  // preventing us from mutating it, so we need to clone it. See #2716
  update = update(assign({}, s), this.props);
  }

  if (update) {
  assign(s, update);
  }

  // Skip update if updater function returned null
  if (update == null) return;

  if (this._vnode) {
  if (callback) {
    this._stateCallbacks.push(callback);
  }
  enqueueRender(this);
  }
};