I am still a bit new to React, and I find it awesome, but one thing is currently on my mind and I would like to know what to think of it.

For instance, I am making a checkout component, where i have a reduction coupon field.

I pass the current price of my event to the coupon so that it can calculate the new:

{this.state.showCouponField ? (
  <CouponForm
    validateCoupon={(coupon) => this.setState({ coupon: coupon })}
    initialValue={this.state.coupon ? this.state.coupon.token : ''}
    initialPrice={this.state.event.final_price}
    setReducedPrice={(reducedPrice) => this.setState({ reducedPrice })}
  />
) : null}

and then display:

<p className="lead">
  Total :
  <span className="pull-right">
    {this.state.reducedPrice ? (
      <span>
        <s>
          {isFree ? 'Gratuit' : `${event.real_final_price * this.state.participants} €`}
        </s>
        &nbsp;
        {this.state.reducedPrice === 0 ? 'Gratuit' : `${this.state.reducedPrice / 100 * this.state.participants} €`}
      </span>
    ) : (
      <span>
        {isFree ? 'Gratuit' : `${event.real_final_price * this.state.participants} €`}
      </span>
    )}
  </span>
</p>

But for instance, the variable reducedPrice is null until someone uses a coupon.

The question is : is it a good practice to set the reducedPrice the initial state

constructor(props) {
  super(props);
  this.state = {
    event: null,
    creditCards: [],
    selectedCardId: null,
    addCreditCard: false,
    participants: 1,
    coupon: null,
    total: 0,
    error: null,
    loading: false,
    bookingId: null,
    user: null,
    showCardOptions: false,
    showModal: false,
    modalUrl: null,
  };

  this.updatePaymentMeans = this.updatePaymentMeans.bind(this);
}

All these null variables seem useless to me as they are empty by default et they are not needed - or I have yet to see a bug - to be initialized because the state is an object so it can be created on the fly.

I know I will forget some of these, so I wonder if I would simply not set them so that it would clearly break if I miss one.

Open question :D