How to get value from a dropdown to a button? - reactjs

Question

I'm using react JS and I have a problem. I don't know how to get the value from my dropdown and put that value into a onclick button. I have read lots of topics but I haven't find anything really useful for a beginner like me.
I am using "scheduler" that helped me built my dropdown and some other stuffs.
So, my dropdown get data from a local file and looks like this:
{values.map(v => (
<option value={this.value}>{v.value}</option>
))}
console.log(ref)
And my button is like this:
<Button onClick={() => this.decrement()}>
Ajouetr la réservation
</Button>
The decrement method was only there to test if it was working, and it is.
Actually, what I want to do is quite simple: I have some values in my dropdown (from 1 to 7). And I have a state that says there is 30 places available. What I want is when I choose a specified item in my dropdown AND validate with my button and then my state to decrement with the specified number. Because right now it only decrement with 1.
I hope it's clear enough for someone to help me, because I spent 2 days on that problem and I don't know what to do.
Thank you :)

Answer 1

Next time, it's nice to provide an interactive example with your question. Here's a CodeSandbox I made that (I hope) illustrates your example (link). If you want to fiddle with the example, just click "Fork" in the top right corner.
Back to the solution:
I think what you're missing is storing the selected value in your state along with the 30 "places". What you want is to make your <select /> tag into a "controlled component". When someone interacts with the <select /> you want to change the internal state so that it matches the selected value. That way, when you call decrement() from your button, you can use the internal state's value rather than getting it from a ref (I think that's what you were trying to do).
Here's a link to the React doc that explains how to use forms, specifically the <select /> tag: (link).
Take care!

Answer 2

I would say that you can think about this in 2 different steps:
SET THE QUANTITY STATE
Set the state with the current dropdown value - For achieving this, you can just use the onChange method in your select:
<select name="quantity"
value={this.state.quantity}
onChange={this.onSelectQuantity}
>
In your constructor, you create a variable quantity inside your state
Create a function called onSelectQuantity where you will set the quantity state with setState.
Do not forget to bind the function onSelectQuantity on the constructor.
With this, every time that you change the value on select, your state would capture its value. You can log it from the function if you want to test if it works.
DECREMENT FROM THE BUTTON
After this, you can just decrease the value of the state again from decrement function
<Button onClick={this.decrement}>
Ajouetr la réservation
</Button>
You will have a function...
decrement() {
const newQuantity = this.state.quantity - 1;
this.setState({
quantity: newQuantity
})
}
Hope it helps!