How to update a second dropdown box, w.r.t values of the first dropdown box in React?

✍🏻Written/Revised : October 2023

Disclaimer: I am fairly very noob to React. So there might be a better way to do this, and if so, please do share it with me in the ‘issues’ in my github repo.

Consider a scenario in your screen where you have 2 dropdowns.

• One is a showing a list of media type. Eg :- Tv Shows, Movies, Anime, Animated…
  
• Other is showing a list of specific items in those types.
  

Now you want the user to have a better navigation in which, if the user selects an option from the first dropdown, it curates the things in the second dropdown accordingly.
For better navigation, lets assume I have a nested json object with the exact mappings as I need.

const itemMapping = {
    "All":[
        "Severance", "loki", "Ahsoka", "The Expanse","Avengers", "Justice League", "Star Wars","Bleach", "Naruto", "One Piece"
    ],
    "Tv Shows":[
        "Severance", "loki", "Ahsoka", "The Expanse"
    ],
    "Movies": [
        "Avengers", "Justice League", "Star Wars"
    ],
    "Anime":[
        "Bleach", "Naruto", "One Piece"
    ]
}

Lets get the first list sorted

const categories = Object.keys(itemMapping);

To get this in the UI, we can add (inside the react-function of your choosing)

const showCategories = (
    <select onChange={handleChange}>
        {categories.map((cat)=>(
            <option value={cat}>{cat}</option>
        ))}
    </select>
)

we can change handleChange and replace it with the action of choosing the array of the selected category and assigning it to an array.
Here is where things get tricky. This will not work, if the second array - the dynamic one - is a static const or let or var It should be a state. i.e

const wrongArray = []; // initializing an empty array
wrongArray = itemMapping['All'] // populating it with the default value of 'all the items'
//👆 This is the wrong way as the react screen will not re-render 
// if there is a change to a static array.
// It needs to detect a change in state to do that. Fortunately, we have the `useState()` react hook
​
​
//👇 This way, we can initialize an array as a react-state. (and also fill it up
// with all the items )
const [catItems, setCatItems] = useState(itemMapping['All']);

Note : Read more about array as react states here

Now, we can use the handleChange to be such that, it updates the state of the catItems array with the corresponding items

const showCategories = (
    <select onChange={(cat) => setCatItems(itemMapping[cat.target.value])}>
        {categories.map((cat)=>(
            <option value={cat}>{cat}</option>
        ))}
    </select>
)

This updated array will be used for the dropdown elements of the second dropdown

  const showCatItems = (
    <select
      onChange={
        // getting a console output so that we know this is working as expected
        (item) => console.log(item.target.value)
      }
    >
      {catItems.map((items) => (
        <option value={items}>{items}</option>
      ))}
    </select>
  );

This should do it.

Now your second dropdown dox elements will be dynamically updated with the selections on the first box.
Thanks for reading👋

---

If you still find the above explanation a bit unsatisfactory, the full code is given below, run that with debug points to understand how it works.

import { useState } from "react";

// A generic react function (now read #56 and come back)
function DependantDropDowns() {
  
  // All the items mapped in a json array for easy usage
  const itemMapping = {
      "All":[
          "Severance", "loki", "Ahsoka", "The Expanse","Avengers", "Justice League", "Star Wars","Bleach", "Naruto", "One Piece"
      ],
      "Tv Shows":[
          "Severance", "loki", "Ahsoka", "The Expanse"
      ],
      "Movies": [
          "Avengers", "Justice League", "Star Wars"
      ],
      "Anime":[
          "Bleach", "Naruto", "One Piece"
      ]
  }
  
  // list of categories
  const categories = Object.keys(itemMapping);
  // default catItems array containing all the items
  const [catItems, setCatItems] = useState(itemMapping["All"]);

  const showCategories = (
    <select onChange={(cat) => setCatItems(itemMapping[cat.target.value])}>
      {categories.map((cat) => (
        <option>{cat}</option>
      ))}
    </select>
  );

  const showCatItems = (
    <select
      onChange={
        // getting a console output so that we know this is working as expected
        (item) => console.log(item.target.value)
      }
    >
      {catItems.map((items) => (
        <option value={items}>{items}</option>
      ))}
    </select>
  );

  return (
    <>
      {showCategories}
      {showCatItems}
    </>
  );
}

export default function App() {
  return <DependantDropDowns />;
}