Fetching JavaScript keys by their values - JavaScript

In JavaScript, you can find an object key by its value using several approaches. This is useful when you need to perform reverse lookups in objects where both keys and values are unique.

Consider this object where we want to find keys by their corresponding values:

const products = {
   "Pineapple": 38,
   "Apple": 110,
   "Pear": 109
};

console.log(products);

{ Pineapple: 38, Apple: 110, Pear: 109 }

Using Object.keys() with find()

The most straightforward approach uses Object.keys() with find() to locate the key:

const products = {
   "Pineapple": 38,
   "Apple": 110,
   "Pear": 109
};

const findKeyByValue = (obj, value) => {
   return Object.keys(obj).find(key => obj[key] === value);
};

console.log(findKeyByValue(products, 110)); // "Apple"
console.log(findKeyByValue(products, 38));  // "Pineapple"
console.log(findKeyByValue(products, 999)); // undefined

Apple
Pineapple
undefined

Using Reverse Mapping

For frequent lookups, create a reverse map once and reuse it:

const products = {
   "Pineapple": 38,
   "Apple": 110,
   "Pear": 109
};

const createReverseMap = (obj) => {
   const reverseMap = {};
   Object.keys(obj).forEach(key => {
       reverseMap[obj[key]] = key;
   });
   return reverseMap;
};

const reverseProducts = createReverseMap(products);
console.log(reverseProducts);

// Now lookup is O(1)
console.log(reverseProducts[110]); // "Apple"
console.log(reverseProducts[38]);  // "Pineapple"

{ '38': 'Pineapple', '109': 'Pear', '110': 'Apple' }
Apple
Pineapple

Using Object.entries()

Another approach uses Object.entries() to work with key-value pairs directly:

const products = {
   "Pineapple": 38,
   "Apple": 110,
   "Pear": 109
};

const findKeyByValue = (obj, value) => {
   const entry = Object.entries(obj).find(([key, val]) => val === value);
   return entry ? entry[0] : undefined;
};

console.log(findKeyByValue(products, 110)); // "Apple"
console.log(findKeyByValue(products, 109)); // "Pear"

Apple
Pear

Comparison

Conclusion

Use Object.keys().find() for simple cases and reverse mapping for performance-critical applications with frequent lookups. Both approaches handle missing values gracefully by returning undefined.