Get key from value in JavaScript

In JavaScript, finding a key from its corresponding value in an object requires iterating through the object's key-value pairs. Unlike accessing values by keys (which is direct), reverse lookup needs a search operation.

Understanding the Problem

Given a JavaScript object and a value, we need to find the key that maps to that value. For example:

{
   key1: 'value1',
   key2: 'value2',
   key3: 'value3'
}

If we search for 'value2', we should get 'key2' as the result.

Using Object.entries() and find()

The most efficient approach uses Object.entries() to convert the object into key-value pairs, then find() to locate the matching value:

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

const appObj = {
   Application1: 'Learning',
   Application2: 'Ecommerce', 
   Application3: 'Software',
   Application4: 'Business'
};

console.log(getKeyByValue(appObj, 'Ecommerce'));  // Application2
console.log(getKeyByValue(appObj, 'Business'));   // Application4
console.log(getKeyByValue(appObj, 'NotFound'));   // null

Application2
Application4
null

Using for...in Loop

A traditional approach using a for...in loop for better readability:

function getKeyByValueLoop(obj, value) {
   for (let key in obj) {
      if (obj[key] === value) {
         return key;
      }
   }
   return null;
}

const colors = {
   red: '#FF0000',
   green: '#00FF00',
   blue: '#0000FF'
};

console.log(getKeyByValueLoop(colors, '#00FF00'));  // green
console.log(getKeyByValueLoop(colors, '#FFFFFF'));  // null

green
null

Finding All Keys for a Value

If multiple keys have the same value, you can find all matching keys:

function getAllKeysByValue(obj, value) {
   return Object.entries(obj)
      .filter(([key, val]) => val === value)
      .map(([key]) => key);
}

const roles = {
   admin: 'full_access',
   editor: 'edit_access', 
   viewer: 'read_access',
   guest: 'read_access'
};

console.log(getAllKeysByValue(roles, 'read_access'));
console.log(getAllKeysByValue(roles, 'full_access'));

[ 'viewer', 'guest' ]
[ 'admin' ]

Comparison of Methods

Time Complexity

All methods have O(n) time complexity in the worst case, where n is the number of properties in the object. Space complexity is O(1) for single result methods and O(k) for methods returning multiple keys, where k is the number of matching keys.

Conclusion

Use Object.entries() with find() for clean, readable code when finding a single key. For performance-critical applications, the for...in loop provides slightly better performance with early termination.