How to Filter Object by Key in JavaScript: Simple Guide

To filter an object by key in JavaScript, use Object.entries() to get key-value pairs, then apply Array.filter() on keys, and finally convert back to an object with Object.fromEntries(). This method keeps only the keys you want based on your condition.

Syntax

Use Object.entries(obj) to get an array of key-value pairs from the object. Then use filter() on this array to keep only pairs where the key matches your condition. Finally, use Object.fromEntries() to convert the filtered pairs back into an object.

Parts explained:

• Object.entries(obj): Converts object to array of [key, value] pairs.
• filter(([key, value]) => condition): Keeps pairs where the key meets the condition.
• Object.fromEntries(filteredArray): Converts filtered pairs back to an object.

javascript

const filteredObject = Object.fromEntries(
  Object.entries(obj).filter(([key, value]) => /* condition on key */)
);

Example

This example filters an object to keep only keys that start with the letter 'a'.

javascript

const obj = {
  apple: 1,
  banana: 2,
  apricot: 3,
  cherry: 4
};

const filtered = Object.fromEntries(
  Object.entries(obj).filter(([key]) => key.startsWith('a'))
);

console.log(filtered);

Output

{"apple":1,"apricot":3}

Common Pitfalls

One common mistake is trying to use filter() directly on an object, which is not possible because objects don't have filter(). Another is forgetting to convert the filtered entries back to an object, resulting in an array instead of an object.

Also, be careful with the condition inside filter() to correctly check the key.

javascript

const obj = { a: 1, b: 2, c: 3 };

// Wrong: objects don't have filter method
// const filteredWrong = obj.filter(key => key === 'a'); // Error

// Right way:
const filteredRight = Object.fromEntries(
  Object.entries(obj).filter(([key]) => key === 'a')
);

console.log(filteredRight);

Output

{"a":1}

Quick Reference

• Object.entries(obj): Get array of [key, value] pairs.
• Array.filter(): Filter pairs by key condition.
• Object.fromEntries(): Convert filtered pairs back to object.

Key Takeaways

Use Object.entries() to convert an object into key-value pairs array before filtering.

Filter the array by checking the key in the filter callback function.

Convert the filtered array back to an object with Object.fromEntries().

Objects do not have filter method directly; always convert to entries first.

Check your filter condition carefully to get the desired keys.