How do you reliably check if an object is empty?

The short answer

The most reliable way is Object.keys(obj).length === 0. This counts the object's own enumerable properties. If it is zero, the object is empty. Make sure you also check that the value is actually an object and not null.

The standard approach

Copy
function isEmpty(obj) {
  return Object.keys(obj).length === 0;
}
isEmpty({}); // true
isEmpty({ name: 'John' }); // false

Handling edge cases

null and undefined are not objects, so check for them first:

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function isEmpty(obj) {
  return (
    obj != null &&
    typeof obj === 'object' &&
    Object.keys(obj).length === 0
  );
}
isEmpty({}); // true
isEmpty(null); // false
isEmpty(undefined); // false
isEmpty([]); // true — arrays are objects with 0 keys

If you want to exclude arrays:

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function isEmptyObject(obj) {
  return (
    obj != null &&
    typeof obj === 'object' &&
    !Array.isArray(obj) &&
    Object.keys(obj).length === 0
  );
}

Why not just use if (obj)

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const obj = {};
if (obj) {
  console.log('This runs!'); // empty objects are truthy
}

An empty object {} is truthy in JavaScript. So if (obj) does not tell you if the object has properties — it only tells you the value is not null, undefined, 0, '', or false.

Other approaches

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// JSON.stringify — works but slower
JSON.stringify(obj) === '{}';
// for...in — slightly more verbose
function isEmpty(obj) {
  for (const key in obj) {
    if (obj.hasOwnProperty(key)) return false;
  }
  return true;
}

Object.keys().length is the cleanest and most widely used approach.

Why interviewers ask this

This tests fundamental JavaScript knowledge about objects, truthiness, and the Object API. It is a quick question that reveals how well you understand JavaScript's type system.