You can use a for…in loop with an Object.hasOwn (ECMA 2022+) test to check whether an object has any own properties:

function isEmpty(obj) {
  for (const prop in obj) {
    if (Object.hasOwn(obj, prop)) {
      return false;
    }
  }

  return true;
}

If you also need to distinguish {}-like empty objects from other objects with no own properties (e.g. Dates), you can do various (and unfortunately need-specific) type checks:

function isEmptyObject(value) {
  if (value == null) {
    // null or undefined
    return false;
  }

  if (typeof value !== 'object') {
    // boolean, number, string, function, etc.
    return false;
  }

  const proto = Object.getPrototypeOf(value);

  // consider `Object.create(null)`, commonly used as a safe map
  // before `Map` support, an empty object as well as `{}`
  if (proto !== null && proto !== Object.prototype) {
    return false;
  }

  return isEmpty(value);
}

Note that comparing against Object.prototype like in this example will fail to recognize cross-realm objects.

Do not use Object.keys(obj).length. It is O(N) complexity because it creates an array containing all the property names only to get the length of that array. Iterating over the object accomplishes the same goal but is O(1).

For compatibility with JavaScript engines that don’t support ES 2022+, const can be replaced with var and Object.hasOwn with Object.prototype.hasOwnProperty.call:

function isEmpty(obj) {
  for (var prop in obj) {
    if (Object.prototype.hasOwnProperty.call(obj, prop)) {
      return false;
    }
  }

  return true
}

Many popular libraries also provide functions to check for empty objects:

jQuery:

jQuery.isEmptyObject({}); // true

lodash:

_.isEmpty({}); // true

Underscore:

_.isEmpty({}); // true

Hoek:

Hoek.deepEqual({}, {}); // true

ExtJS:

Ext.Object.isEmpty({}); // true

AngularJS (version 1):

angular.equals({}, {}); // true

Ramda:

R.isEmpty({}); // true