Object constructor

The Object constructor coerces the passed argument to an object. If it's already an object, the same object is returned.

Therefore, you can use it to coerce the value to an object, and strictly compare that object with the original value.

The following function requires ECMAScript 3, which introduced ===:

 function isObject(value) { /* Requires ECMAScript 3 or later */
   return Object(value) === value;
 }

I like this approach because it's simple and self-descriptive, and an analogous check will also work for booleans, numbers and strings. However, be aware it relies on the global Object not being shadowed nor altered.

Constructors

When you instantiate a constructor, it can return a value different than the just-created instance. But that value will be ignored unless it's an object.

The following function requires ECMAScript 3, which allowed constructors to return non-objects. Before ECMAScript 3 that threw an error, but try statements didn't exist back then.

 function isObject(value) { /* Requires ECMAScript 3 or later */
   return new function() { return value; }() === value;
 }

While a bit less simple than the previous example, this one does not rely on any global property, and thus might be the safest.

this value

Old ECMAScript specifications required the this value to be an object. ECMAScript 3 introduced Function.prototype.call, which allowed to call a function with an arbitrary this value, but coerced to an object.

ECMAScript 5 introduced a strict mode which removed this behavior, but in sloppy mode we still can (but arguably shouldn't) rely on it.

 function isObject(value) { /* Requires ECMAScript 3 or later in sloppy mode */
   return function() { return this === value; }.call(value);
 }

[[Prototype]]

All ordinary objects have an internal slot called [[Prototype]], whose value determines from which other object it inherits from. The value can only be an object or null. Therefore, you can attempt to create an object which inherits from the desired value, and check if it worked.

Both Object.create and Object.getPrototypeOf require ECMAScript 5.

 function isObject(value) { /* Requires ECMAScript 5 or later */
   try {
     Object.create(value);
     return value !== null;
   } catch(err) {
     return false;
   }
 }

 function isObject(value) { /* Requires ECMAScript 5 or later */
   function Constructor() {}
   Constructor.prototype = value;
   return Object.getPrototypeOf(new Constructor()) === value;
 }

Some new ECMAScript 6 ways

ECMAScript 6 introduces some new indirect ways to check is a value is an object. They use the previously seen approach to pass the value to some code which requires an object, wrapped inside a try statement to catch errors. Some hidden examples, not worth commenting

function isObject(value) { /* Requires ECMAScript 6 or later */
  try {
    Object.setPrototypeOf({}, value);
    return value !== null;
  } catch(err) {
    return false;
  }
}

function isObject(value) { /* Requires ECMAScript 6 or later */
  try {
    new Proxy(value, value);
    return value;
  } catch(err) {
    return false;
  }
}

function isObject(value) { /* Requires ECMAScript 6 or later */
  try {
    new WeakSet([value]);
    return value;
  } catch(err) {
    return false;
  }
}

Object constructor

The Object constructor coerces the passed argument to an object. If it's already an object, the same object is returned.

Therefore, you can use it to coerce the value to an object, and strictly compare that object with the original value.

The following function requires ECMAScript 3, which introduced ===:

 function isObject(value) { /* Requires ECMAScript 3 or later */
   return Object(value) === value;
 }

I like this approach because it's simple and self-descriptive, and an analogous check will also work for booleans, numbers and strings. However, be aware it relies on the global Object not being shadowed nor altered.

Constructors

When you instantiate a constructor, it can return a value different than the just-created instance. But that value will be ignored unless it's an object.

The following function requires ECMAScript 3, which allowed constructors to return non-objects. Before ECMAScript 3 that threw an error, but try statements didn't exist back then.

 function isObject(value) { /* Requires ECMAScript 3 or later */
   return new function() { return value; }() === value;
 }

While a bit less simple than the previous example, this one does not rely on any global property, and thus might be the safest.

this value

Old ECMAScript specifications required the this value to be an object. ECMAScript 3 introduced Function.prototype.call, which allowed to call a function with an arbitrary this value, but coerced to an object.

ECMAScript 5 introduced a strict mode which removed this behavior, but in sloppy mode we still can (but arguably shouldn't) rely on it.

 function isObject(value) { /* Requires ECMAScript 3 or later in sloppy mode */
   return function() { return this === value; }.call(value);
 }

[[Prototype]]

All ordinary objects have an internal slot called [[Prototype]], whose value determines from which other object it inherits from. The value can only be an object or null. Therefore, you can attempt to create an object which inherits from the desired value, and check if it worked.

Both Object.create and Object.getPrototypeOf require ECMAScript 5.

 function isObject(value) { /* Requires ECMAScript 5 or later */
   try {
     Object.create(value);
     return value !== null;
   } catch(err) {
     return false;
   }
 }

 function isObject(value) { /* Requires ECMAScript 5 or later */
   function Constructor() {}
   Constructor.prototype = value;
   return Object.getPrototypeOf(new Constructor()) === value;
 }

Some new ECMAScript 6 ways

ECMAScript 6 introduces some new indirect ways to check is a value is an object. They use the previously seen approach to pass the value to some code which requires an object, wrapped inside a try statement to catch errors. Some hidden examples, not worth commenting

function isObject(value) { /* Requires ECMAScript 6 or later */
  try {
    Object.setPrototypeOf({}, value);
    return value !== null;
  } catch(err) {
    return false;
  }
}

function isObject(value) { /* Requires ECMAScript 6 or later */
  try {
    new WeakSet([value]);
    return true;
  } catch(err) {
    return false;
  }
}