[javascript] The most accurate way to check JS object's type?

I'd argue that most of the solutions shown here suffer from being over-engineerd. Probably the most simple way to check if a value is of type [object Object] is to check against the .constructor property of it:

function isObject (a) { return a != null && a.constructor === Object; }

or even shorter with arrow-functions:

const isObject = a => a != null && a.constructor === Object;

The a != null part is necessary because one might pass in null or undefined and you cannot extract a constructor property from either of these.

It works with any object created via:

• the Object constructor
• literals {}

Another neat feature of it, is it's ability to give correct reports for custom classes which make use of Symbol.toStringTag. For example:

class MimicObject {
  get [Symbol.toStringTag]() {
    return 'Object';
  }
}

The problem here is that when calling Object.prototype.toString on an instance of it, the false report [object Object] will be returned:

let fakeObj = new MimicObject();
Object.prototype.toString.call(fakeObj); // -> [object Object]

But checking against the constructor gives a correct result:

let fakeObj = new MimicObject();
fakeObj.constructor === Object; // -> false