The Distinction Between proto and constructor.prototype

When working with JavaScript objects, understanding the difference between proto and constructor.prototype is crucial. While proto points to the direct prototype of an object, constructor.prototype refers to the object from which the function that created the object was created. This distinction can lead to different results when traversing prototype chains.

Prototype Chain Traversal

As demonstrated in the provided code, proto can be used to traverse the prototype chain of an object. In the example, proto is used to traverse the prototype chain of newtoy, an instance of the Gadget function. Each subsequent proto call ascends one level in the chain, eventually returning null since there is no prototype beyond Object.prototype.

However, if constructor.prototype.constructor.prototype.constructor.prototype is used, it also ascends the prototype chain but eventually returns the initial Gadget function, since it refers to the prototype from which the Gadget constructor was created.

Null Checking in Internet Explorer

In Internet Explorer, where the proto property is not available, the prototype chain can be traversed using the prototype getter on the Function object. The code below demonstrates how to check for null:

function checkNull(obj) {
  while (obj = obj.prototype) {
    // Check if the prototype is null
  }
}