Understanding the JS code while setting up redis with Mongoose

What will classify mongoose.Query.prototype.exec; as? value type or reference type? Because if it is a reference type then when we change mongoose.Query.prototype.exec = function

exec is of reference type, but it is assigned the value of another reference variable mongoose.Query.prototype.exec. You can think of it like this : mongoose.Query.prototype.exec is itself pointing to an object (a Function object) in memory, and now after the assignment, exec is also pointing to the same object - in other words, the memory address of the object is copied (by value) from mongoose.Query.prototype.exec to exec during assignment. So the value of the variable mongoose.Query.prototype.exec itself i.e. the memory address stored in it, can be changed without affecting the other variable exec. They both will just end up pointing to two different objects.

Can someone explain this in stretch i.e this in apply points to where?

In this case, it'll be the object on which this function will be invoked i.e. the Query instance.

and he is passing argument (this, argument); where does that Argument come from?

Unless there is some code you missed to copy paste in the question, argument appears to be a typo. He was probably referring to the built-in object arguments which is accessible inside every function and consists of the arguments passed to the function. Here is a reference.

At a high level, what the instructor is trying to do is to override the built-in behavior of the function Query.exec() to add some of his own custom processing. He first creates a "backup" of the original function, then points Query.exec to his custom function which adds the custom processing (the log statement) and then hands over control to the backup i.e. proceed with built-in behavior. Whoever invokes exec() on a Query instance after this point will see the overridden functionality - first a log statement, then built-in behavior of exec()