App Lab/JS confusion between strings and variables
As always with javascript, show the right&amp;wrong:

Show them, using the exact piece of code from your example, but add another line:

var greeting = "Hello ";
var first_name = prompt("What's your first name?");
var last_name = prompt("What's your last name?");

var reply = greeting+" "+last_name+", "+first_name;
console.log("reply");
console.log(reply);


And run it. 

Then they see that when they log something with " ", the output is literally what's inside the quotes.

However, if they have a variable (this is important to note that reply is defined with var), the output is the contents of the variable. Truth be told, the output is the string representation of the object, but I fear that's too advanced at this point. 

Further explaining that when they use the word var (or const etc.), you create a something that holds information. On the other hand, using quotes "", you are simply representing that information.

But no matter how well you explain it, it's still far more effective to show examples like the one above, and to give the students exercises that will allow them to experience the point you try to teach.

For example, let them try to do that with a Date object:

As an exercise, teach them about Date in JS:

var d = new Date()


and then explain that d holds info about the date and time that the line was executed. 

Once the students understand the very basics of the date object, ask them to print it. The difference between

console.log(d)


and 

console.log("d")


are very, very big.