JavaScript: while loop inside a for loop

while (cond)
  statement1;
  statement2;

does not do what you think (and the indentation is lying to you). Reformating the code to be true to the logical flow gives you this:

var hash = "";
var count = 1;
var n = 3;
for (var pound = 1; pound <= 7; pound++) {
  while (hash.length != count)
    hash += "#";
  hash += "\n";
  count += n;
  n++;
}
console.log(hash);

While repeats only the very next statement. If you need it to repeat more, you have to use a compound statement (statement block) using the curly braces.

Here, for loop will go once for each row; within that, while will go once for each hash character.

Any loop without brackets { <-code here-> } only loops looking the following line.

So your while is doing only this:

while (hash.length != count)
    hash += "#";

So, removing your for loop, you will have hash = "#" (because count starts at 1, and hash.length is 0), and then the following will happen:

hash += "\n";
count += n;
n++;

Basically, hash will be # with a line break.

The answer to your question is that since the while loop has no brackets to indicate the code inside the loop. only one line, the
hash += "#";
gets execute, and when you get to the line that modifies the value of the count:
count += n;,
the while loop wont repeat since you already exited that loop.
Usually you want to keep the code as simple as possible. you can get the same result as the code you posted by simply writing it this way:

   var hash="";
   for (var pound = 1; pound <=7; pound ++)
        {
            hash += "#";
            console.log(hash);            
         }

you will use far less variables, the length of hash will greatly be reduce. sometimes too many string concatenation slows down code execution.

what the code above does is run a for loop seven times, each time, it appends a '#' to the hash string and displays it on the console...
same result, fewer lines, faster execution, and much easier to read.

if your are new to programming, I would recommend you start with a programming language the encourage good programming practice.

although PASCAL is not very popular anymore, learning it first before any C style language, would most likely get you up and running in writing very good code.

I know Pascal haters will probably criticize.. but, this language is good to get you to concentrate on algorithms and good programming style. which should be your first step before jumping into any language.