Adding the line console.log causes a different return

Quick solution:

function findNextSquare(sq) {
  var rt = Math.sqrt(sq);
  return rt % 1 !== 0 ? -1 : (rt + 1)**2; 
}

Explanation:

This has nothing to do with asynchronicity.

rt++ in console.log((rt++)**2) causes the rt variable to be incremented. Therefore, rt will be different (one more than it should be) when it gets to the return line.

Note the difference:

• rt + 1 is an expression that adds 1 to rt and returns the result — rt stays unchanged
• rt++ adds 1 to rt, saves the result back to rt, and returns the old rt
• ++rt adds 1 to rt, saves the result back to rt, and returns the new rt

Let's see what your original code did with sq = 16, with the console.log not commented out:

function findNextSquare(sq) {            // sq = 16
  var rt = Math.sqrt(sq);                // rt = 4
  console.log((rt++) ** 2);              // logs 4^2 = 16, rt incremented to 5
  return rt % 1 !== 0 ? -1 : (rt++)**2;  // returns 5^2 = 25, rt incremented to 6 (unneeded)
}

When you remove console.log, this is the result:

function findNextSquare(sq) {            // sq = 16
  var rt = Math.sqrt(sq);                // rt = 4
  return rt % 1 !== 0 ? -1 : (rt++)**2;  // returns 4^2 = 16, THEN rt is incremented to 5
}

The solution is to add 1 to rt and then square the new value. We don't actually need to save the new value back to rt:

function findNextSquare(sq) {              // sq = 16
  var rt = Math.sqrt(sq);                  // rt = 4
  return rt % 1 !== 0 ? -1 : (rt + 1)**2;  // returns (4 + 1)^2 = 25
}

If you want a console.log in there, you can have it, just make sure it does not change rt:

function findNextSquare(sq) {
  var rt = Math.sqrt(sq);
  console.log((rt + 1) ** 2);
  return rt % 1 !== 0 ? -1 : (rt + 1) ** 2; 
}

The ++ operator automatically increments the given variable. And its not just for the console.log().

What happens when you do variableName++; is equal to writing

variableName = variableName + 1;

so whether you have used the ++ operator inside a console.log() or somewhere else, the variable gets incremented for good.

Be careful how you use it. If you want to still use the console log with incremented variable, I would suggest using console.log(variableName + 1);

Hope this helps!

Expression used in console.log increments rt value.