Question

Once you notice that there is something wrong with your program because it misbehaves or produces errors, the next step is to figure out what the problem is.

Sometimes it is obvious. The error message will point at a specific line of your program, and if you look at the error description and that line of code, you can often see the problem.

But not always. Sometimes the line that triggered the problem is simply the first place where a bogus value produced elsewhere gets used in an invalid way. And sometimes there is no error message at all—just an invalid result. If you have been solving the exercises in the earlier chapters, you will probably have already experienced such situations.

The following example program tries to convert a whole number to a string in any base (decimal, binary, and so on) by repeatedly picking out the last digit and then dividing the number to get rid of this digit. But the insane output that it currently produces suggests that it has a bug.

function numberToString(n, base) {
  var result = "", sign = "";
  if (n < 0) {
    sign = "-";
    n = -n;
  }
  do {
    result = String(n % base) + result;
    n /= base;
  } while (n > 0);
  return sign + result;
}
console.log(numberToString(13, 10));
// → 1.5e-3231.3e-3221.3e-3211.3e-3201.3e-3191.3e-3181.3…

Even if you see the problem already, pretend for a moment that you don’t. We know that our program is malfunctioning, and we want to find out why.

This is where you must resist the urge to start making random changes to the code. Instead, think. Analyze what is happening and come up with a theory of why it might be happening. Then, make additional observations to test this theory—or, if you don’t yet have a theory, make additional observations that might help you come up with one.

Putting a few strategic console.log calls into the program is a good way to get additional information about what the program is doing. In this case, we want n to take the values 13 , 1 , and then 0 . Let’s write out its value at the start of the loop.

13
1.3
0.13
0.013
…
1.5e-323

Right. Dividing 13 by 10 does not produce a whole number. Instead of n /= base , what we actually want is n = Math.floor(n / base) so that the number is properly “shifted” to the right.

An alternative to using console.log is to use the debugger capabilities of your browser. Modern browsers come with the ability to set a breakpoint on a specific line of your code. This will cause the execution of the program to pause every time the line with the breakpoint is reached and allow you to inspect the values of variables at that point. I won’t go into details here since debuggers differ from browser to browser, but look in your browser’s developer tools and search the Web for more information. Another way to set a breakpoint is to include a debugger statement (consisting of simply that keyword) in your program. If the developer tools of your browser are active, the program will pause whenever it reaches that statement, and you will be able to inspect its state.

This is a book about getting computers to do what you want them to do. Computers are about as common as screwdrivers today, but they contain a lot more hidden complexity and thus are harder to operate and understand. To many, they remain alien, slightly threatening things.