Debugging

Code does what you wrote, not what you meant. Once you internalize that — really internalize it — debugging stops being a panic and starts being a routine. This lesson teaches the move you'll use forever, then has you fix four small bugs that walk through the most common patterns.

What you'll learn

• Use console.log to see what variables actually hold mid-function
• Read code top-down, looking for the gap between intent and behavior
• Recognize a few bug shapes the first time you meet them so they don't fool you twice

Instructions

The function findMax(numbers) is supposed to return the largest number in an array. The starter code looks reasonable, and it even passes some tests. But it's wrong — and the test that catches it is all-negative inputs.

Your job:

1. Read the failing test in the Examples tab. It tells you what the bug looks like.
2. Add a console.log inside the loop to see how max evolves on the all-negative case. (You can leave it in for now; you can clean up at the end.)
3. Find the wrong assumption. Fix it.
4. Run again. Tests pass.

The bug is in ONE line. You've been writing this kind of code already in earlier lessons.

Key concepts

console.log is your microscope

You can't see what's happening inside a running function. Add console.log to make the invisible visible:

function findMax(numbers) {
  let max = 0;
  for (const n of numbers) {
    console.log("checking", n, "current max:", max);  // ← this is the trick
    if (n > max) max = n;
  }
  return max;
}

The labels matter: "checking", n, "current max:", max is much easier to read than n alone. When the printout looks wrong, you've found the bug.

Read what's there, not what you meant

Beginners (and pros, honestly) reread their own code and see what they meant to write, not what's actually there. The trick is to read each line as if a stranger wrote it:

• "What does this line literally do?"
• "What's in each variable when this line runs?"
• "Is that what I expected?"

console.log is how you turn the second question from a guess into a fact.

The four practice exercises

The exercises in the Practice tab each contain a small bug. They're modeled on the four most common bug shapes — one per exercise. You don't need to know which is which up front; just read the failing test, run the code, drop in a console.log, and watch what happens. By the time you finish all four, the patterns will start jumping out at you.

Hints

1. The bug in findMax is the line that initializes max. What if every number in the array is smaller than the initial value?
2. Initialize max to numbers[0] instead of a hardcoded value.
3. let max = numbers[0]; — now whatever the first element is, that's the starting point, and the loop lifts it from there.