What is debugging?

Learn the debugging process and effective approaches.

What is debugging?

Debugging is the process of finding and fixing unexpected behavior in your code. A bug usually shows up as a wrong result, a crash or unexpected behavior. A good process has five steps: reproduce, isolate, understand the cause, fix and verify.

The most important step is reducing the bug to a small, repeatable example. If you cannot reproduce the bug, you cannot prove the fix works.

The five-step process

Let us apply the five steps in order. Reproduce: find the smallest input that triggers the bug. Isolate: narrow down to the line or function that causes it. Understand the cause: compare expected and actual behavior.

Then fix: make the smallest change that targets the root cause. Finally verify: run both the bug case and other cases again to make sure the fix did not break anything else.

Exercise Log values to the console

Define a function called sumPositive(nums). It should loop through nums with for...of and add numbers greater than or equal to 0 to total. Inside the loop, log each value with console.log(num). Then assign the result of sumPositive([3, -2, 5]) to a variable result and log it with console.log(result).

Show example solution

function sumPositive(nums) {
  let total = 0;
  for (const num of nums) {
    console.log(num);
    if (num >= 0) {
      total += num;
    }
  }
  return total;
}

const result = sumPositive([3, -2, 5]);
console.log(result);

Test yourself

Which is the first step of the debugging process?
- Reproduce the bug
- Rewrite all the code
- Delete every variable
- Restart the computer
Show answer

Reproduce the bug — Correct. You must reproduce the bug first; a bug you cannot repeat is hard to fix.

Complete the process step

console.log("step 1: reproduce the ____");

Show answer

console.log("step 1: reproduce the bug");

Match the step to its meaning

Reproduce

Trigger the bug again with a small input
Isolate

Narrow down to the line that causes it
Fix

Change the code and remove the bug
Verify

Prove that the fix really works

Next lesson: Call traces with console.trace

We Are Enhancing Your Experience

Cookie Preferences

Strictly Necessary Cookies

Analytical Cookies