Logging

Logging is the practice of printing out useful information that helps you understand what your program is doing. It’s one of the simplest but most powerful tools for debugging, learning, and tracking your application’s behavior.

Whenever you're unsure what your code is doing — or why something is breaking — logs act like a flashlight showing you what's happening step by step.

Why logging matters

Logs help you:

see values change over time
confirm if a function is running
find where something breaks
inspect unexpected input or output
understand how your program flows

Without logging, you're “programming in the dark.”

console.log("Function started");
console.log("User age:", age);
console.log("Calculating total price...");

💡 Logging is often the first step in solving ANY debugging problem.

console.log() — The developer’s best friend

This is the most common and simplest way to create logs:

console.log("Hello!");
console.log("The value of x is", x);
console.log(user);

But logging is more powerful when used intentionally, not randomly.

Log Levels

JavaScript provides different log methods to express the importance of a message:

1. console.log() → General info

Used for normal, everyday information.

console.log("App started");

2. console.warn() → Warning

Used when something is not broken but could be a problem.

console.warn("User email is missing. Using default value.");

3. console.error() → Error

Used when something is broken or failed.

console.error("Failed to load data from API");

4. console.info() → Informational messages

Similar to log but semantically clearer.

console.info("User logged in:", username);

5. console.debug() → Developer debugging

Useful for detailed debug information.

console.debug("Loop iteration:", i);

💡 Using the right level makes logs more meaningful and easier to filter.

Logging variables and objects

You can log values directly:

console.log(score);
console.log(isLoggedIn);
console.warn(user);

Or label them for clarity:

console.log("Current score:", score);
console.log("User data:", user);

Logging objects is extremely useful for inspection.

Logging during debugging

When debugging a function, logs help you track:

function entry and exit
parameter values
conditional branches
loop iterations
unexpected states

Example:

function calculateTotal(cart) {
  console.log("calculateTotal called with:", cart);

  let total = 0;

  for (const item of cart) {
    console.log("Adding item:", item);
    total += item.price;
  }

  console.log("Final total:", total);
  return total;
}

💡 Logs should tell a story: what is happening, in what order, and with what values.

Logging best practices

✔️ Write meaningful logs, not noisy logs

Bad:

console.log("x");

Good:

console.log("Current x value:", x);

✔️ Don't log too much

Oversharing logs:

makes debugging harder
hides the important information
slows down programs

Use logs with intention.

✔️ Remove unnecessary logs after debugging

Temporary logs should be deleted:

❌ during production

❌ inside loops in final version

❌ inside sensitive areas (passwords, tokens!)

✔️ Log edge cases during debugging

if (!user) {
  console.error("User object missing:", user);
}

⚠️ Never log sensitive information like passwords, tokens, or credit card numbers.

Examples of practical logging

Tracking function flow

console.log("Starting save process...");
console.log("Saving user:", user.id);
console.log("Process completed.");

Checking conditional behavior

if (age < 18) {
  console.warn("User is under 18. Access denied.");
}

Debugging a loop

for (let i = 0; i < items.length; i++) {
  console.debug("Loop index:", i, "Item:", items[i]);
}

Debugging unexpected values

if (!email) {
  console.error("Email is undefined:", email);
}

Week 5 - Problem solving