Debugging with console.log()

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Quick Questions

Shade in the A or B circle for each question.

1. Your program prints the wrong total. What is a good first debugging step?

A Add console.log("total:", total) to see the value as the program runs

B Delete the whole program and start again

2. Which debug message is easier to understand in the console?

A console.log("score:", score);

B console.log(score);

3. You want to see how total changes on every repeat of a loop. Where should the debug line go?

A Inside the loop body, after total is updated

B Only after the loop has finished

4. A function add(a, b) gives the wrong answer. Which debug line helps most?

A console.log("add is a function");

B console.log("a:", a, "b:", b);

5. Your debug output shows count: 1, count: 2, count: 4 when you expected 3. What does this tell you?

A The bug must be somewhere after the last log line

B The value first goes wrong between the second and third logs

Match the Pairs

Draw a line connecting each item on the left to its match on the right.

Left column		Right column
1console.log("total:", total);		AA comma is missing between the label and the variable, so this is a syntax error and nothing is logged. It should be console.log("total:", total);
2console.log("n =", n, "result =", result);		Bfor (let i = 0; i < 5; i++) { console.log("i:", i); }
3Log the counter inside a loop to see how its value changes on every repeat		CThe variable total is inside the quotes, so only the word is printed, not the value. It should be console.log("total:", total);
4Once the bug is fixed, remove or comment out the debug line so the console stays clean		D// console.log("score:", score);
5console.log("total:" total);		ELogs a function's parameter and its result together so you can see what went in and what came out
6console.log("total:");		FPrints a labelled value of total so you can check what it holds at that point in the program

Definitions

console.log("total:", total)

Fill in the Blanks

Complete each line of code by filling in the missing parts.

let total = 0; total = total + 5; console.___("total:", total);

for (let i = 1; i <= 3; i++) { console.log("i:", _); }

function double(n) { return n * 2; } const result = double(4); console.log("result:", ______);

function greet(name) { console.log("name:", ____); }

let score = 0; score += 5; __ console.log("score:", score);

Spot the Error

Shade in the letter of the line that contains an error.

Question 1

A let total = 0;

B let total = total + 5;

C console.log("total:", total);

Question 2

A let sum = 0;

B for (let i = 1; i <= 3; i++) {

C sum =+ i;

D console.log("sum:", sum);

E }

Question 3

A function add(a, b) {

B console.log("a:", a);

C let sum = a + b;

D console.log("sum:" sum);

E return sum;

F }

Question 4

A let score = 10;

B score = score + 5;

C console.log("score:", score;

Question 5

A let price = 20;

B let qty = 3;

C let cost = price * qty;

D console.log("cost:", Cost);

Answer Key

Debugging with console.log()

Section A: Quick Questions

A — Add console.log("total:", total) to see the value as the program runs

A — console.log("score:", score);

A — Inside the loop body, after total is updated

B — console.log("a:", a, "b:", b);

B — The value first goes wrong between the second and third logs

Section B: Match the Pairs

console.log("total:", total); → Prints a labelled value of total so you can check what it holds at that point in the program

console.log("n =", n, "result =", result); → Logs a function's parameter and its result together so you can see what went in and what came out

Log the counter inside a loop to see how its value changes on every repeat → for (let i = 0; i < 5; i++) { console.log("i:", i); }

Once the bug is fixed, remove or comment out the debug line so the console stays clean → // console.log("score:", score);

console.log("total:" total); → A comma is missing between the label and the variable, so this is a syntax error and nothing is logged. It should be console.log("total:", total);

console.log("total:"); → The variable total is inside the quotes, so only the word is printed, not the value. It should be console.log("total:", total);

Section C: Definitions

console.log("total:", total) — Prints a labelled variable value while the program runs, so you can spot the line where the value first goes wrong. Remove these debug lines once the bug is fixed.

Section D: Fill in the Blanks

let total = 0;
total = total + 5;
console.log("total:", total);

for (let i = 1; i <= 3; i++) {
  console.log("i:", i);
}

function double(n) { return n * 2; }
const result = double(4);
console.log("result:", result);

function greet(name) {
  console.log("name:", name);
}

let score = 0;
score += 5;
// console.log("score:", score);

Section E: Spot the Error

B — total is already declared on line 1, so let cannot declare it again. Line 2 should reassign it with total = total + 5; so the console.log on line 3 can show the new value.

C — =+ sets sum to positive i instead of adding to it. The logged sum would show 1, 2, 3, not 1, 3, 6. It should be sum += i; and the log inside the loop is how you spot where the value goes wrong.

D — The comma between the label and the variable is missing. It should be console.log("sum:", sum); so the function's result is logged with its label.

C — The closing bracket is missing. It should be console.log("score:", score); so the debug line runs and shows the value of score.

D — JavaScript is case sensitive. The variable is cost, not Cost, so this causes a ReferenceError. The debug line should log cost.