Commenting out code to find a bug

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Date:

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Quick Questions

Shade in the A or B circle for each question.

1. You want JavaScript to skip this line while you hunt for a bug: console.log(total); Which version does that?

A // console.log(total);

B console.log(total); //

2. What happens when you comment out a line with //?

A JavaScript skips the line but it stays in your file

B JavaScript deletes the line from your file

3. You commented out a line and the bug disappeared. What does this suggest?

A The bug is probably caused by that line

B The bug is definitely somewhere else

4. Which is the best way to find a bug by commenting out?

A Comment out every line at once, then run the program

B Comment out one line, run the program, then check the result

5. You finished testing. How do you restore the line // let score = 10;?

A Add another // so it reads //// let score = 10;

B Remove the // so it reads let score = 10;

Match the Pairs

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

Left column		Right column
1// console.log(total);		Aconsole.log(total);
2/* let bonus = 5; score += bonus; */		BThis line is commented out, so JavaScript skips it and does not print total.
3To test if a line is causing a bug, switch it off by putting // in front of it.		CThe block comment is never closed. It needs */ at the end, or the rest of the program is swallowed and an error appears.
4After you finish testing, restore the code by removing the // so the line runs again.		DOnly one slash was typed. A comment needs two slashes //, so JavaScript sees a syntax error instead of a skipped line.
5/ console.log(total);		EA block comment that switches off several lines at once so none of them run.
6/* let bonus = 5; score += bonus;		F// score = score * 2;

Definitions

// console.log(total);

Fill in the Blanks

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

__ console.log(total);

let total = 0; __ total = total - 5; console.log(total);

__ console.log(a); console.log(b); */

/* total = total - 5; __ console.log(total);

let total = 4 * 5; // Line below is switched back on ___________(total);

Spot the Error

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

Question 1

A # console.log(score * 2);

B let score = 10;

C score = score + 5;

D console.log(score);

Question 2

A let total = 0;

B total = total + 5;

C / total = total * 2;

D console.log(total);

Question 3

A */

B console.log("one");

C console.log("two");

D */

E console.log("three");

Question 4

A let total = 0;

B for (let i = 1; i < 4; i++) {

C total += i;

D // }

E console.log(total);

Question 5

A let a = 3;

B console.log(a + b); //

C let b = 4;

D console.log(a * b);

Answer Key

Commenting out code to find a bug

Section A: Quick Questions

A — // console.log(total);

A — JavaScript skips the line but it stays in your file

A — The bug is probably caused by that line

B — Comment out one line, run the program, then check the result

B — Remove the // so it reads let score = 10;

Section B: Match the Pairs

// console.log(total); → This line is commented out, so JavaScript skips it and does not print total.

/* let bonus = 5; score += bonus; */ → A block comment that switches off several lines at once so none of them run.

To test if a line is causing a bug, switch it off by putting // in front of it. → // score = score * 2;

After you finish testing, restore the code by removing the // so the line runs again. → console.log(total);

/ console.log(total); → Only one slash was typed. A comment needs two slashes //, so JavaScript sees a syntax error instead of a skipped line.

/* let bonus = 5; score += bonus; → The block comment is never closed. It needs */ at the end, or the rest of the program is swallowed and an error appears.

Section C: Definitions

// console.log(total); — Putting // in front of a line makes JavaScript skip it, so you can switch code off to find a bug. Remove the // to switch it back on.

Section D: Fill in the Blanks

// console.log(total);

let total = 0;
// total = total - 5;
console.log(total);

/* console.log(a);
console.log(b); */

/* total = total - 5; */
console.log(total);

let total = 4 * 5;
// Line below is switched back on
console.log(total);

Section E: Spot the Error

A — # is not a comment in JavaScript, so this line is a syntax error. To switch a line off you put // in front of it: // console.log(score * 2);

C — A single / does not start a comment, so JavaScript tries to read it as division and gives a syntax error. A comment needs two slashes: //

A — A block comment must start with /* and end with */. Starting with */ is a syntax error, so the first line should be /*

D — Commenting out the closing brace leaves the loop open and causes a syntax error. Only comment out whole statements you want to test, and keep paired symbols like { } together. The line should be }

B — The // only switches off the text after it, so console.log(a + b); still runs and causes an error because b is not ready yet. Put // at the start of the line: // console.log(a + b);