Common bugs in functions and arrays
Name:
Date:
Class:
A
Quick Questions
Shade in the A or B circle for each question.
1. Which loop correctly visits every item in the array scores?
2. A function should give back the total of two numbers. Which version works?
3. Where should the total be set to 0 when adding up an array?
4. You want to run the function greet. Which line actually calls it?
5. An array has 5 items. What does arr[5] give?
B
Match the Pairs
Draw a line connecting each item on the left to its match on the right.
| Left column | Right column | |
|---|---|---|
| 1for (let i = 0; i <= arr.length; i++) | AThe total is set to 0 inside the loop, so it resets every pass and ends up holding only the last number. | |
| 2function add(a, b) { a + b; } | BMissing return: the sum is worked out but never sent back, so add(2, 3) gives undefined. | |
| 3A total must be set to 0 once, before the loop, so it keeps adding up across every pass. | CLoop limit bug: <= runs one step too far, so the last pass reads arr[arr.length], which is undefined. | |
| 4To run a function and get its result you must call it with brackets, even if it has no parameters. | Dconst score = getScore(); | |
| 5for (const n of nums) { let total = 0; total += n; } | EThe function is not called because the () are missing, so it prints the function itself, not its result. | |
| 6console.log(getScore); | Flet total = 0; for (const n of nums) { total += n; } |
C
Definitions
bug
D
Fill in the Blanks
Complete each line of code by filling in the missing parts.
1.
for (let i = 0; i _ scores.length; i++) {
console.log(scores[i]);
}
2.
function double(n) {
______ n * 2;
}
3.
let total = _;
for (const n of nums) { total += n; }
console.log(total);
4.
function greet() { return "Hi"; }
console.log(greet_));
5.
for (let i = 0; i __ nums.length - 1; i++) {
console.log(nums[i]);
}
E
Spot the Error
Shade in the letter of the line that contains an error.
Question 1
A
const nums = [4, 7, 9];
B
const n = nums.length;
C
for (let i = 0; i <= n; i++) {
D
console.log(nums[i]);
E
}
Question 2
A
function cube(n) {
B
const result = n * n * n;
C
console.log("Cubing...");
D
result;
E
}
F
console.log(cube(3));
Question 3
A
function sumAll() {
B
let total = 0;
C
for (const n of nums) {
D
total += n;
E
}
F
return total;
Question 4
A
const bonus = () => 10;
B
const total = 50 + bonus;
C
console.log(total);
Question 5
A
function isAdult(age) {
B
const ok = age >= 18;
C
console.log(ok);
D
}
E
console.log(isAdult(20));
Answer Key
Common bugs in functions and arrays
Section A: Quick Questions
Q1
A — for (let i = 0; i < scores.length; i++)
Q2
A — function add(a, b) { return a + b; }
Q3
A — Before the loop starts
Q4
B — greet();
Q5
B — undefined, because the last index is 4
Section B: Match the Pairs
Q1
for (let i = 0; i <= arr.length; i++) → Loop limit bug: <= runs one step too far, so the last pass reads arr[arr.length], which is undefined.
Q2
function add(a, b) { a + b; } → Missing return: the sum is worked out but never sent back, so add(2, 3) gives undefined.
Q3
A total must be set to 0 once, before the loop, so it keeps adding up across every pass. → let total = 0;
for (const n of nums) { total += n; }
Q4
To run a function and get its result you must call it with brackets, even if it has no parameters. → const score = getScore();
Q5
for (const n of nums) {
let total = 0;
total += n;
} → The total is set to 0 inside the loop, so it resets every pass and ends up holding only the last number.
Q6
console.log(getScore); → The function is not called because the () are missing, so it prints the function itself, not its result.
Section C: Definitions
Q1
bug — A mistake in code that gives wrong results. Common ones: a missing return, <= instead of < in an index loop, a total set inside the loop, or calling a function without ().Section D: Fill in the Blanks
Q1
for (let i = 0; i < scores.length; i++) {
console.log(scores[i]);
}Q2
function double(n) {
return n * 2;
}Q3
let total = 0;
for (const n of nums) { total += n; }
console.log(total);Q4
function greet() { return "Hi"; }
console.log(greet());Q5
for (let i = 0; i <= nums.length - 1; i++) {
console.log(nums[i]);
}Section E: Spot the Error
Q1
C — The last valid index is n - 1, so i <= n runs one time too many and prints undefined. The condition should be i < n.
Q2
D — The function works out the answer but never returns it, so cube(3) gives undefined. The line should be return result;
Q3
A — The parameter nums is missing, so the name nums does not exist inside the function and the loop causes a ReferenceError. It should be function sumAll(nums) {
Q4
B — Without () the function is not called. JavaScript adds the function itself to 50 instead of its result. It should be 50 + bonus();
Q5
C — console.log only shows the value. It does not send it back to the caller, so isAdult(20) is undefined. It should be return ok;