Reading an error message and its line number

Name:

Date:

Class:

Quick Questions

Shade in the A or B circle for each question.

1. An error message says 'script.js:5'. What does the 5 tell you?

A The program has exactly 5 errors in it

B JavaScript noticed the error on line 5, so check there first

2. Which part of an error message names the kind of problem, such as SyntaxError or ReferenceError?

A The line number, at the end of the file name

B The error name, at the start of the message

3. The console shows 'script.js:4 Uncaught SyntaxError: Unexpected token'. Where should you start looking?

A Line 4, and then the line above it

B Line 1, because errors always start there

4. Line 2 is: let name = "Sam and line 3 is: console.log(name);. The error is reported on line 3. Which is true?

A The real mistake may be on line 2, the missing closing quote

B The real mistake must be on line 3, because that is where the error was reported

5. An error message says 'ReferenceError: score is not defined'. What does the description tell you?

A JavaScript could not find a variable called score

B The variable score has the wrong data type

Match the Pairs

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

Left column		Right column
1script.js:3		AThe closing } is missing. JavaScript reports the error at the end of the file (Unexpected end of input), not on line 1 where the function started.
2ReferenceError: score is not defined		BThe file name and line number where JavaScript noticed the error. Check line 3 first, but the real mistake may be a line earlier.
3An error message names the type of error and describes what went wrong		CTypeError: Assignment to constant variable.
4The real mistake may be a line earlier than the line number in the error message		DThe message says ReferenceError: agee is not defined at line 2. The variable name is misspelled and should be age.
5let age = 12; console.log(agee);		Elet nums = [1, 2 console.log(nums);
6function hi() { console.log("Hi"); hi();		FThe error name and description: JavaScript cannot find anything called score, so check for a typo or a missing let.

Definitions

script.js:3

Fill in the Blanks

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

console.log(scoree); // ______________: scoree is not defined

let age = 12; age = ; // Error is reported at script.js:_

const max = 5; max = 6; // _________ : Assignment to constant variable.

// Error at script.js:5 // Check line 5 first, then line _

console.log(x); // _________:1 ReferenceError

Spot the Error

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

Question 1

A let score = 10;

B let bonus = 5;

C console.log(score + bonus;

Question 2

A const name = "Mia";

B console.log("Hi + name);

C console.log(name.length);

Question 3

A let total = 0;

B total = total + 5;

C console.log(totl);

Question 4

A const lives = 3;

B let score = 0;

C score += 10;

D lives = lives - 1;

Question 5

A let items = ["pen", "ruler";

B console.log(items[0]);

C console.log(items.length);

Answer Key

Reading an error message and its line number

Section A: Quick Questions

B — JavaScript noticed the error on line 5, so check there first

B — The error name, at the start of the message

A — Line 4, and then the line above it

A — The real mistake may be on line 2, the missing closing quote

A — JavaScript could not find a variable called score

Section B: Match the Pairs

script.js:3 → The file name and line number where JavaScript noticed the error. Check line 3 first, but the real mistake may be a line earlier.

ReferenceError: score is not defined → The error name and description: JavaScript cannot find anything called score, so check for a typo or a missing let.

An error message names the type of error and describes what went wrong → TypeError: Assignment to constant variable.

The real mistake may be a line earlier than the line number in the error message → let nums = [1, 2 console.log(nums);

let age = 12; console.log(agee); → The message says ReferenceError: agee is not defined at line 2. The variable name is misspelled and should be age.

function hi() { console.log("Hi"); hi(); → The closing } is missing. JavaScript reports the error at the end of the file (Unexpected end of input), not on line 1 where the function started.

Section C: Definitions

script.js:3 — Part of an error message: the file and line number where JavaScript noticed the error. Check that line first, but the real mistake may be a line earlier.

Section D: Fill in the Blanks

console.log(scoree);
// ReferenceError: scoree is not defined

let age = 12;
age = ;
// Error is reported at script.js:2

const max = 5;
max = 6;
// TypeError
: Assignment to constant variable.

// Error at script.js:5
// Check line 5 first, then line 4

console.log(x);
// script.js:1 ReferenceError

Section E: Spot the Error

C — The closing bracket is missing, so JavaScript reports a SyntaxError and gives line 3 as the place to look. It should be console.log(score + bonus);

B — The closing quote after Hi is missing, so the string never ends. The SyntaxError (Invalid or unexpected token) points to line 2. It should be console.log("Hi " + name);

C — The variable name is misspelt, so JavaScript throws a ReferenceError: totl is not defined, and names line 3. It should be console.log(total);

D — lives is a const, so it cannot be reassigned. JavaScript shows TypeError: Assignment to constant variable on line 4. It should create a new variable: let left = lives - 1;

A — The closing square bracket is missing on line 1. The error message may point at the end of this line or even a line later, so check the line before it too. It should be let items = ["pen", "ruler"];