JavaScript lesson 2: Errors and comments
US version. US grades and US spelling. · UK version of this lesson
Errors are normal – every programmer gets them, every day. The message names the kind of error, and that is your first clue. A complete 40-minute JavaScript lesson for beginners (Grades 7–8), from Unit 1: Getting started. It runs right here in your browser – nothing to install.
- Time
- 40 minutes + 20 min homework
- Grade
- Grades 7–8 (ages 12–13)
- You need
- A computer or tablet, paper and a pencil
- Graded
- Instantly, with explanations
For parents: how to use this lesson
You don't need to know JavaScript yourself. The lesson explains each step on screen, the way Mr Babbage, our Head of Computing, likes it: a worked example first, then a go for your child. Code questions run your child's program in the browser – graded straight away.
- Read what your child will learn together.
- Press Start the lesson below. Sit with your child for the teaching and the worked example (about 16 minutes): ask them to say what each line does.
- Let them do the practice on their own. When code goes wrong, the error message is a clue – let them read it out and try again before you help.
- Finish with the talk question: Which error message have you met most today? What usually causes it?
Watch out for
- The program runs up to the line with a SyntaxError. A SyntaxError stops everything before the first line runs.
Console.logis fine. Capital letters matter: onlyconsoleworks.
Miss Lovelace's tip for home: ask your child to explain one line of their program to you in their own words. If they can explain it, they understand it – and every error message is a clue, not a failure.
What your child will learn
Learning goal: read an error message and name the kind of error.
Success looks like
- I can read an error message and name the kind of error.
- I can tell a SyntaxError from a ReferenceError and a TypeError.
- I can write comments with
//and comment a line out.
Key words
bug · debugging · SyntaxError · ReferenceError · TypeError · comment
What to have ready
- A device with a web browser (the code runs in the browser – nothing to install)
- Paper for tracing
- A keyboard helps for typing code (a tablet or phone works too)
The big ideas (for the adult)
Errors are normal – every programmer gets them, every day. The message names the kind of error, and that is your first clue.
A SyntaxError means JavaScript can't read the code (a missing quote mark or bracket), so NOTHING runs. A ReferenceError means it doesn't know a name – often a spelling or capital-letter slip. A TypeError means a value was used the wrong way.
A comment starts with //. JavaScript ignores the rest of that line, so comments are notes for people. Putting // in front of a line "comments it out" without deleting it.
Worked example – see it
// console.log("guitar")
console.log("9" + 8)It prints:
98
The lesson, step by step
| Part | Time | How it is checked |
|---|---|---|
| Starter | 5 min | Graded by Classfern |
| Teaching input | 8 min | Read, watch and talk |
| See it, then predict | 8 min | Graded by Classfern |
| Put it in order, write it, fix it | 14 min | Graded by Classfern |
| Closure and exit ticket | 5 min | Graded by Classfern + self-check |
The lesson
It runs here on the page. No sign-up and no names – progress is kept only in this browser. Each visit picks a new mix of questions.
Homework
20 minutes: 7 questions – predict what code prints, write or fix a short program, and check the key ideas – then a short reflection. New questions every time, all graded instantly.
Practice tests
Curriculum
CSTA K–12 Computer Science Standards (2017), grades 6–8: 2-AP-17 – Systematically test and refine programs using a range of test cases; 2-AP-19 – Document programs so they are easier to follow, test and debug.
The standard codes are CSTA's; the descriptions are our own words.
Our lessons and questions are our own. They are not written or endorsed by any government department, testing agency or exam board. JavaScript is a trademark or registered trademark of Oracle Corporation in the U.S. and other countries; Classfern is not affiliated with or endorsed by it.