Comments and Fixing Mistakes

Notes for humans and how to read errors

5 min read · runnable Javascript

Code is read by computers, but it is also read by people, including you next week when you have forgotten what you wrote. To help, you can leave little notes called comments.

Writing comments

Anything after // on a line is ignored by the computer. It is just for humans.

// This line is a note and does nothing.
console.log("This line runs."); // notes can go at the end too

Comments are great for explaining why you did something, or for temporarily turning off a line of code without deleting it.

Everyone makes mistakes

When code has a problem, JavaScript shows an error message. This is not the computer being mean, it is trying to help you find the issue.

A very common beginner mistake is forgetting the quotes around text, or misspelling a word like console. When you see an error, read it slowly and check the line it mentions.

Tip: Errors are normal and expected. Even experts see dozens a day. The skill is not avoiding them, it is reading them calmly and fixing them one at a time.

Try it yourself

Run the code below. Try adding your own comment, and try removing the // from the third line to see what changes.

Try it yourself

// This is a comment. The computer skips it.
console.log("Comments help humans read code.");
// console.log("I am switched off for now.");

console.log("Try un-commenting the line above!");

Output

Press “Run code” to see the result.

Quick check

Q1 What happens to text that comes after // on a line?

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