Reading an error message

You are going to see far more errors than working programs, especially at first. That is not a sign you are doing badly; it is the job. What separates people who progress quickly is that they read the message instead of panicking at the colour red.

Make a mistake on purpose. Put this in a file and run it:

console.log(greeting);

Node answers something close to this:

ReferenceError: greeting is not defined
    at Object.<anonymous> (/Users/you/js/hello.js:1:13)

Three separate pieces of information are in there, and each is useful.

• ReferenceError — the kind of problem. This one means a name was used that nothing was ever attached to.
• greeting is not defined — the specific detail. It even names the culprit.
• hello.js:1:13 — the file, the line, and the column. Line 1, character 13.

The error kinds you will meet first

• SyntaxError — the text is not valid JavaScript at all, usually an unclosed bracket or quote. Nothing ran, not even line 1.
• ReferenceError — a name was used that does not exist. Frequently a typo.
• TypeError — the value exists but cannot do what you asked, such as calling something that is not a function.

The difference between the first and the other two matters. A SyntaxError stops everything before a single line executes, so if you see output before the error, it cannot be a syntax problem.

Try these

1. Cause a SyntaxError deliberately — leave a quote unclosed. Does anything print before it?
2. Cause a TypeError by writing const x = 5; x();. What does the message say the problem is?
3. Put a console.log before and after a deliberate error. Which of the two appears?