Errors, throw, and try/catch

lesson 8-3 · ~11 min · 27/29

Reading an error like a pro

Every JavaScript error message has the same anatomy, and reading it calmly is half of debugging:

TypeError: Cannot read properties of undefined (reading 'name')
    at main.js:3

The name (TypeError) says what category went wrong.
The message says exactly what happened: something was undefined and you asked for its .name.
The stack trace lines point at the file and line number. Start at the top one.

The three you will meet most:

Error	Usual meaning
`ReferenceError: x is not defined`	typo in a name, or the variable is out of scope (lesson 6-3)
`TypeError: ... of undefined`	a lookup returned undefined and you kept going
`SyntaxError`	the code itself is malformed, often a missing brace or quote

The anatomy of a JavaScript error: category, explanation, and the exact line to look at.

throw and try/catch

Your own code can refuse bad input by throwing an error, and callers can recover with try/catch. This is Python's raise and try/except mechanism with different keywords — raise becomes throw, except becomes catch — and the control flow is identical:

function divide(a, b) {
  if (b === 0) {
    throw new Error("Cannot divide by zero");
  }
  return a / b;
}

A throw stops the function on the spot. If nothing catches it, the whole program crashes with the message you wrote. A try block volunteers to handle trouble: when anything inside throws, execution jumps straight to catch, receives the error object, and the program lives on. err.message holds the text you passed to new Error(...).

Code exercise · javascript

Run it. The second divide call throws, so the line after it inside try never runs. catch receives the error and the program continues normally.

Code exercise · javascript

Your turn. JSON.parse (lesson 5-3) throws on malformed text. Wrap the parse in try/catch: on success print the parsed object's name, on failure print Invalid JSON.

Code exercise · javascript

Second practice. Write checkAge(age) that throws new Error("Invalid age") when age is NaN or negative, and returns it otherwise. The try block calls it with Number("twenty") — which is NaN (lesson 8-2) — so catch should fire. The program must print Invalid age, then Still running.

Quiz

Your program prints: ReferenceError: userName is not defined. What is the most likely cause?