# Debugging Fundamentals: Reading Errors Like a Developer

The Number One Debugging Skill: Read the Error

Most beginners see an error and immediately panic or paste the entire message into a search engine. Senior developers read the error systematically:

TypeError: Cannot read properties of undefined (reading 'map')
    at UserList (src/components/UserList.tsx:14:22)
    at renderWithHooks (node_modules/react-dom/...)
    at mountIndeterminateComponent (node_modules/react-dom/...)

How to read this: 1. Error type: TypeError — you are trying to use something that does not exist 2. Message: Cannot read properties of undefined (reading 'map') — you called .map() on something that is undefined 3. Location: UserList.tsx:14:22 — line 14, column 22 of UserList.tsx 4. Stack trace: Shows the call chain that led to the error

The fix is almost always at the first line that references YOUR code (not node_modules). In this case: line 14 of UserList.tsx. Look there, find what is undefined, and figure out why.

Common Error Categories