The Common Error Families

You're not facing infinite different errors - just a small number of families, dressed in different words each time. Recognize the family from the type and message, and you already know roughly what went wrong and where to look, like a doctor hearing "sharp pain when you breathe in" pointing to a category before the exact diagnosis.

The first fork: syntax errors vs runtime errors

One big split tells you when things went wrong, which changes how you hunt.

Syntax errors - "I can't even read this." You wrote something the language cannot parse - a missing bracket, a stray comma, a misspelled keyword. The grammar didn't make sense, so it gave up before running a single line. Nothing runs. The friendly kind of failure: caught instantly, almost always a small typo near the spot it names.

$ python app.py
  File "app.py", line 4
    if user == "admin"
                      ^
SyntaxError: expected ':'

What just happened: Python expected a colon after the if condition, didn't find one, and refused to go further - ^ points at where it gave up. Fix: add the : - if user == "admin":. It never ran your program; it couldn't get past reading it.

Runtime errors - "I read it fine, but then something went wrong while doing it." Grammar valid, so it started executing - then hit something impossible partway through: a missing value, a missing file, a number divided by zero. It starts, does some work, then crashes at the bad moment; the output before the crash shows how far it got.

$ python app.py
Starting up...
Traceback (most recent call last):
  File "app.py", line 8, in <module>
    average = total / count
              ~~~~~~^~~~~~~
ZeroDivisionError: division by zero

What just happened: The code ran fine, even printing Starting up..., until line 8 divided by count, which was 0 this time - and dividing by zero is undefined. The crash is about the data it met along the way, not the grammar.

💡 Key point. Syntax error = couldn't read your code (nothing ran; look for a typo near the named line). Runtime error = read fine, went wrong while running (look at the data and conditions at that line). This tells you whether you're hunting a typo or a bad value.

The runtime families you'll meet most.

Family 1: null / undefined - "nothing where I expected something"

The most common runtime error in the world. You asked for something - a property, a value, an item - and what was actually there was nothing: null, None, undefined, nil, depending on the language. You used that nothing as if it were real: expected a user object, but the variable held nothing, and reaching for user.name crashed because nothing has no .name.

TypeError: Cannot read properties of null (reading 'name')

AttributeError: 'NoneType' object has no attribute 'name'

What just happened: Both lines (JavaScript, then Python) say the same thing: the value you reached into was empty. The fix is rarely at the crash line - it's upstream: figure out why the value was empty (a failed lookup? a function returning nothing? data that never arrived?).

⚠️ The crash location is the symptom, not the cause. Null/undefined errors point at where you used the emptiness, but the real bug is wherever the value became empty. Ask "why was this nothing?" rather than just guarding the crash line.

Family 2: type mismatches - "that's the wrong kind of thing"

You gave an operation a value of a type it can't work with: adding a number to text, calling something that isn't a function, indexing into something that isn't a list. Often the data arrived in an unexpected shape - like a number that came in as text from a form or file.

$ node total.js
TypeError: amount.toFixed is not a function

What just happened: .toFixed() is a method numbers have, but amount was probably a string like "19.99", and strings don't have .toFixed. is not a function is the classic tell you called a method on a value that doesn't have it. Fix: convert the value to the right type first.

📝 Terminology. A type is the kind of value something is - number, string, list, boolean. Type mismatches are the computer enforcing that you don't accidentally treat one kind as another.

Family 3: not-found - "I looked for it and it isn't there"

You referred to something by name or path, and it doesn't exist where you pointed. "Something" can be a file, a module/package, a key, a variable, a column - anything addressed by name. Usually the cause is one of three things: a typo, a wrong path or working directory, or a thing you forgot to create/install.

$ python report.py
FileNotFoundError: [Errno 2] No such file or directory: 'data/sales.csv'

What just happened: The program tried to open data/sales.csv and there's no such file at the place it looked. Often the file exists but you're running from a different folder, so the relative path points somewhere empty. First check: does the file exist, and are you in the directory you expect? (ls / dir shows what's actually around you.)

Same family, different costume - a missing package:

$ python app.py
ModuleNotFoundError: No module named 'requests'

What just happened: Your code said import requests, but the package isn't installed. Same family, different fix: pip install requests. One more flavor, a missing key:

KeyError: 'email'

What just happened: You asked a dictionary for key 'email', and there's no such key - maybe the data lacked an email, or it's spelled 'e-mail' or 'Email'. Same family: named thing, not present.

⚠️ "Not found" almost always means one of: typo, wrong location, or not-yet-created. Run through those three before assuming anything deeper is wrong - nine times out of ten it's one of them.

Family 4: permission denied - "you're not allowed to do that"

The thing exists and your request makes sense - but the OS (or a service) refused to let you do it: write to that folder, open that file, bind to that port, access that resource. Common on shared machines, system folders, protected files, or reserved ports.

$ ./deploy.sh
bash: ./deploy.sh: Permission denied

What just happened: deploy.sh is there and the path is right - but it isn't marked executable, so it won't run. A permissions problem, not a not-found one. (Fix: chmod +x deploy.sh - the real skill is knowing "Permission denied" is a rights issue, not a typo.)

💡 Key point. "Permission denied" differs fundamentally from "not found." Not-found means the thing isn't there; permission-denied means it's there but you can't touch it that way. Knowing which one you got saves you hunting for a missing file when the real issue is access.

How the families connect to the anatomy

The error type (from Phase 1) told you the family; the message told you the specifics: FileNotFoundError → not-found → "which file? where am I running from?" TypeError: ... not a function → type-mismatch → "what type is this really?" Type names the family, message names the case.

Recap

1. First fork: syntax error (couldn't read your code - nothing ran, look for a typo) vs runtime error (ran fine, then hit bad data or a missing thing).
2. Null/undefined - nothing where you expected something; the cause is upstream of the crash line.
3. Type mismatch - the wrong kind of value for the operation; usually data arrived in an unexpected shape.
4. Not-found - a named thing (file, module, key, variable) isn't there; check typo, location, not-yet-created.
5. Permission denied - the thing exists but you're not allowed; an access problem, not a missing one.
6. The type names the family; the message names the case. Recognize the family first.

Now you can read an error and recognize its anatomy and its family. The last phase covers the calm, repeatable method for resolving one.

← Phase 1: An Error Is Information · Phase 3: What to Actually Do With One →