Developer Errors Library — Cause, Environment & Fix

A growing library of 28 high-intent developer errors. Each entry is a reproducible symptom with a root-cause diagnosis and a concrete, ordered fix from the DevFixPro 2026 error dataset (retrieved 2026-08-29). Filter by language or framework, then open any error for the full step-by-step fix.

JavaScript

SyntaxError: Unexpected token in JSON at position N

JSON.parse() or await response.json() throws 'Unexpected token < in JSON at position 0' (or another token/position).

Cause, environments & fix →JavaScript

TypeError: Cannot read properties of undefined (reading 'x')

TypeError: Cannot read properties of undefined (reading 'x') — or 'Cannot read properties of null'.

Cause, environments & fix →

TypeScript

error TS2345: Argument of type 'X' is not assignable to parameter of type 'Y'

error TS2345: Argument of type 'X' is not assignable to parameter of type 'Y'.

Cause, environments & fix →

Node.js

Error [ERR_REQUIRE_ESM]: require() of ES Module not supported

Error [ERR_REQUIRE_ESM]: require() of ES Module '...' is not supported. Instead change the require to a dynamic import().

Cause, environments & fix →Node.js

Error: listen EADDRINUSE — address already in use

Error: listen EADDRINUSE: address already in use :::3000 (or another port).

Cause, environments & fix →

Python

ModuleNotFoundError: No module named 'x'

ModuleNotFoundError: No module named 'requests' (or another package).

Cause, environments & fix →Python

IndentationError: unexpected indent / unindent does not match

IndentationError: unexpected indent — or 'unindent does not match any outer indentation level'.

Cause, environments & fix →Python

KeyError: 'x' when accessing a dict

KeyError: 'name' (or another key) when indexing a dict.

Cause, environments & fix →Python

TypeError: unsupported operand type(s) for +: 'int' and 'str'

TypeError: unsupported operand type(s) for +: 'int' and 'str' (or a similar type combination).

Cause, environments & fix →

Go

panic: runtime error: invalid memory address or nil pointer dereference

panic: runtime error: invalid memory address or nil pointer dereference [signal SIGSEGV].

Cause, environments & fix →

Rust

error[E0502]: cannot borrow as mutable because also borrowed as immutable

error[E0502]: cannot borrow `x` as mutable because it is also borrowed as immutable.

Cause, environments & fix →

Java / JVM

Eclipse launch error: cannot connect to VM / timeout

Application launch fails with 'Could not connect to the virtual machine' or a timeout.

Cause, environments & fix →Java / JVM

Troubleshoot Kotlin compilation / unresolved reference

Unresolved reference, or compilation succeeds in IDE but fails in Gradle.

Cause, environments & fix →Java / JVM

java.lang.NullPointerException

java.lang.NullPointerException (or 'Cannot invoke "X.method()" because ... is null').

Cause, environments & fix →Java / JVM

java.lang.ClassNotFoundException / NoClassDefFoundError

java.lang.ClassNotFoundException — or NoClassDefFoundError — for com.example.Foo.

Cause, environments & fix →

Web / HTTP

Error: Babylon.js render context is not available

Engine fails to start; 'render context is not available' on mount.

Cause, environments & fix →Web / HTTP

Canva (or similar web app) keeps crashing / blank canvas

Editor tab freezes, canvas goes blank, or the tab crashes under load.

Cause, environments & fix →Web / HTTP

CORS policy blocked the cross-origin request

Access to fetch at 'https://api' from origin 'https://app' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present.

Cause, environments & fix →Web / HTTP

HTTP 404 Not Found

The server returns HTTP 404 Not Found for a request.

Cause, environments & fix →

Docker

Docker: permission denied connecting to the daemon socket

docker: permission denied while trying to connect to the Docker daemon socket at unix:///var/run/docker.sock.

Cause, environments & fix →

npm / Git

npm error code ERESOLVE — unable to resolve dependency tree

npm error code ERESOLVE / unable to resolve dependency tree / peer dependency conflict.

Cause, environments & fix →npm / Git

git push rejected — non-fast-forward / updates were rejected

failed to push some refs to '...' / Updates were rejected because the remote contains work that you do not have locally.

Cause, environments & fix →

ML / CUDA

tf.config.experimental.set_memory_growth undefined / no effect

CUDA OOM even on small batches; set_memory_growth call throws or is ignored.

Cause, environments & fix →ML / CUDA

AUTOMATIC1111 / Stable Diffusion CUDA error

CUDA out of memory or 'Torch not compiled with CUDA' at startup.

Cause, environments & fix →ML / CUDA

assertion `t >= 0 && t < n_classes` failed

Training or inference aborts with an assertion about class index bounds.

Cause, environments & fix →

CI / Build

Jenkins pipeline is unexpectedly slow

Stages take minutes longer than expected; idle gaps between steps.

Cause, environments & fix →

Shell

bash: ulimit -n: open files limit too low

EMFILE, 'too many open files', or watch/build processes crash under load.

Cause, environments & fix →

Game engine

cocos2d plist parsing / texture atlas load failure

Sprite frame not found, or plist fails to parse at load.

Cause, environments & fix →

● Data updated 2026-08-29

Sources & attribution

Error records aggregated from Google Search Console query gaps (2026-05-21~2026-08-18) for devfixpro.com, plus official framework docs. License: CC BY 4.0 — attribute DevFixPro (devfixpro.com).