Runtime
Runtime refers to the environment where your code actually executes, and 'runtime errors' are problems that only appear when the code runs — as opposed to errors caught by your editor or compiler beforehand. For vibe coders, runtime errors are the bugs that slip past TypeScript and linting to crash your app in action.
Example
Your code compiles without errors, but when a user submits a form with an empty field, the app crashes with 'Cannot read property of undefined.' The code was syntactically correct but failed at runtime because it didn't handle missing data.
Runtime is when theory meets reality. Your code might look perfect in the editor but only proves itself when it actually runs.
Compile Time vs Runtime
| Compile Time | Runtime |
|---|---|
| Before code runs | While code runs |
| Syntax and type errors | Logic and data errors |
| Caught by editor/compiler | Caught by testing or users |
| Easy to fix | May need debugging |
Common Runtime Errors
| Error | What It Means | Common Cause |
|---|---|---|
undefined is not a function | Calling something that doesn't exist | Typo or missing import |
Cannot read property of null | Accessing data that isn't there | Data not loaded yet |
Network error | API call failed | Wrong URL or server down |
Out of memory | Too much data processed | Infinite loop or huge dataset |
Why AI Code Gets Runtime Errors
AI generates code that's syntactically perfect but sometimes:
- Assumes data exists when it might not
- Doesn't handle network failures
- Misses edge cases in user input
- Uses APIs slightly incorrectly
Prevention Strategies
- TypeScript — Catches many potential runtime errors at compile time
- Testing — Run code with different inputs to find failures
- Error boundaries — Graceful error handling instead of crashes
- Null checks — Verify data exists before using it
- AI review — Ask AI to "find potential runtime errors in this code"
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Related Terms:
Emergent — An agentic vibe-coding platform that builds production-ready apps