TypeError: Cannot read properties of undefined (reading 'length')
Code checked .length on something that is undefined — typically an array or string that hasn’t loaded.
Exact message you may see
reading 'length'Cannot read property 'length' of undefined
Seen on:
JavaScript
Meaning
A specific, very common case of the undefined-property error: data from an API, props or state is undefined during the first render or when a field is missing.
Common causes
- Array/string not initialised
- API field missing
- Accessing .length before async data arrives
⚡ Quick fix
- Default to empty: (items ?? []).length
- Initialise state with [] or ''
- Guard with optional chaining: items?.length
Detailed fix by platform
React
const [items, setItems] = useState([]);
How to diagnose
- Value — Which variable is undefined?
- Timing — Before data loaded?
🔧 Still not fixed?
Many errors look alike. If the steps above didn’t solve it, one of these is probably what you’re facing:
Similar errors
Most viewed in JavaScript
- Failed to fetch TypeError: Failed to fetch / NetworkError when attempting to fetch resource
- QuotaExceededError QuotaExceededError: Failed to execute 'setItem' on 'Storage': Setting the value exceeded the quota
- module MIME type Failed to load module script: Expected a JavaScript module script but the server responded with a MIME type of "text/html"
Other ways to find it
Where this error sits
Origin: On your device. Errors that are often confused with or caused by this one (click to open):
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Last updated 14 Oct 2026