I've seen this happening when a hook was using after a conditional statement. Moving the hook usage above the conditional statement fixed the problem.

Reference: https://reactjs.org/docs/hooks-rules.html

Answer from fpintos on Stack Overflow
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GitHub
github.com › facebook › react › issues › 18372
Bug: Combination of standard hooks causes "Cannot read property 'length' of undefined" in areHookInputsEqual · Issue #18372 · facebook/react
March 23, 2020 - Found a combination of fairly trivial hooks that causes "Cannot read property 'length' of undefined" in areHookInputsEqual" error. React version: 16.12 Steps To Reproduce Ope...
Author: facebook
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prevDep.length undefined Cannot read properties of undefined (reading 'length')
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April 27, 2024
React - Cannot read property 'length' of undefined
Hi guys! So I’ve got a Contact form component with multiple Input components. When I run the app and type anything into the FormInput components, I get the following error… TypeError: Cannot read property ‘length’ of undefined. I cannot see why this would be undefined? More on forum.freecodecamp.org
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December 9, 2020
`@preact/signals-react` 1.3.6 is brokes useSyncExternalStoreWithSelector
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September 10, 2023
[Compiler Bug]: TypeError: Cannot read properties of undefined (reading 'length')
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GitHub
github.com › facebook › react › issues › 22049
Bug: "Cannot read property 'length' of undefined" in areHookInputsEqual · Issue #22049 · facebook/react
August 9, 2021 - React version: 17.0.2 Steps To Reproduce When testing my component where I'm using this usecallback hook. I'm getting the error. const getUrlWithPrefix = useCallback( (href: string) => { let url = href; // logic return url; }, [prefix], ...
Author: facebook
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Tabnine
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Code Library - Tabnine
July 25, 2024 - Get the answers and suggestions you need from our AI code assistant. Get started in minutes with a free 90 day trial of Tabnine Pro.
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freeCodeCamp
forum.freecodecamp.org › curriculum help
React - Cannot read property 'length' of undefined - Curriculum Help - The freeCodeCamp Forum
December 9, 2020 - Hi guys! So I’ve got a Contact form component with multiple Input components. When I run the app and type anything into the FormInput components, I get the following error… TypeError: Cannot read property ‘length’ of undefined. I cannot see why this would be undefined?
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GitHub
github.com › preactjs › signals › issues › 411
`@preact/signals-react` 1.3.6 is brokes useSyncExternalStoreWithSelector · Issue #411 · preactjs/signals
September 10, 2023 - react-dom.development.js:16249 Uncaught TypeError: Cannot read properties of undefined (reading 'length') at areHookInputsEqual (react-dom.development.js:16249:38) at updateMemo (react-dom.development.js:17240:11) at Object.useMemo (react-dom.development.js:17886:16) at useMemo (react.development.js:1650:21) at useSyncExternalStoreWithSelector (with-selector.development.js:64:18) at useStore (index.tsx:17:17) at useRouterState (react.tsx:568:10) at useMatch (react.tsx:673:24) at useSearch (react.tsx:802:10) at _Task (Task.tsx:23:57) areHookInputsEqual @ react-dom.development.js:16249 updateMem
Author: preactjs
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GitHub
github.com › react › react › issues › 31205
[Compiler Bug]: TypeError: Cannot read properties of undefined (reading 'length') · Issue #31205 · react/react
October 11, 2024 - What kind of issue is this? React Compiler core (the JS output is incorrect, or your app works incorrectly after optimization) babel-plugin-react-compiler (build issue installing or using the Babel plugin) eslint-plugin-react-compiler (b...
Author: react
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Rollbar
rollbar.com › home › how to fix typeerror: cannot read property length of undefined in javascript
How to Fix TypeError: Cannot Read Property Length of Undefined in JavaScript
Verify that nested properties exist ... TypeError: Cannot read property 'length' of undefined happens when you try to access the length property on something that doesn't exist....
Published: July 13, 2025
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TrackJS
trackjs.com › javascript answers › how to fix `cannot read properties of undefined (reading 'length')`
How to fix `Cannot read properties of undefined (reading 'length')` • TrackJS
April 8, 2026 - Common JavaScript error when accessing length property on undefined values. Usually occurs with arrays, strings, or API responses that haven't loaded yet. Quick fixes: add null checks, use optional chaining, handle loading states.
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Reddit
reddit.com › r/learnjavascript › [deleted by user]
[deleted by user] : r/learnjavascript
July 15, 2022 - You’re getting this error when calling the length property on something. That means that something you have use .length on is undefined at some point.
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Stack Overflow
stackoverflow.com › questions › 78264410 › javascript-cannot-read-properties-of-undefined-reading-length-code-does
typeerror - JavaScript - Cannot read properties of undefined (reading 'length') - (Code does work and provided) - Stack Overflow
Typing in 100.1 will show 1 However, when you first click on the field, you will get the Uncaught TypeError. Cannot read properties of undefined (reading 'length') The code is originally written with a hardcoded value.
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Stack Overflow
stackoverflow.com › questions › 75408156 › getting-uncaught-typeerror-cannot-read-properties-of-undefined-reading-length
Getting Uncaught TypeError: cannot read properties of undefined (reading 'length')
I'm getting this error: Here's my code: const Search = () => { const [searchBox, setSearchBox] = useState(false) const [searchIcon, setSearchIcon] = useState(true) const {searchText,
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GitHub
github.com › davidkpiano › react-redux-form › issues › 1101
Uncaught TypeError: Cannot read property 'length' of undefined · Issue #1101 · davidkpiano/react-redux-form
March 5, 2018 - The Problem In the console, I am getting 'Uncaught TypeError: Cannot read property 'length' of undefined' for the line specified below. Steps to Reproduce render(){ const validateEmail = ({email}) => { var VALID_EMAIL_REGEX = /\A[\w+\-.]...
Author: davidkpiano
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Dynamics Community
community.dynamics.com › forums › thread › details
Uncaught TypeError: Cannot read property 'length' of undefined
on the Contact Form, I sometimes get the following error when leaving/reloading it: · This happens quite a lot to me. Is this a CRM bug or is some of my custom code causing a problem here