JavaScript Errors Explained

14 errors, each with the code that makes it, the exact output and the fix.

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JS TypeError: Cannot read properties of undefinedUncaught TypeError: Cannot read properties of undefined (reading 'name')
JS ReferenceError: variable is not definedUncaught ReferenceError: totl is not defined
JS TypeError: x is not a functionUncaught TypeError: numbers.push2 is not a function
JS TypeError: Assignment to constant variableUncaught TypeError: Assignment to constant variable.
JS TypeError: Cannot set properties of nullUncaught TypeError: Cannot set properties of null (setting 'textContent')
JS SyntaxError: Unexpected token in JSONUncaught SyntaxError: Unexpected token '<', "<!DOCTYPE "... is not valid JSON
JS RangeError: Maximum call stack size exceededUncaught RangeError: Maximum call stack size exceeded
JS fetch blocked by CORS policyAccess to fetch at 'http://127.0.0.1:9282/data.json' from origin 'http://127.0.0.1:9281' has been bl
JS TypeError: Failed to fetchUncaught (in promise) TypeError: Failed to fetch
JS SyntaxError: Cannot use import outside a moduleUncaught SyntaxError: Cannot use import statement outside a module
JS SyntaxError: Unexpected end of inputUncaught SyntaxError: Unexpected end of input
JS SyntaxError: Unexpected identifierUncaught SyntaxError: Unexpected identifier 'b'
JS TypeError: value is not iterableUncaught TypeError: getPair is not a function or its return value is not iterable
JS RangeError: Invalid array lengthUncaught RangeError: Invalid array length

JavaScript errors appear in the browser console, and the words depend on the browser. The pages below use the messages of Google Chrome (the same words are used by Microsoft Edge and Node.js); Firefox and Safari word several of them differently, and the paste box on the home page knows some of those wordings. Each example was run as a real web page in Chrome, and the message shown is the one that Chrome printed.

Start with how to read a JavaScript error in the console if you have not used the developer tools before.

This is an independent guide. It is not connected with Python, Microsoft, Google, GitHub or the Git project. Product names belong to their owners. Examples were run on 13 November 2026 in a practice folder or a blank workbook; your version may word a message slightly differently.