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JavaScript Errors

8 errors

JavaScript runtime errors — ReferenceError, TypeError, SyntaxError, and common beginner mistakes.

8 / 8

  1. 1

    ReferenceError: x is not defined

    You referenced a variable name that JavaScript doesn't know about — it was never declared with `let`, `const`, or `var` in any accessible scope.

  2. 2

    TypeError: x is not a function

    You used parentheses to call a value, but that value is not a function — it's a number, string, object, or `undefined`.

  3. 3

    TypeError: Cannot read properties of undefined

    You used dot notation on a value that is `undefined` — the property doesn't exist because the object itself is missing.

  4. 4

    TypeError: Cannot read properties of null

    You used dot notation on a value that is `null` — `null` has no properties, so the access fails.

  5. 5

    SyntaxError: Unexpected token

    JavaScript found a symbol where it shouldn't be — usually a missing bracket, comma, or quote earlier in the code.

  6. 6

    ReferenceError: Cannot access 'x' before initialization

    You used a `let` or `const` variable before it was declared — JavaScript knows it exists (hoisting) but won't let you access it yet.

  7. 7

    NaN Result

    Your calculation resulted in `NaN` — usually because you mixed numbers with `undefined`, non-numeric strings, or other invalid values.

  8. 8

    Promise Not Awaited

    You called an async function without `await`, so you got a Promise object instead of the actual result you expected.

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