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This TypeError is thrown when the value after 'extends' is not a constructor function or class. JavaScript allows any expression after 'extends', so a variable holding null, undefined, or a plain object will cause this error at class definition time.
Why it happens
- •The parent class variable is null because it was not imported correctly.
- •A circular dependency causes the parent module to export undefined at the time of evaluation.
- •Extending a value returned by a factory function that returned null on failure.
- •A typo in the import name resolves to undefined.
Examples
JavaScript
Code that triggers the error
const Base = null;
class Child extends Base {
greet() { return "hi"; }
}Error output
TypeError: Class extends value null is not a constructor or null
How to fix it
Check that the value you are extending is a class or constructor function. Debug circular imports if the parent is undefined at evaluation time. Add a runtime assertion: if (typeof Base !== 'function') throw new Error('Base is not a constructor').
Fixed code
class Base {
greet() { return "hello"; }
}
class Child extends Base {
greet() { return super.greet() + " there"; }
}Related errors
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