JavaScript error“TypeError: Class constructor X cannot be invoked without 'new'”

Why the class was called without new, and the fix

1. 01

   Calling a class like a function

   “All class constructors must be called with new.” MDN, TypeError: class constructors must be invoked with 'new'

   A class can only be constructed. Add new where it is created.

   // wrong
   const user = User(data);
   
   // right
   const user = new User(data);

2. 02

   A class passed where a function is expected

   “However, class constructors are always constructors and cannot be called as normal functions.” MDN, TypeError: class constructors must be invoked with 'new'

   rows.map(Row), a callback, or a factory option calls the class without new. Wrap it in a function that constructs it.

   // wrong
   const items = rows.map(Row);
   
   // right
   const items = rows.map((r) => new Row(r));

3. 03

   TypeScript compiling to ES5 and extending a real class

   “Modern browsers support all ES6 features, so ES6 is a good choice.” TypeScript, TSConfig: target

   With target ES5, class Child extends Base becomes a function that calls Base.call(this). If Base comes from a package that ships real ES2015+ classes, that call throws. Raise the target so your classes stay classes. Current Babel handles this case; a very old Babel (6) or a custom ES5 transform can fail the same way, so update it and drop IE-era targets.

   // tsconfig.json
   {
     "compilerOptions": {
       "target": "ES2017"
     }
   }

4. 04

   Old-style inheritance with Parent.call(this)

   Pre-class code inherits with Parent.call(this, ...) and Object.create. Once Parent is rewritten as a class, that call throws. Convert the child to class Child extends Parent with super().

   class Child extends Parent {
     constructor(name) {
       super(name);
     }
   }

Checked against the vendor’s docs on November 8, 2026: MDN: TypeError: class constructors must be invoked with 'new'; TypeScript: TSConfig target.

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Related: “x is not a constructor” · JavaScript “x is not a function” · TypeScript “Type is not assignable to type”