TS2564

Property never assigned

Property 'x' has no initializer and is not definitely assigned in the constructor.

The compiler’s own words. Not translated — this is the string you pasted into a search box.

The class declares a property that is never given a value, so every instance would start out with `undefined` in a slot the type says is always a number.

Reproduction

class C {
  x: number
}

The build asserts this emits exactly this code.

Why the compiler says this

This comes from `strictPropertyInitialization`, and it closes a hole that existed for years: a declared property was trusted on its word. The compiler will accept an initializer, a definite assignment in the constructor, or an admission that the property is optional — what it will not accept is a type that claims more than the class can deliver.

Fixes

01
```
class C {
  x = 0
}
```
Initialize it inline. The type is inferred from the value, so there is nothing to keep in sync.

class C {
  x: number

  constructor(x: number) {
    this.x = x
  }
}

Or assign it in the constructor, where the compiler can see every path reaches it.

Takeaway

The `!` definite-assignment modifier also silences this, and it is a promise the compiler cannot check. Reach for it only when something outside the class genuinely does the assigning.

Errors