# TS2729: Property '…' is used before its initialization.

> A class field's initialiser reads another field that is initialised later. Reorder the fields, or compute the value in the constructor.

## What it means

Class fields are initialised in the order they're written. An initialiser that reads `this.x`
before `x`'s own initialiser has run sees `undefined`.

## Example

```ts
class Box {
  area = this.side * 2;
  side = 2;
}
export {};
```

## How to fix

- Declare the field that's read first.
- Or compute the value in the constructor, or make it a getter so it's read on use.

Source: https://ts-r.dev/errors/TS2729/