# unicorn/prefer-class-fields

> Require class fields instead of `this` assignments of fixed values in the constructor

A property that starts with the same value in every instance doesn't need the constructor. As a class field, it sits at the top of the class, where you look to see what an instance has. If the constructor did nothing else, you can delete it.

The rule only checks the constructor's first statement. It's reported if it assigns a string, number, `BigInt`, boolean, `null` or regular expression literal to a property of `this`.

`--fix` moves the value to a field.

{/* Generated by src/codegen/lint-rule-docs.ts. Do not edit this section. */}

## Configure

Off by default. To turn it on:

```toml bunfig.toml
[lint.rules]
"unicorn/prefer-class-fields" = "error" # or "warn"
```

{/* End of the generated section. */}

## Examples

Reported:

{/* bad */}
```js
class Counter {
  constructor() {
    this.count = 0;
  }
}
```

Not reported:

{/* good */}
```js
class Counter {
  count = 0;
}
```

A value that depends on the arguments, or that's built when the constructor runs, stays where it is:

{/* good */}
```js
class Counter {
  constructor(start) {
    this.count = start;
  }
}

class Queue {
  constructor() {
    this.items = [];
  }
}
```

An assignment that comes after another statement, such as `super()`, isn't reported:

{/* good */}
```js
class Timer extends Emitter {
  constructor() {
    super();
    this.ticks = 0;
  }
}
```

If the field already has a value, the constructor overwrites it right away, and that's reported too. `--fix` leaves this alone. `--fix-suggestions` keeps the value from the constructor.

{/* bad */}
```js
class Counter {
  count = 1;

  constructor() {
    this.count = 0;
  }
}
```