# Custom serialisation

Borrowing the example from [Gson's `TypeAdapter`](https://google.github.io/gson/apidocs/com/google/gson/TypeAdapter.html), we will create a `Point` class that can revive strings like `"5,8"`. Let's see how the reviver looks like:

```javascript
const reviver = (k, v) => Point.of(...v.split(','))
```

*Note: a third `path` argument is forwarded by the built-in revivers, mostly to help display more informative error messages when necessary. `JSON.parse(..., reviver)` would not forward a path as it is not part of the native API.*

With that, we are ready to create our `Point` class:

```javascript
class Point extends M.Base {
  constructor(props) {
    super(props)

    this.x = () => props.x
    this.y = () => props.y
  }

  distanceTo(point) {
    const {x: x1, y: y1} = this
    const {x: x2, y: y2} = point

    return Math.sqrt((x2() - x1()) ** 2 + (y2() - y1()) ** 2)
  }

  toJSON() {
    return `${this.x()},${this.y()}`
  }

  static of(x, y) {
    return new Point({x, y})
  }

  static metadata() {
    return Object.freeze({type: Point, reviver})
  }
}
```

We can now use it as follows:

```javascript
const pointA = M.fromJSON(Point, '"2,3"')
const pointB = Point.of(3, 4)

pointA.distanceTo(pointB)
// => 1.4142135623730951 = Math.SQRT2 ≈ √2

JSON.stringify(pointB)
// => "3,4"
```


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