docs/basics/custom_serialisation.md
# 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:
```js
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:
```js
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:
```js
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"
```