Don't use var

Example form the interpolate method:

```typescript
interpolate(scroll: number, to:Waypoint) {
var totalDelta = to.scroll - this.scroll;
var amount = totalDelta == 0 ? 0 : Math.max(0.0, Math.min(1.0,(scroll - this.scroll) / totalDelta));
var result : any = {};
for (let key in this.style) {
const fromValue = this.style[key];
var toValue = to.style[key];
if(toValue == undefined) {
toValue = fromValue;
}
var valueDelta = toValue - fromValue;
var current = (fromValue == toValue) ? fromValue : (fromValue + amount * valueDelta);
result[key] = spring(current);
}
return result;
}
```

Don't use var. A quick reminder on the best practice about this topic:
- Always declare you variable as **const** first (immutable).
- Switch **const** to **let** if a mutation is needed.
- Never use **var** to ensure you will never have context problems (or just for an exception).