optional using a validator function

[ioanmolnar]

If necessary, check changes to objects using a validator. The validator function must return the value true or false.

Example:

var test = {};
var p = ObservableSlim.create(test, true,
function(changes) {
console.log(JSON.stringify(changes));
},
function(changing) {
console.log(JSON.stringify(changing));
let result = false;
// ... check or validate
return result;
});