value.from( object, keyPath )

In the example below, a keyObservable is created that is one-way bound to the value at outer.inner.key. When outer.inner.key changes, keyObservable.value is updated, but changes to keyObservable.value do not update outer.inner.key.

import {DefineMap, value} from "can";

const outer = new DefineMap({
  inner: {
    key: "hello"
  }
});

const keyObservable = value.from(outer, "inner.key");
console.log( keyObservable.value ); //-> "hello"

try {
  keyObservable.value = "aloha";
} catch(error) {
  // Error thrown because the value isn't settable
  console.log(error.message); //-> "Cannot set property value
  // of #<Observation> which has only a getter"
}