tko.observable) Trigger immediately with current with TC39 {next}-s…

…tyle subscribe

So when a TC39 style `subscribe` call is made, we trigger with the current value immediately, contrasting with the prior behaviour - namely triggering only on changes to the state.

tc39/proposal-observable#190

CHANGELOG.md

@@ -25,6 +25,8 @@ For TODO between alpha and release, see https://github.com/knockout/tko/issues/1
 * Expose `createViewModel` on Components registered with `Component.register`
 * Changed `Component.elementName` to `Component.customElementName` and use a kebab-case version of the class name for the custom element name by default
 * Pass `{element, templateNodes}` to the `Component` constructor as the second parameter of descendants of the `Component` class
+* Add support for `<ko binding='...'>`
+* Add basic support for `ko.subscribable` as TC39-Observables
 
 ## 🚚  Alpha-4a (8 Nov 2017)
 

packages/tko.observable/spec/observableBehaviors.js

@@ -348,8 +348,18 @@ describe('Observable', function () {
     expect(myObservable.customFunction1).toBe(customFunction1)
     expect(myObservable.customFunction2).toBe(customFunction2)
   })
+
+  it('immediately emits any value when called with {next: ...}', function () {
+    const instance = observable(1)
+    let x
+    instance.subscribe({next: v => (x = v)})
+    expect(x).toEqual(1)
+    observable(2)
+    expect(x).toEqual(1)
+  })
 })
 
+
 describe('unwrap', function () {
   it('Should return the supplied value for non-observables', function () {
     var someObject = { abc: 123 }

packages/tko.observable/src/observable.js

@@ -58,9 +58,16 @@ observable.fn = {
   valueWillMutate () {
     this.notifySubscribers(this[observableLatestValue], 'beforeChange')
   },
-  // subscribe (callback, callbackTarget, event) {
+
+  // Note that TC39 `subscribe` ought to immediately emit.
   // https://github.com/tc39/proposal-observable/issues/190
-  // }
+  subscribe (callback, callbackTarget, event) {
+    const isTC39Callback = typeof callback === 'object' && callback.next
+    if (isTC39Callback) {
+      callback.next(this())
+    }
+    return subscribable.fn.subscribe.call(this, callback, callbackTarget, event)
+  },
 
   // Some observables may not always be writeable, notably computeds.
   isWriteable: true