Do not expect full state in the "leaves" array

If there's an update to the state then we always have a "join". If it's
just leave, then the state doesn't have to change, and we can use
previously known state.

assets/js/phoenix/presence.js

@@ -198,7 +198,7 @@ export default class Presence {
       if (changes[key]) {
         changes[key].leftMetas = leftPresence.metas;
       } else {
-        changes[key] = {joinedMetas: [], leftMetas: leftPresence.metas, update: leftPresence}
+        changes[key] = {joinedMetas: [], leftMetas: leftPresence.metas, update: {}}
       }
     })
 
@@ -207,7 +207,7 @@ export default class Presence {
       const refsToRemove = leftMetas.map(m => m.phx_ref)
       const oldPresence = state[key];
 
-      const newPresence = {metas: oldPresence ? oldPresence.metas : []};
+      const newPresence = oldPresence ? { ...oldPresence } : {metas: []}
       newPresence.metas = newPresence.metas
         .filter(m => joinedRefs.indexOf(m.phx_ref) === -1)
         .concat(joinedMetas)

assets/test/presence_test.js

@@ -15,7 +15,7 @@ let fixtures = {
     return {u1: {metas: [{id: 1, phx_ref: "1.2"}]}}
   },
   leaves(){
-    return {u2: {metas: [{id: 2, phx_ref: "2"}]}}
+    return {u2: {metas: [{phx_ref: "2"}]}}
   },
   state(){
     return {
@@ -183,7 +183,7 @@ describe("synchronizeDiff", () => {
     let state = {
       u1: {metas: [{id: 1, phx_ref: "1"}, {id: 1, phx_ref: "1.2"}]}
     }
-    Presence.synchronizeDiff(state, {joins: {}, leaves: {u1: {metas: [{id: 1, phx_ref: "1"}]}}})
+    Presence.synchronizeDiff(state, {joins: {}, leaves: {u1: {metas: [{phx_ref: "1"}]}}})
 
     assert.deepEqual(state, {
       u1: {metas: [{id: 1, phx_ref: "1.2"}]},
@@ -198,7 +198,7 @@ describe("synchronizeDiff", () => {
     }
     let update2 = {
       joins: {u1: {metas: [{id: 1, phx_ref_prev: 1, phx_ref: 1.1}]}},
-      leaves: {u1: {metas: [{id: 1, phx_ref: 1}]}}
+      leaves: {u1: {metas: [{phx_ref: 1}]}}
     }
 
     let stateAfterUpdate1 = {metas: [{id: 1, phx_ref: 1}, {id: 2, phx_ref: 2}]}
@@ -223,7 +223,7 @@ describe("synchronizeDiff", () => {
     }
     let update2 = {
       joins: {},
-      leaves: {u1: {metas: [{id: 1, phx_ref: 1}]}}
+      leaves: {u1: {metas: [{phx_ref: 1}]}}
     }
 
     let stateAfterUpdate1 = {metas: [{id: 1, phx_ref: 1}, {id: 2, phx_ref: 2}]}
@@ -248,7 +248,7 @@ describe("synchronizeDiff", () => {
     }
     let update2 = {
       joins: {u1: {foo: 'baz', metas: [{id: 1, phx_ref_prev: 1, phx_ref: 1.1}]}},
-      leaves: {u1: {foo: 'bar', metas: [{id: 1, phx_ref: 1}]}}
+      leaves: {u1: {metas: [{phx_ref: 1}]}}
     }
 
     let stateAfterUpdate1 = {foo: 'bar', metas: [{id: 1, phx_ref: 1}]}
@@ -264,6 +264,31 @@ describe("synchronizeDiff", () => {
     Presence.synchronizeDiff(state, update1)
     Presence.synchronizeDiff(state, update2, onChange)
   });
+
+  it("uses the latest known state on leave when there are no joins", done => {
+    let state = {}
+    let update1 = {
+      joins: {u1: {foo: 'bar', metas: [{id: 1, phx_ref: 1}, {id: 2, phx_ref: 2}]}},
+      leaves: {}
+    }
+    let update2 = {
+      joins: {},
+      leaves: {u1: {metas: [{phx_ref: 1}]}}
+    }
+
+    let stateAfterUpdate1 = {foo: 'bar', metas: [{id: 1, phx_ref: 1}, {id: 2, phx_ref: 2}]}
+    let expectedFinalState = {foo: 'bar', metas: [{id: 2, phx_ref: 2}]}
+
+    let onChange = (key, oldPresence, newPresence) => {
+      assert.deepEqual(key, "u1");
+      assert.deepEqual(oldPresence, stateAfterUpdate1)
+      assert.deepEqual(newPresence, expectedFinalState)
+      done();
+    };
+
+    Presence.synchronizeDiff(state, update1)
+    Presence.synchronizeDiff(state, update2, onChange)
+  });
 })
 
 describe("syncDiff", () => {
@@ -290,7 +315,7 @@ describe("syncDiff", () => {
     let state = {
       u1: {metas: [{id: 1, phx_ref: "1"}, {id: 1, phx_ref: "1.2"}]}
     }
-    state = Presence.syncDiff(state, {joins: {}, leaves: {u1: {metas: [{id: 1, phx_ref: "1"}]}}})
+    state = Presence.syncDiff(state, {joins: {}, leaves: {u1: {metas: [{phx_ref: "1"}]}}})
 
     assert.deepEqual(state, {
       u1: {metas: [{id: 1, phx_ref: "1.2"}]},