diff --git a/js/src/dataflow.js b/js/src/dataflow.js
new file mode 100644
index 0000000..af06642
--- /dev/null
+++ b/js/src/dataflow.js
@@ -0,0 +1,100 @@
+"use strict";
+// Property-based "dataflow"
+
+var Immutable = require("immutable");
+var MapSet = require("./mapset.js");
+
+function Graph() {
+  this.edgesForward = Immutable.Map();
+  this.edgesReverse = Immutable.Map();
+  this.damagedNodes = Immutable.Set();
+  this.currentSubjectId = null;
+  this.observablePropertyCounter = 0;
+}
+
+Graph.prototype.withSubject = function (subjectId, f) {
+  var oldSubjectId = this.currentSubjectId;
+  this.currentSubjectId = subjectId;
+  var result;
+  try {
+    result = f();
+  } catch (e) {
+    this.currentSubjectId = oldSubjectId;
+    throw e;
+  }
+  this.currentSubjectId = oldSubjectId;
+  return result;
+};
+
+Graph.prototype.recordObservation = function (objectId) {
+  if (this.currentSubjectId) {
+    this.edgesForward = MapSet.add(this.edgesForward, objectId, this.currentSubjectId);
+    this.edgesReverse = MapSet.add(this.edgesReverse, this.currentSubjectId, objectId);
+  }
+};
+
+Graph.prototype.recordDamage = function (objectId) {
+  this.damagedNodes = this.damagedNodes.add(objectId);
+};
+
+Graph.prototype.forgetSubject = function (subjectId) {
+  var self = this;
+  var subjectObjects = self.edgesReverse.get(subjectId) || Immutable.Set();
+  self.edgesReverse = self.edgesReverse.remove(subjectId);
+  subjectObjects.forEach(function (objectId) {
+    self.edgesForward = MapSet.remove(self.edgesForward, objectId, subjectId);
+  });
+};
+
+Graph.prototype.repairDamage = function (repairNode) {
+  var self = this;
+  var repairedThisRound = Immutable.Set();
+  while (true) {
+    var workSet = self.damagedNodes;
+    self.damagedNodes = Immutable.Set();
+
+    var alreadyDamaged = workSet.intersect(repairedThisRound);
+    if (!alreadyDamaged.isEmpty()) {
+      console.warn('Cyclic dependencies involving', alreadyDamaged);
+    }
+
+    workSet = workSet.subtract(repairedThisRound);
+    repairedThisRound = repairedThisRound.union(workSet);
+
+    if (workSet.isEmpty()) break;
+
+    workSet.forEach(function (objectId) {
+      var subjects = self.edgesForward.get(objectId) || Immutable.Set();
+      subjects.forEach(function (subjectId) {
+        self.forgetSubject(subjectId);
+        self.withSubject(subjectId, function () {
+          repairNode(subjectId);
+        });
+      });
+    });
+  }
+};
+
+Graph.prototype.defineObservableProperty = function (obj, prop, value, options) {
+  var graph = this;
+  var objectId = (options.baseId || prop) + '_' + (graph.observablePropertyCounter++);
+  Object.defineProperty(obj, prop, {
+    configurable: true,
+    enumerable: true,
+    get: function () {
+      graph.recordObservation(objectId);
+      return value;
+    },
+    set: function (newValue) {
+      if (!options.noopGuard || !options.noopGuard(value, newValue)) {
+        graph.recordDamage(objectId);
+        value = newValue;
+      }
+    }
+  });
+  return objectId;
+};
+
+///////////////////////////////////////////////////////////////////////////
+
+module.exports.Graph = Graph;
diff --git a/js/src/mapset.js b/js/src/mapset.js
new file mode 100644
index 0000000..fb411a7
--- /dev/null
+++ b/js/src/mapset.js
@@ -0,0 +1,26 @@
+"use strict";
+// Utilities for Maps of Sets
+
+var Immutable = require('immutable');
+
+function add(ms, key, val) {
+  return ms.set(key, (ms.get(key) || Immutable.Set()).add(val));
+}
+
+function remove(ms, key, val) {
+  var oldSet = ms.get(key);
+  if (oldSet) {
+    var newSet = oldSet.remove(val);
+    if (newSet.isEmpty()) {
+      ms = ms.remove(key);
+    } else {
+      ms = ms.set(key, newSet);
+    }
+  }
+  return ms;
+}
+
+///////////////////////////////////////////////////////////////////////////
+
+module.exports.add = add;
+module.exports.remove = remove;
diff --git a/js/test/test-dataflow.js b/js/test/test-dataflow.js
new file mode 100644
index 0000000..8cea7c2
--- /dev/null
+++ b/js/test/test-dataflow.js
@@ -0,0 +1,129 @@
+"use strict";
+
+var expect = require('expect.js');
+var Immutable = require('immutable');
+
+var Dataflow = require('../src/dataflow.js');
+
+function Cell(graph, initialValue, name) {
+  this.objectId = graph.defineObservableProperty(this, 'value', initialValue, {
+    baseId: name,
+    noopGuard: function (a, b) {
+      return a === b;
+    }
+  });
+}
+
+function DerivedCell(graph, name, valueThunk) {
+  var c = new Cell(graph, undefined, name);
+  c.refresh = function () { c.value = valueThunk(); };
+  graph.withSubject(c, function () { c.refresh(); });
+  return c;
+}
+
+function expectSetsEqual(a, bArray) {
+  return expect(Immutable.is(a, Immutable.Set(bArray))).to.be(true);
+}
+
+function checkDamagedNodes(g, expectedObjects) {
+  return expectSetsEqual(g.damagedNodes, expectedObjects);
+}
+
+describe('dataflow edges, damage and subjects', function () {
+  it('should be recorded', function () {
+    var g = new Dataflow.Graph();
+    var c = new Cell(g, 123);
+
+    g.withSubject('s', function () { c.value; });
+    g.withSubject('t', function () { c.value; });
+    g.withSubject('s', function () { c.value; });
+
+    c.value = 234;
+    expect(g.damagedNodes.size).to.equal(1);
+
+    var subjects = Immutable.Set();
+    g.repairDamage(function (subjectId) { subjects = subjects.add(subjectId); });
+    expectSetsEqual(subjects, ['s', 't']);
+  });
+});
+
+describe('DerivedCell', function () {
+  describe('simple case', function () {
+    var g = new Dataflow.Graph();
+    var c = DerivedCell(g, 'c', function () { return 123; });
+    var d = DerivedCell(g, 'd', function () { return c.value * 2; });
+    it('should be properly initialized', function () {
+      expect(c.value).to.equal(123);
+      expect(d.value).to.equal(246);
+    });
+    it('should lead initially to damaged everything', function () {
+      expect(g.damagedNodes.size).to.equal(2);
+    });
+    it('should repair idempotently after initialization', function () {
+      g.repairDamage(function (c) { c.refresh(); });
+      expect(c.value).to.equal(123);
+      expect(d.value).to.equal(246);
+    });
+    it('should be inconsistent after modification but before repair', function () {
+      c.value = 124;
+      expect(c.value).to.equal(124);
+      expect(d.value).to.equal(246);
+    });
+    it('should repair itself properly', function () {
+      g.repairDamage(function (c) { c.refresh(); });
+      expect(c.value).to.equal(124);
+      expect(d.value).to.equal(248);
+    });
+  });
+
+  describe('a more complex case', function () {
+    var g = new Dataflow.Graph();
+
+    function add(a, b) { return a + b; }
+    var xs = new Cell(g, Immutable.List.of(1, 2, 3, 4), 'xs');
+    var sum = DerivedCell(g, 'sum', function () { return xs.value.reduce(add, 0); });
+    var len = DerivedCell(g, 'len', function () { return xs.value.size; });
+    var avg = DerivedCell(g, 'avg', function () {
+      if (len.value === 0) return null;
+      return sum.value / len.value;
+    });
+    var scale = new Cell(g, 1, 'scale');
+    var ans = DerivedCell(g, 'ans', function () {
+      if (scale.value === 0) return null;
+      return typeof avg.value === 'number' && avg.value / scale.value;
+    });
+
+    function expectValues(vs) {
+      g.repairDamage(function (c) { c.refresh(); });
+      expect([xs.value.toJS(), sum.value, len.value, avg.value, scale.value, ans.value]).to.eql(vs);
+    }
+
+    it('initially', function () {
+      expectValues([ [1,2,3,4], 10, 4, 2.5, 1, 2.5 ]);
+    });
+    it('at scale zero', function () {
+      scale.value = 0;
+      expectValues([ [1,2,3,4], 10, 4, 2.5, 0, null ]);
+    });
+    it('with nine and zero', function () {
+      xs.value = xs.value.concat([9, 0]);
+      expectValues([ [1,2,3,4,9,0], 19, 6, 19/6, 0, null ]);
+    });
+    it('with five and four', function () {
+      xs.value = xs.value.skipLast(2).concat([5, 4]);
+      expectValues([ [1,2,3,4,5,4], 19, 6, 19/6, 0, null ]);
+    });
+    it('at scale one', function () {
+      scale.value = 1;
+      expectValues([ [1,2,3,4,5,4], 19, 6, 19/6, 1, 19/6 ]);
+    });
+    it('empty', function () {
+      xs.value = Immutable.List();
+      expectValues([ [], 0, 0, null, 1, false ]);
+    });
+    it('four, five, and six', function () {
+      xs.value = Immutable.List.of(4, 5, 6);
+      expectValues([ [4,5,6], 15, 3, 15/3, 1, 15/3 ]);
+    });
+  });
+});