simplecell.js

const depGraph = new Map(),
      suspended = new Set();
let inTransaction = false;

function addEdge(g, from, to) {
  let s = g.has(from) ? g.get(from) : new Set();
  s.add(to);
  g.set(from, s);
}

function propagate(g, from, walked = new Set()) {
  if (!g.has(from)) return;
  g.get(from).forEach(to => {
    if (!walked.has(to)) {
      walked.add(to);
      if (to.update()) {
        propagate(g, to, walked);
      }
    }
  });
}

function transaction(thunk) {
  if (inTransaction) {
    thunk();
  } else {
    try {
      inTransaction = true;
      thunk();
    } finally {
      inTransaction = false;
      let walked = new Set();
      suspended.forEach(s => propagate(depGraph, s, walked));
      suspended.clear();
    }
  }
}

class Input {
  constructor(value) {
    this.value = this.previousValue = value;
  }
  set(value) {
    if (this.value !== value) {
      this.previousValue = this.value;
      this.value = value;
      if (inTransaction) {
        suspended.add(this);
      } else {
        propagate(depGraph, this);
      }
    }
    return value;
  }
}

class Formula {
  constructor(f, sources) {
    this.f = f;
    this.sources = sources;
    sources
      .filter(Formula.isReactiveSource)
      .forEach(source => addEdge(depGraph, source, this));
    this.value = this.previousValue = f.apply(null, this.sourceValues());
  }
  static isReactiveSource(x) {
    return x instanceof Input || x instanceof Formula;
  }
  sourceValues() {
    return this.sources.map(x => Formula.isReactiveSource(x) ? x.value : x);
  }
  update() {
    let value = this.f.apply(null, this.sourceValues());
    if (this.value !== value) {
      this.previousValue = this.value;
      this.value = value;
      return true;
    }
    return false;
  }
}

function input(initialValue) {
  return new Input(initialValue);
}

function formula(f) {
  return (...sources) => new Formula(f, sources);
}

let A = input(100),
    B = input(200),
    C = formula(console.log)(A, B);

console.log("Setting A and B in succession")
A.set(A.value + 1);
B.set(B.value + 1);

setTimeout(() => transaction(() => {
  console.log("Setting A and B in a transaction")
  A.set(A.value + 1);
  B.set(B.value + 1);
}), 1000);