Don’t compute unreachable outputs.

Author: mbostock

src/runtime.js

@@ -112,7 +112,7 @@ function runtime_computeNow() {
       variable._indegree = 0;
       variable._outputs.forEach(variables.add, variables);
     } else {
-      variable._indegree = -1;
+      variable._indegree = NaN;
       variables.delete(variable);
     }
   });

src/variable.js

@@ -17,7 +17,7 @@ export default function Variable(type, module, observer) {
     _definition: {value: variable_undefined, writable: true},
     _duplicate: {value: undefined, writable: true},
     _duplicates: {value: undefined, writable: true},
-    _indegree: {value: -1, writable: true}, // The number of computing inputs.
+    _indegree: {value: NaN, writable: true}, // The number of computing inputs.
     _inputs: {value: [], writable: true},
     _invalidate: {value: noop, writable: true},
     _module: {value: module},