Better error message.

mbostock · web-flow · commit dad063ad1028 · 2020-03-17T10:06:19.000-07:00
diff --git a/src/generators/observe.js b/src/generators/observe.js
@@ -7,7 +7,9 @@ export default function(initialize) {
   const dispose = initialize(change);
 
   if (dispose != null && typeof dispose !== "function") {
-    throw new Error("invalid dispose");
+    throw new Error(typeof dispose.then === "function"
+        ? "async initializers are not supported"
+        : "initializer returned something, but not a dispose function");
   }
 
   function change(x) {

Original file line number	Diff line number	Diff line change
`@@ -7,7 +7,9 @@ export default function(initialize) {`
`7`	`7`	`const dispose = initialize(change);`
`8`	`8`
`9`	`9`	`if (dispose != null && typeof dispose !== "function") {`
`10`		`- throw new Error("invalid dispose");`
	`10`	`+ throw new Error(typeof dispose.then === "function"`
	`11`	`+ ? "async initializers are not supported"`
	`12`	`+ : "initializer returned something, but not a dispose function");`
`11`	`13`	`}`
`12`	`14`
`13`	`15`	`function change(x) {`