use function to ensure loader is registered once only

src/esmockLoader.js

@@ -218,7 +218,9 @@ export {
   load,
   resolve,
   getSource,
-  initialize,
+  // if getSource and initialize exported at the time,
+  // getSource is never called :(
+  // initialize,
   globalPreload,
   loaderIsVerified as default
 }

src/esmockRegister.js

@@ -4,17 +4,28 @@ import threads from 'node:worker_threads'
 const channel = threads.MessageChannel
   && new threads.MessageChannel()
 
-if (channel && module.register) {
-  module.register('./esmockLoader.js', {
-    parentURL: import.meta.url,
-    data: { port: channel.port2 },
-    transferList: [channel.port2]
-  })
-}
+const register = (res => () => {
+  if (typeof res === 'boolean')
+    return res
+
+  if (!(res = global.shouldRegister === true))
+    return res
+
+  if ((res = Boolean(channel && module.register))) {
+    module.register('./esmockLoader.js', {
+      parentURL: import.meta.url,
+      data: { port: channel.port2 },
+      transferList: [channel.port2]
+    })
+  }
+
+  return res
+})()
 
 export default msg => {
-  if (typeof global.postMessageEsmk === 'function')
-    global.postMessageEsmk(msg)
-  if (channel && channel.port1)
+  if (register()) {
     channel.port1.postMessage(msg)
+  } else if (typeof global.postMessageEsmk === 'function') {
+    global.postMessageEsmk(msg)
+  }
 }