Normative: use null return instead of error for un-syncable imports

README.md

@@ -44,10 +44,10 @@ We propose to expose an explicit synchronous import function for ES modules, as
 const ns = import.sync('./mod.js');
 ```
 
-The major design of the proposal is a new `Error` which is thrown when a module is not synchronously
-available, or uses top-level await.
+When a module is not synchronously available (e.g. it requires async I/O to fetch, or uses top-level
+await), `import.sync` returns `null` rather than throwing an error.
 
-Whether a module is synchronously available would otherwise be a host-determined property.
+Whether a module is synchronously available is a host-determined property.
 
 ## Use Cases
 
@@ -66,16 +66,17 @@ const app = import.sync('app');
 
 ### Conditional Loading
 
-Just like with dynamic import, with a synchronous import, it's possible to check if a module or builtin is available, but synchronously.
-
-For example, checking if host builtins are available:
+Because `import.sync` returns `null` when a module is not synchronously available, optional chaining
+can be used to conditionally access features:
 
 ```js
-let fs;
-try {
-  fs = import.sync('node:fs');
-} catch {}
+import.sync('lib')?.feature();
+```
+
+More generally, it's possible to check if a module or builtin is available synchronously:
 
+```js
+const fs = import.sync('node:fs');
 if (fs) {
   // Use node:fs, only if it is available
 }
@@ -84,11 +85,7 @@ if (fs) {
 Or a library that conditionally binds to a framework dependency:
 
 ```js
-let react;
-try {
-  react = import.sync('react');
-} catch {}
-
+const react = import.sync('react');
 if (react) {
   // Bind to the React framework, if available
 }

spec.emu

@@ -62,7 +62,7 @@ contributors: Guy Bedford
         <h1>
           <ins>ImportCallSync (
             _arguments_: a Parse Node
-          ): either a normal completion or an abrupt completion</ins>
+          ): either a normal completion containing either a Module Namespace Object or *null*, or an abrupt completion</ins>
         </h1>
         <dl class="header"></dl>
         <emu-alg>
@@ -94,10 +94,13 @@ contributors: Guy Bedford
           1. Let _moduleCompletion_ be NormalCompletion(~empty~).
           1. Perform HostLoadImportedModule(_referrer_, _moduleRequest_, ~empty~, _moduleCompletion_).
           1. NOTE: HostLoadImportedModule will update _moduleCompletion_ synchronously either updating its value to be a Module Record, or setting it to be a throw completion.
-          1. Let _module_ be ? _moduleCompletion_.
-          1. Perform ? _module_.LoadRequestedModulesSync().
-          1. Perform ? _module_.Link().
-          1. If ReadyForSyncExecution(_module_) is *false*, throw a *TypeError* exception.
+          1. If _moduleCompletion_ is a throw completion, return *null*.
+          1. Let _module_ be _moduleCompletion_.[[Value]].
+          1. Let _loadResult_ be Completion(_module_.LoadRequestedModulesSync()).
+          1. If _loadResult_ is a throw completion, return *null*.
+          1. Let _linkResult_ be Completion(_module_.Link()).
+          1. If _linkResult_ is a throw completion, return *null*.
+          1. If ReadyForSyncExecution(_module_) is *false*, return *null*.
           1. Perform ? EvaluateModuleSync(_module_).
           1. Return GetModuleNamespace(_module_, ~evaluation~).
         </emu-alg>