Refactor unambiguous to track state during parsing.

packages/babylon/src/index.js

@@ -22,11 +22,12 @@ export function parse(input: string, options?: Options): File {
     options = Object.assign({}, options);
     try {
       options.sourceType = "module";
-      const ast = getParser(options, input).parse();
+      const parser = getParser(options, input);
+      const ast = parser.parse();
 
       // Rather than try to parse as a script first, we opt to parse as a module and convert back
       // to a script where possible to avoid having to do a full re-parse of the input content.
-      if (!hasModuleSyntax(ast)) ast.program.sourceType = "script";
+      if (!parser.sawUnambiguousESM) ast.program.sourceType = "script";
       return ast;
     } catch (moduleError) {
       try {
@@ -111,16 +112,3 @@ function getParserClass(
   }
   return cls;
 }
-
-function hasModuleSyntax(ast) {
-  return ast.program.body.some(
-    child =>
-      (child.type === "ImportDeclaration" &&
-        (!child.importKind || child.importKind === "value")) ||
-      (child.type === "ExportNamedDeclaration" &&
-        (!child.exportKind || child.exportKind === "value")) ||
-      (child.type === "ExportAllDeclaration" &&
-        (!child.exportKind || child.exportKind === "value")) ||
-      child.type === "ExportDefaultDeclaration",
-  );
-}

packages/babylon/src/parser/base.js

@@ -11,6 +11,7 @@ export default class BaseParser {
   inModule: boolean;
   plugins: { [key: string]: boolean };
   filename: ?string;
+  sawUnambiguousESM: boolean = false;
 
   // Initialized by Tokenizer
   state: State;

packages/babylon/src/parser/statement.js

@@ -146,8 +146,25 @@ export default class StatementParser extends ExpressionParser {
         let result;
         if (starttype == tt._import) {
           result = this.parseImport(node);
+
+          if (
+            result.type === "ImportDeclaration" &&
+            (!result.importKind || result.importKind === "value")
+          ) {
+            this.sawUnambiguousESM = true;
+          }
         } else {
           result = this.parseExport(node);
+
+          if (
+            (result.type === "ExportNamedDeclaration" &&
+              (!result.exportKind || result.exportKind === "value")) ||
+            (result.type === "ExportAllDeclaration" &&
+              (!result.exportKind || result.exportKind === "value")) ||
+            result.type === "ExportDefaultDeclaration"
+          ) {
+            this.sawUnambiguousESM = true;
+          }
         }
 
         this.assertModuleNodeAllowed(node);