Allow optional chaining in ecmascript (#619)

Fixes #618

ecmascript/parser/src/lib.rs

@@ -137,6 +137,17 @@ impl Syntax {
         }
     }
 
+    pub fn optional_chaining(self) -> bool {
+        match self {
+            Syntax::Es(EsConfig {
+                optional_chaining: true,
+                ..
+            })
+            | Syntax::Typescript(TsConfig { .. }) => true,
+            _ => false,
+        }
+    }
+
     pub fn dynamic_import(self) -> bool {
         match self {
             Syntax::Es(EsConfig {
@@ -361,6 +372,9 @@ pub struct EsConfig {
     #[serde(default)]
     pub nullish_coalescing: bool,
 
+    #[serde(default)]
+    pub optional_chaining: bool,
+
     /// Stage 3.
     #[serde(default)]
     pub import_meta: bool,

ecmascript/parser/src/parser/expr.rs

@@ -888,7 +888,7 @@ impl<'a, I: Tokens> Parser<'a, I> {
         }
 
         let is_optional_chaining =
-            self.input.syntax().typescript() && is!('?') && peeked_is!('.') && eat!('?');
+            self.input.syntax().optional_chaining() && is!('?') && peeked_is!('.') && eat!('?');
 
         /// Wrap with optional chaining
         macro_rules! wrap {