Note method precondition

Author: kateinoigakukun

Sources/JavaScriptKit/JSValue.swift

@@ -82,17 +82,20 @@ public enum JSValue: Equatable {
 
 extension JSValue {
     /// An unsafe convenience method of `JSObject.subscript(_ name: String) -> ((ConvertibleToJSValue...) -> JSValue)?`
+    /// - Precondition: `self` must be a JavaScript Object and specified member should be a callable object.
     public subscript(dynamicMember name: String) -> ((ConvertibleToJSValue...) -> JSValue) {
         object![dynamicMember: name]!
     }
 
     /// An unsafe convenience method of `JSObject.subscript(_ index: Int) -> JSValue`
+    /// - Precondition: `self` must be a JavaScript Object.
     public subscript(dynamicMember name: String) -> JSValue {
         get { self.object![name] }
         set { self.object![name] = newValue }
     }
 
     /// An unsafe convenience method of `JSObject.subscript(_ index: Int) -> JSValue`
+    /// - Precondition: `self` must be a JavaScript Object.
     public subscript(_ index: Int) -> JSValue {
         get { object![index] }
         set { object![index] = newValue }