fix(guards): union type string or enum with one value only

Author: vmasek

src/parser.ts

@@ -417,7 +417,7 @@ function parseInterfaceProperties(
                 ),
               )
               .join(' && ')})`
-          : / \| /.test(typescriptType)
+          : 'enum' in propSchema
           ? `[${(typescriptType || '').replace(
               / \| /g,
               ', ',

tests/gcloud-firestore/api/guards/index.ts

@@ -765,7 +765,7 @@ export function isValue(arg: any): arg is models.Value {
     // mapValue?: MapValue
     ( typeof arg.mapValue === 'undefined' || isMapValue(arg.mapValue) ) &&
     // nullValue?: 'NULL_VALUE'
-    ( typeof arg.nullValue === 'undefined' || is'NULL_VALUE'(arg.nullValue) ) &&
+    ( typeof arg.nullValue === 'undefined' || ['NULL_VALUE'].includes(arg.nullValue) ) &&
     // referenceValue?: string
     ( typeof arg.referenceValue === 'undefined' || typeof arg.referenceValue === 'string' ) &&
     // stringValue?: string