fix: avoid schema field mutation when passing selectedFields opt (#1437)

Author: alvarowolfx

src/bigquery.ts

@@ -588,14 +588,16 @@ export class BigQuery extends Service {
       parseJSON?: boolean;
     }
   ) {
+    // copy schema fields to avoid mutation when filtering selected fields
+    let schemaFields = schema.fields ? [...schema.fields] : [];
     if (options.selectedFields && options.selectedFields!.length > 0) {
       const selectedFieldsArray = options.selectedFields!.map(c => {
         return c.split('.');
       });
 
       const currentFields = selectedFieldsArray.map(c => c.shift());
       //filter schema fields based on selected fields.
-      schema.fields = schema.fields?.filter(
+      schemaFields = schemaFields.filter(
         field =>
           currentFields
             .map(c => c!.toLowerCase())
@@ -610,7 +612,7 @@ export class BigQuery extends Service {
 
     function mergeSchema(row: TableRow) {
       return row.f!.map((field: TableRowField, index: number) => {
-        const schemaField = schema.fields![index];
+        const schemaField = schemaFields[index];
         let value = field.v;
         if (schemaField.mode === 'REPEATED') {
           value = (value as TableRowField[]).map(val => {

src/table.ts

@@ -1830,6 +1830,10 @@ class Table extends ServiceObject {
     delete options.wrapIntegers;
     const parseJSON = options.parseJSON ? options.parseJSON : false;
     delete options.parseJSON;
+    const selectedFields = options.selectedFields
+      ? options.selectedFields.split(',')
+      : [];
+    delete options.selectedFields;
     const onComplete = (
       err: Error | null,
       rows: TableRow[] | null,
@@ -1841,10 +1845,8 @@ class Table extends ServiceObject {
         return;
       }
       rows = BigQuery.mergeSchemaWithRows_(this.metadata.schema, rows || [], {
-        wrapIntegers: wrapIntegers,
-        selectedFields: options.selectedFields
-          ? options.selectedFields!.split(',')
-          : [],
+        wrapIntegers,
+        selectedFields,
         parseJSON,
       });
       callback!(null, rows, nextQuery, resp);