Cleanup spread operators in reduce calls (elastic#157471)

## Summary

The spread operator is costly and put pressure on GC. It should be
avoided when possible, especially in loops.

This PR adapts a lot of `reduce` calls in the codebase to remove the
usages of the diabolic spread operator, when possible.

Note: the PR is not fully exhaustive. I focused on the server-side, as
we're more directly impacted than on browser-side code regarding
performances.

## Removing `...` usages in `kittens.reduce()`

For `reduce` loops, the spread operator can usually easily be replaced:

#### - setting a value on the accum object and returning it

#### BAD
```ts
  return this.toArray().reduce(
      (acc, renderer) => ({
        ...acc,
        [renderer.name]: renderer,
      }),
      {} as Record<string, ExpressionRenderer>
    );
```

#### GOOD
```ts
  return this.toArray().reduce((acc, renderer) => {
      acc[renderer.name] = renderer;
      return acc;
    }, {} as Record<string, ExpressionRenderer>);
```


#### - assigning values to the accum object and returning it 

#### BAD
```ts
  const allAggs: Record<string, any> = fieldAggRequests.reduce(
      (aggs: Record<string, any>, fieldAggRequest: unknown | null) => {
        return fieldAggRequest ? { ...aggs, ...(fieldAggRequest as Record<string, any>) } : aggs;
      },
      {}
    );
```

#### GOOD
```ts
  const allAggs = fieldAggRequests.reduce<Record<string, any>>(
      (aggs: Record<string, any>, fieldAggRequest: unknown | null) => {
        if (fieldAggRequest) {
          Object.assign(aggs, fieldAggRequest);
        }
        return aggs;
      },
      {}
    );
```

#### - pushing items to the accum list and returning it 

#### BAD
```ts
  const charsFound = charToArray.reduce(
    (acc, char) => (value.includes(char) ? [...acc, char] : acc),
    [] as string[]
  );
```

#### GOOD
```ts
  const charsFound = charToArray.reduce((acc, char) => {
    if (value.includes(char)) {
      acc.push(char);
    }
    return acc;
  }, [] as string[]);
```

## Questions

#### Are you sure all the changes in this are strictly better for
runtime performances?

Yes, yes I am.

#### How much better?

Likely not much.

#### Are you planning on analyzing the perf gain?

Nope.

#### So why did you do it?

I got tired of seeing badly used spread operators in my team's owned
code, and I had some extra time during on-week, so I spent a few hours
adapting the usages in all our runtime/production codebase.

#### Was it fun?

Take your best guess.

---------

Co-authored-by: Kibana Machine <[email protected]>

packages/core/http/core-http-server-internal/src/http_config.ts

@@ -262,12 +262,12 @@ export class HttpConfig implements IHttpConfig {
     this.securityResponseHeaders = securityResponseHeaders;
     this.customResponseHeaders = Object.entries(rawHttpConfig.customResponseHeaders ?? {}).reduce(
       (headers, [key, value]) => {
-        return {
-          ...headers,
-          [key]: Array.isArray(value) ? value.map((e) => convertHeader(e)) : convertHeader(value),
-        };
+        headers[key] = Array.isArray(value)
+          ? value.map((e) => convertHeader(e))
+          : convertHeader(value);
+        return headers;
       },
-      {}
+      {} as Record<string, string | string[]>
     );
     this.maxPayload = rawHttpConfig.maxPayload;
     this.name = rawHttpConfig.name;

packages/core/node/core-node-server-internal/src/node_service.ts

@@ -70,7 +70,8 @@ export class NodeService {
 
     // We assume the combination of node roles has been validated and avoid doing additional checks here.
     this.roles = NODE_ALL_ROLES.reduce((acc, curr) => {
-      return { ...acc, [camelCase(curr)]: (roles as string[]).includes(curr) };
+      acc[camelCase(curr) as keyof NodeRoles] = (roles as string[]).includes(curr);
+      return acc;
     }, {} as NodeRoles);
 
     return {

packages/core/saved-objects/core-saved-objects-api-server-internal/src/lib/search/aggregations/validation.ts

@@ -194,11 +194,9 @@ const recursiveRewrite = (
       newValue = recursiveRewrite(value, nestedContext, [...parents, key]);
     }
 
-    return {
-      ...memo,
-      [newKey]: newValue,
-    };
-  }, {});
+    memo[newKey] = newValue;
+    return memo;
+  }, {} as Record<string, unknown>);
 };
 
 const childContext = (context: ValidationContext, path: string): ValidationContext => {

packages/core/saved-objects/core-saved-objects-api-server-internal/src/lib/search/search_dsl/query_params.ts

@@ -290,7 +290,7 @@ const getMatchPhrasePrefixFields = ({
     fields = types.reduce((typeFields, type) => {
       const defaultSearchField = registry.getType(type)?.management?.defaultSearchField;
       if (defaultSearchField) {
-        return [...typeFields, `${type}.${defaultSearchField}`];
+        typeFields.push(`${type}.${defaultSearchField}`);
       }
       return typeFields;
     }, [] as string[]);

packages/core/saved-objects/core-saved-objects-import-export-server-internal/src/import/saved_objects_importer.ts

@@ -41,10 +41,7 @@ export class SavedObjectsImporter implements ISavedObjectsImporter {
     this.#importSizeLimit = importSizeLimit;
     this.#importHooks = typeRegistry.getAllTypes().reduce((hooks, type) => {
       if (type.management?.onImport) {
-        return {
-          ...hooks,
-          [type.name]: [type.management.onImport],
-        };
+        hooks[type.name] = [type.management.onImport];
       }
       return hooks;
     }, {} as Record<string, SavedObjectsImportHook[]>);

packages/core/saved-objects/core-saved-objects-migration-server-internal/src/core/build_types_mappings.ts

@@ -15,14 +15,12 @@ import type { SavedObjectsTypeMappingDefinitions } from '@kbn/core-saved-objects
 export const buildTypesMappings = (
   types: SavedObjectsType[]
 ): SavedObjectsTypeMappingDefinitions => {
-  return types.reduce((acc, { name: type, mappings }) => {
+  return types.reduce<SavedObjectsTypeMappingDefinitions>((acc, { name: type, mappings }) => {
     const duplicate = acc.hasOwnProperty(type);
     if (duplicate) {
       throw new Error(`Type ${type} is already defined.`);
     }
-    return {
-      ...acc,
-      [type]: mappings,
-    };
+    acc[type] = mappings;
+    return acc;
   }, {});
 };

packages/core/saved-objects/core-saved-objects-migration-server-internal/src/document_migrator/build_active_migrations.ts

@@ -48,14 +48,11 @@ export function buildActiveMigrations({
       referenceTransforms,
     });
 
-    if (!typeTransforms.transforms.length) {
-      return migrations;
+    if (typeTransforms.transforms.length) {
+      migrations[type.name] = typeTransforms;
     }
 
-    return {
-      ...migrations,
-      [type.name]: typeTransforms,
-    };
+    return migrations;
   }, {} as ActiveMigrations);
 }
 

packages/core/saved-objects/core-saved-objects-migration-server-internal/src/document_migrator/document_migrator.ts

@@ -99,13 +99,16 @@ export class DocumentMigrator implements VersionedTransformer {
       throw new Error('Migrations are not ready. Make sure prepareMigrations is called first.');
     }
 
-    return Object.entries(this.migrations).reduce(
+    return Object.entries(this.migrations).reduce<SavedObjectsMigrationVersion>(
       (acc, [type, { latestVersion, immediateVersion }]) => {
         const version = includeDeferred ? latestVersion : immediateVersion;
         const latestMigrationVersion =
           migrationType === 'core' ? version.core : maxVersion(version.migrate, version.convert);
 
-        return latestMigrationVersion ? { ...acc, [type]: latestMigrationVersion } : acc;
+        if (latestMigrationVersion) {
+          acc[type] = latestMigrationVersion;
+        }
+        return acc;
       },
       {}
     );

src/plugins/chart_expressions/expression_partition_vis/public/utils/layers/sort_predicate.ts

@@ -22,13 +22,10 @@ type SortPredicatePieDonutFn = (visParams: PartitionVisParams) => SortFn;
 type SortPredicatePureFn = () => SortFn;
 
 export const extractUniqTermsMap = (dataTable: Datatable, columnId: string) =>
-  [...new Set(dataTable.rows.map((item) => item[columnId]))].reduce(
-    (acc, item, index) => ({
-      ...acc,
-      [item]: index,
-    }),
-    {}
-  );
+  [...new Set(dataTable.rows.map((item) => item[columnId]))].reduce((acc, item, index) => {
+    acc[item] = index;
+    return acc;
+  }, {});
 
 export const sortPredicateSaveSourceOrder: SortPredicatePureFn =
   () =>

src/plugins/chart_expressions/expression_xy/public/helpers/layers.ts

@@ -153,17 +153,18 @@ export const getLayerFormats = (
       }),
       {}
     ),
-    splitSeriesAccessors: splitColumnAccessors?.reduce((formatters, splitAccessor) => {
-      const formatObj = getAccessorWithFieldFormat(splitAccessor, table.columns);
-      const accessor = Object.keys(formatObj)[0];
-      return {
-        ...formatters,
-        [accessor]: {
+    splitSeriesAccessors: splitColumnAccessors?.reduce<SplitAccessorsFieldFormats>(
+      (formatters, splitAccessor) => {
+        const formatObj = getAccessorWithFieldFormat(splitAccessor, table.columns);
+        const accessor = Object.keys(formatObj)[0];
+        formatters[accessor] = {
           format: formatObj[accessor],
           formatter: formatFactory(formatObj[accessor]),
-        },
-      };
-    }, {}),
+        };
+        return formatters;
+      },
+      {}
+    ),
     splitColumnAccessors: getAccessorWithFieldFormat(splitColumnAccessor, table.columns),
     splitRowAccessors: getAccessorWithFieldFormat(splitRowAccessor, table.columns),
   };

src/plugins/console/server/services/spec_definitions_service.ts

@@ -128,17 +128,15 @@ export class SpecDefinitionsService {
       if (overrideFile) {
         merge(loadedSpec, JSON.parse(readFileSync(overrideFile, 'utf8')));
       }
-      const spec: Record<string, EndpointDescription> = {};
       Object.entries(loadedSpec).forEach(([key, value]) => {
         if (acc[key]) {
           // add time to remove key collision
-          spec[`${key}${Date.now()}`] = value;
+          acc[`${key}${Date.now()}`] = value;
         } else {
-          spec[key] = value;
+          acc[key] = value;
         }
       });
-
-      return { ...acc, ...spec };
+      return acc;
     }, {} as Record<string, EndpointDescription>);
   }
 

src/plugins/controls/server/options_list/options_list_cheap_suggestion_queries.ts

@@ -53,7 +53,8 @@ const cheapSuggestionAggSubtypes: { [key: string]: OptionsListSuggestionAggregat
     parse: (rawEsResult) => ({
       suggestions: get(rawEsResult, 'aggregations.suggestions.buckets')?.reduce(
         (acc: OptionsListSuggestions, suggestion: EsBucket) => {
-          return [...acc, { value: suggestion.key, docCount: suggestion.doc_count }];
+          acc.push({ value: suggestion.key, docCount: suggestion.doc_count });
+          return acc;
         },
         []
       ),
@@ -76,7 +77,8 @@ const cheapSuggestionAggSubtypes: { [key: string]: OptionsListSuggestionAggregat
     parse: (rawEsResult) => ({
       suggestions: get(rawEsResult, 'aggregations.suggestions.buckets')?.reduce(
         (acc: OptionsListSuggestions, suggestion: EsBucket & { key_as_string: string }) => {
-          return [...acc, { value: suggestion.key_as_string, docCount: suggestion.doc_count }];
+          acc.push({ value: suggestion.key_as_string, docCount: suggestion.doc_count });
+          return acc;
         },
         []
       ),
@@ -151,7 +153,8 @@ const cheapSuggestionAggSubtypes: { [key: string]: OptionsListSuggestionAggregat
         suggestions: sortedSuggestions
           .slice(0, 10) // only return top 10 results
           .reduce((acc: OptionsListSuggestions, suggestion: EsBucket) => {
-            return [...acc, { value: suggestion.key, docCount: suggestion.doc_count }];
+            acc.push({ value: suggestion.key, docCount: suggestion.doc_count });
+            return acc;
           }, []),
       };
     },
@@ -189,7 +192,8 @@ const cheapSuggestionAggSubtypes: { [key: string]: OptionsListSuggestionAggregat
     parse: (rawEsResult) => ({
       suggestions: get(rawEsResult, 'aggregations.nestedSuggestions.suggestions.buckets')?.reduce(
         (acc: OptionsListSuggestions, suggestion: EsBucket) => {
-          return [...acc, { value: suggestion.key, docCount: suggestion.doc_count }];
+          acc.push({ value: suggestion.key, docCount: suggestion.doc_count });
+          return acc;
         },
         []
       ),

src/plugins/controls/server/options_list/options_list_expensive_suggestion_queries.ts

@@ -93,7 +93,8 @@ const expensiveSuggestionAggSubtypes: { [key: string]: OptionsListSuggestionAggr
 
       const suggestions = get(rawEsResult, `${basePath}.suggestions.buckets`)?.reduce(
         (acc: OptionsListSuggestions, suggestion: EsBucket) => {
-          return [...acc, { value: suggestion.key, docCount: suggestion.doc_count }];
+          acc.push({ value: suggestion.key, docCount: suggestion.doc_count });
+          return acc;
         },
         []
       );
@@ -120,7 +121,8 @@ const expensiveSuggestionAggSubtypes: { [key: string]: OptionsListSuggestionAggr
     parse: (rawEsResult) => {
       const suggestions = get(rawEsResult, 'aggregations.suggestions.buckets')?.reduce(
         (acc: OptionsListSuggestions, suggestion: EsBucket & { key_as_string: string }) => {
-          return [...acc, { value: suggestion.key_as_string, docCount: suggestion.doc_count }];
+          acc.push({ value: suggestion.key_as_string, docCount: suggestion.doc_count });
+          return acc;
         },
         []
       );
@@ -200,7 +202,8 @@ const expensiveSuggestionAggSubtypes: { [key: string]: OptionsListSuggestionAggr
       const suggestions = sortedSuggestions
         .slice(0, request.size)
         .reduce((acc: OptionsListSuggestions, suggestion: EsBucket) => {
-          return [...acc, { value: suggestion.key, docCount: suggestion.doc_count }];
+          acc.push({ value: suggestion.key, docCount: suggestion.doc_count });
+          return acc;
         }, []);
       const totalCardinality =
         (get(rawEsResult, `aggregations.suggestions.buckets.ipv4.unique_terms.value`) ?? 0) +

src/plugins/data/common/search/expressions/es_raw_response.ts

@@ -26,7 +26,10 @@ function flatten(obj: any, keyPrefix = '') {
       nestedRows.push(
         ...obj[key]
           .map((nestedRow: any) => flatten(nestedRow, prefix + key))
-          .reduce((acc: any, object: any) => [...acc, ...object], [])
+          .reduce((acc: unknown[], object: unknown[]) => {
+            acc.push(...object);
+            return acc;
+          }, [])
       );
     } else if (typeof obj[key] === 'object' && obj[key] !== null) {
       const subRows = flatten(obj[key], prefix + key);

src/plugins/data/server/search/collectors/search_session/fetch.ts

@@ -48,7 +48,8 @@ export function fetchProvider(getIndexForType: (type: string) => Promise<string>
       const { transientCount = 0, persistedCount = 0 } = buckets.reduce(
         (usage: Partial<ReportedUsage>, bucket: SessionPersistedTermsBucket) => {
           const key = bucket.key_as_string === 'false' ? 'transientCount' : 'persistedCount';
-          return { ...usage, [key]: bucket.doc_count };
+          usage[key] = bucket.doc_count;
+          return usage;
         },
         {}
       );

src/plugins/data_views/server/fetcher/lib/field_capabilities/field_caps_response.ts

@@ -110,13 +110,10 @@ export function readFieldCapsResponse(
           searchable: isSearchable,
           aggregatable: isAggregatable,
           readFromDocValues: false,
-          conflictDescriptions: types.reduce(
-            (acc, esType) => ({
-              ...acc,
-              [esType]: capsByType[esType].indices,
-            }),
-            {}
-          ),
+          conflictDescriptions: types.reduce((acc, esType) => {
+            acc[esType] = capsByType[esType].indices;
+            return acc;
+          }, {} as Record<string, estypes.Indices | undefined>),
           metadata_field: capsByType[types[0]].metadata_field,
         };
         // This is intentionally using a "hash" and a "push" to be highly optimized with very large indexes

src/plugins/es_ui_shared/static/forms/hook_form_lib/hooks/use_form_is_modified.ts

@@ -61,10 +61,10 @@ export const useFormIsModified = ({
       return;
     }
 
-    return fieldPathsToDiscard.reduce(
-      (acc, path) => ({ ...acc, [path]: true }),
-      {} as { [key: string]: {} }
-    );
+    return fieldPathsToDiscard.reduce((acc, path) => {
+      acc[path] = true;
+      return acc;
+    }, {} as { [key: string]: {} });
 
     // discardArrayToString === discard, we don't want to add it to the dependencies so
     // the consumer does not need to memoize the "discard" array they provide.

src/plugins/es_ui_shared/static/forms/hook_form_lib/lib/utils.ts

@@ -67,10 +67,11 @@ export const stripOutUndefinedValues = <R>(obj: GenericObject): R => {
     .filter(({ 1: value }) => value !== undefined)
     .reduce((acc, [key, value]) => {
       if (typeof value === 'object' && value !== null && !Array.isArray(value)) {
-        return { ...acc, [key]: stripOutUndefinedValues(value) };
+        acc[key as keyof R] = stripOutUndefinedValues(value);
+      } else {
+        acc[key as keyof R] = value;
       }
-
-      return { ...acc, [key]: value };
+      return acc;
     }, {} as R);
 };
 

src/plugins/es_ui_shared/static/validators/string/contains_chars.ts

@@ -9,10 +9,12 @@
 export const containsChars = (chars: string | string[]) => (value: string) => {
   const charToArray = Array.isArray(chars) ? (chars as string[]) : ([chars] as string[]);
 
-  const charsFound = charToArray.reduce(
-    (acc, char) => (value.includes(char) ? [...acc, char] : acc),
-    [] as string[]
-  );
+  const charsFound = charToArray.reduce((acc, char) => {
+    if (value.includes(char)) {
+      acc.push(char);
+    }
+    return acc;
+  }, [] as string[]);
 
   return {
     charsFound,

src/plugins/event_annotation/common/fetch_event_annotations/utils.ts

@@ -179,10 +179,10 @@ export const postprocessAnnotations = (
 
         let extraFields: Record<string, string | number | string[] | number[]> = {};
         if (annotationConfig?.extraFields?.length) {
-          extraFields = annotationConfig.extraFields.reduce(
-            (acc, field) => ({ ...acc, [`field:${field}`]: row[fieldsColIdMap[field]] }),
-            {}
-          );
+          extraFields = annotationConfig.extraFields.reduce((acc, field) => {
+            acc[`field:${field}`] = row[fieldsColIdMap[field]];
+            return acc;
+          }, {} as typeof extraFields);
         }
         if (annotationConfig?.textField) {
           extraFields[`field:${annotationConfig.textField}`] =

src/plugins/expressions/common/expression_renderers/expression_renderer_registry.ts

@@ -27,13 +27,10 @@ export class ExpressionRendererRegistry implements IRegistry<ExpressionRenderer>
   }
 
   public toJS(): Record<string, ExpressionRenderer> {
-    return this.toArray().reduce(
-      (acc, renderer) => ({
-        ...acc,
-        [renderer.name]: renderer,
-      }),
-      {} as Record<string, ExpressionRenderer>
-    );
+    return this.toArray().reduce((acc, renderer) => {
+      acc[renderer.name] = renderer;
+      return acc;
+    }, {} as Record<string, ExpressionRenderer>);
   }
 
   public toArray(): ExpressionRenderer[] {