[8.7] [@kbn/handlebars] Minor refactor (#150440) (#150679)

# Backport

This will backport the following commits from `main` to `8.7`:
- [[@kbn/handlebars] Minor refactor
(#150440)](https://github.com/elastic/kibana/pull/150440)

<!--- Backport version: 8.9.7 -->

### Questions ?
Please refer to the [Backport tool
documentation](https://github.com/sqren/backport)

<!--BACKPORT [{"author":{"name":"Thomas
Watson","email":"watson@elastic.co"},"sourceCommit":{"committedDate":"2023-02-09T11:21:32Z","message":"[@kbn/handlebars]
Minor refactor
(#150440)","sha":"5f0b56d3bc470eba4753dfa806154814609deee0","branchLabelMapping":{"^v8.8.0$":"main","^v(\\d+).(\\d+).\\d+$":"$1.$2"}},"sourcePullRequest":{"labels":["release_note:skip","backport:prev-minor","v8.8.0"],"number":150440,"url":"https://github.com/elastic/kibana/pull/150440","mergeCommit":{"message":"[@kbn/handlebars]
Minor refactor
(#150440)","sha":"5f0b56d3bc470eba4753dfa806154814609deee0"}},"sourceBranch":"main","suggestedTargetBranches":[],"targetPullRequestStates":[{"branch":"main","label":"v8.8.0","labelRegex":"^v8.8.0$","isSourceBranch":true,"state":"MERGED","url":"https://github.com/elastic/kibana/pull/150440","number":150440,"mergeCommit":{"message":"[@kbn/handlebars]
Minor refactor
(#150440)","sha":"5f0b56d3bc470eba4753dfa806154814609deee0"}}]}]
BACKPORT-->

Co-authored-by: Thomas Watson <watson@elastic.co>

packages/kbn-handlebars/index.test.ts

@@ -506,12 +506,13 @@ describe('blocks', () => {
     it('should call multiple decorators in the same program body in the expected order and get the expected output', () => {
       let decoratorCall = 0;
       let progCall = 0;
-      expectTemplate('{{*decorator}}con{{*decorator}}tent')
-        .beforeRender(() => {
+      expectTemplate('{{*decorator}}con{{*decorator}}tent', {
+        beforeRender() {
           // ensure the counters are reset between EVAL/AST render calls
           decoratorCall = 0;
           progCall = 0;
-        })
+        },
+      })
         .withInput({
           decoratorCall: 0,
           progCall: 0,

packages/kbn-handlebars/src/__jest__/test_bench.ts

@@ -24,6 +24,7 @@ global.kbnHandlebarsEnv = null;
 
 interface TestOptions {
   beforeEach?: Function;
+  beforeRender?: Function;
 }
 
 export function expectTemplate(template: string, options?: TestOptions) {
@@ -39,7 +40,6 @@ export function forEachCompileFunctionName(
 class HandlebarsTestBench {
   private template: string;
   private options: TestOptions;
-  private beforeRenderFn: Function = () => {};
   private compileOptions?: ExtendedCompileOptions;
   private runtimeOptions?: ExtendedRuntimeOptions;
   private helpers: { [name: string]: Handlebars.HelperDelegate | undefined } = {};
@@ -52,11 +52,6 @@ class HandlebarsTestBench {
     this.options = options;
   }
 
-  beforeRender(fn: Function) {
-    this.beforeRenderFn = fn;
-    return this;
-  }
-
   withCompileOptions(compileOptions?: ExtendedCompileOptions) {
     this.compileOptions = compileOptions;
     return this;
@@ -159,16 +154,14 @@ class HandlebarsTestBench {
   private compileAndExecuteEval() {
     const renderEval = this.compileEval();
 
-    const runtimeOptions: ExtendedRuntimeOptions = Object.assign(
-      {
-        helpers: this.helpers,
-        partials: this.partials,
-        decorators: this.decorators,
-      },
-      this.runtimeOptions
-    );
+    const runtimeOptions: ExtendedRuntimeOptions = {
+      helpers: this.helpers as Record<string, Function>,
+      partials: this.partials as Record<string, HandlebarsTemplateDelegate>,
+      decorators: this.decorators,
+      ...this.runtimeOptions,
+    };
 
-    this.beforeRenderFn();
+    this.execBeforeRender();
 
     return renderEval(this.input, runtimeOptions);
   }
@@ -176,16 +169,14 @@ class HandlebarsTestBench {
   private compileAndExecuteAST() {
     const renderAST = this.compileAST();
 
-    const runtimeOptions: ExtendedRuntimeOptions = Object.assign(
-      {
-        helpers: this.helpers,
-        partials: this.partials,
-        decorators: this.decorators,
-      },
-      this.runtimeOptions
-    );
+    const runtimeOptions: ExtendedRuntimeOptions = {
+      helpers: this.helpers as Record<string, Function>,
+      partials: this.partials as Record<string, HandlebarsTemplateDelegate>,
+      decorators: this.decorators,
+      ...this.runtimeOptions,
+    };
 
-    this.beforeRenderFn();
+    this.execBeforeRender();
 
     return renderAST(this.input, runtimeOptions);
   }
@@ -200,6 +191,10 @@ class HandlebarsTestBench {
     return handlebarsEnv.compileAST(this.template, this.compileOptions);
   }
 
+  private execBeforeRender() {
+    this.options.beforeRender?.();
+  }
+
   private execBeforeEach() {
     if (this.options.beforeEach) {
       this.options.beforeEach();

packages/kbn-handlebars/src/handlebars.ts

@@ -7,7 +7,7 @@
 // https://www.typescriptlang.org/docs/handbook/modules.html#export--and-import--require
 import Handlebars from 'handlebars';
 
-import type { DecoratorsHash, ExtendedCompileOptions, ExtendedRuntimeOptions } from './types';
+import type { ExtendedCompileOptions, ExtendedRuntimeOptions } from './types';
 import { ElasticHandlebarsVisitor } from './visitor';
 
 const originalCreate = Handlebars.create;
@@ -46,11 +46,8 @@ Handlebars.compileAST = function (
   }
 
   // If `Handlebars.compileAST` is reassigned, `this` will be undefined.
-  const helpers = (this ?? Handlebars).helpers;
-  const partials = (this ?? Handlebars).partials;
-  const decorators = (this ?? Handlebars).decorators as DecoratorsHash;
+  const visitor = new ElasticHandlebarsVisitor(this ?? Handlebars, input, options);
 
-  const visitor = new ElasticHandlebarsVisitor(this, input, options, helpers, partials, decorators);
   return (context: any, runtimeOptions?: ExtendedRuntimeOptions) =>
     visitor.render(context, runtimeOptions);
 };

packages/kbn-handlebars/src/visitor.ts

@@ -27,7 +27,6 @@ import type {
   HelpersHash,
   NodeType,
   NonBlockHelperOptions,
-  PartialsHash,
   ProcessableBlockStatementNode,
   ProcessableNode,
   ProcessableNodeWithPathParts,
@@ -51,9 +50,6 @@ export class ElasticHandlebarsVisitor extends Handlebars.Visitor {
   private template?: string;
   private compileOptions: ExtendedCompileOptions;
   private runtimeOptions?: ExtendedRuntimeOptions;
-  private initialHelpers: HelpersHash;
-  private initialPartials: PartialsHash;
-  private initialDecorators: DecoratorsHash;
   private blockParamNames: any[][] = [];
   private blockParamValues: any[][] = [];
   private ast?: hbs.AST.Program;
@@ -65,10 +61,7 @@ export class ElasticHandlebarsVisitor extends Handlebars.Visitor {
   constructor(
     env: typeof Handlebars,
     input: string | hbs.AST.Program,
-    options: ExtendedCompileOptions = {},
-    helpers: HelpersHash,
-    partials: PartialsHash,
-    decorators: DecoratorsHash
+    options: ExtendedCompileOptions = {}
   ) {
     super();
 
@@ -80,13 +73,7 @@ export class ElasticHandlebarsVisitor extends Handlebars.Visitor {
       this.template = input as string;
     }
 
-    this.compileOptions = Object.assign(
-      {
-        data: true,
-      },
-      options
-    );
-
+    this.compileOptions = { data: true, ...options };
     this.compileOptions.knownHelpers = Object.assign(
       Object.create(null),
       {
@@ -102,10 +89,6 @@ export class ElasticHandlebarsVisitor extends Handlebars.Visitor {
       this.compileOptions.knownHelpers
     );
 
-    this.initialHelpers = Object.assign({}, helpers);
-    this.initialPartials = Object.assign({}, partials);
-    this.initialDecorators = Object.assign({}, decorators);
-
     const protoAccessControl = createProtoAccessControl({});
 
     const container: Container = (this.container = {
@@ -156,13 +139,16 @@ export class ElasticHandlebarsVisitor extends Handlebars.Visitor {
   render(context: any, options: ExtendedRuntimeOptions = {}): string {
     this.contexts = [context];
     this.output = [];
-    this.runtimeOptions = Object.assign({}, options);
-    this.container.helpers = Object.assign(this.initialHelpers, options.helpers);
-    this.container.partials = Object.assign(this.initialPartials, options.partials);
-    this.container.decorators = Object.assign(
-      this.initialDecorators,
-      options.decorators as DecoratorsHash
-    );
+    this.runtimeOptions = { ...options };
+    this.container.helpers = {
+      ...this.env.helpers,
+      ...(options.helpers as HelpersHash),
+    };
+    this.container.partials = { ...this.env.partials, ...options.partials };
+    this.container.decorators = {
+      ...(this.env.decorators as DecoratorsHash),
+      ...(options.decorators as DecoratorsHash),
+    };
     this.container.hooks = {};
     this.processedRootDecorators = false;
     this.processedDecoratorsForProgram.clear();
@@ -540,7 +526,7 @@ export class ElasticHandlebarsVisitor extends Handlebars.Visitor {
       };
 
       if (fn.partials) {
-        options.partials = Object.assign({}, options.partials, fn.partials);
+        options.partials = { ...options.partials, ...fn.partials };
       }
     }
 
@@ -694,7 +680,7 @@ export class ElasticHandlebarsVisitor extends Handlebars.Visitor {
       nextContext: any,
       runtimeOptions: ExtendedRuntimeOptions = {}
     ) => {
-      runtimeOptions = Object.assign({}, runtimeOptions);
+      runtimeOptions = { ...runtimeOptions };
 
       // inherit data in blockParams from parent program
       runtimeOptions.data = runtimeOptions.data || this.runtimeOptions!.data;