[8.6] [@kbn/handlebars] Refactor internal context handling (#148029) (#…

…148035)

# Backport

This will backport the following commits from `main` to `8.6`:
- [[@kbn/handlebars] Refactor internal context handling
(#148029)](#148029)

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

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

<!--BACKPORT [{"author":{"name":"Thomas
Watson","email":"[email protected]"},"sourceCommit":{"committedDate":"2022-12-22T21:23:43Z","message":"[@kbn/handlebars]
Refactor internal context handling
(#148029)","sha":"0d2e8156bf8c0e5d38a40fb02f94662348e9377a","branchLabelMapping":{"^v8.7.0$":"main","^v(\\d+).(\\d+).\\d+$":"$1.$2"}},"sourcePullRequest":{"labels":["release_note:skip","backport:prev-minor","v8.7.0"],"number":148029,"url":"https://github.com/elastic/kibana/pull/148029","mergeCommit":{"message":"[@kbn/handlebars]
Refactor internal context handling
(#148029)","sha":"0d2e8156bf8c0e5d38a40fb02f94662348e9377a"}},"sourceBranch":"main","suggestedTargetBranches":[],"targetPullRequestStates":[{"branch":"main","label":"v8.7.0","labelRegex":"^v8.7.0$","isSourceBranch":true,"state":"MERGED","url":"https://github.com/elastic/kibana/pull/148029","number":148029,"mergeCommit":{"message":"[@kbn/handlebars]
Refactor internal context handling
(#148029)","sha":"0d2e8156bf8c0e5d38a40fb02f94662348e9377a"}}]}]
BACKPORT-->

Co-authored-by: Thomas Watson <[email protected]>

packages/kbn-handlebars/README.md

@@ -59,7 +59,7 @@ To instruct the `Visitor` code to traverse any child nodes of a given node, our
 
 We keep state internally in the `ElasticHandlebarsVisitor` object using the following private properties:
 
-- `scopes`: An array (stack) of `context` objects. In a simple template this array will always only contain a single element: The main `context` object. In more complicated scenarios, new `context` objects will be pushed and popped to and from the `scopes` stack as needed.
+- `contexts`: An array (stack) of `context` objects. In a simple template this array will always only contain a single element: The main `context` object. In more complicated scenarios, new `context` objects will be pushed and popped to and from the `contexts` stack as needed.
 - `output`: An array containing the "rendered" output of each node (normally just one element per node). In the most simple template, this is simply joined together into a the final output string after the AST has been traversed. In more complicated templates, we use this array temporarily to collect parameters to give to helper functions (see the `getParams` function).
 
 ## Testing

packages/kbn-handlebars/index.ts

@@ -187,7 +187,7 @@ interface Container {
 }
 
 class ElasticHandlebarsVisitor extends Handlebars.Visitor {
-  private scopes: any[] = [];
+  private contexts: any[] = [];
   private output: any[] = [];
   private template?: string;
   private compileOptions: ExtendedCompileOptions;
@@ -288,7 +288,7 @@ class ElasticHandlebarsVisitor extends Handlebars.Visitor {
   }
 
   render(context: any, options: ExtendedRuntimeOptions = {}): string {
-    this.scopes = [context];
+    this.contexts = [context];
     this.output = [];
     this.runtimeOptions = options;
     this.container.helpers = Object.assign(this.initialHelpers, options.helpers);
@@ -364,7 +364,7 @@ class ElasticHandlebarsVisitor extends Handlebars.Visitor {
     } else if (path.data) {
       result = this.lookupData(this.runtimeOptions!.data, path);
     } else {
-      result = this.resolvePath(this.scopes[path.depth], path);
+      result = this.resolvePath(this.contexts[path.depth], path);
     }
 
     this.output.push(result);
@@ -514,7 +514,7 @@ class ElasticHandlebarsVisitor extends Handlebars.Visitor {
     // @ts-expect-error strict is not a valid property on PathExpression, but we used in the same way it's also used in the original handlebars
     path.strict = true;
     const result = this.resolveNodes(path)[0];
-    const lambdaResult = this.container.lambda(result, this.scopes[0]);
+    const lambdaResult = this.container.lambda(result, this.context);
 
     if (isBlock(node)) {
       this.blockValue(node, lambdaResult);
@@ -531,8 +531,7 @@ class ElasticHandlebarsVisitor extends Handlebars.Visitor {
     const name = node.path.original;
     const options = this.setupParams(node, name);
 
-    const context = this.scopes[0];
-    const result = this.container.hooks.blockHelperMissing!.call(context, value, options);
+    const result = this.container.hooks.blockHelperMissing!.call(this.context, value, options);
 
     this.output.push(result);
   }
@@ -647,8 +646,7 @@ class ElasticHandlebarsVisitor extends Handlebars.Visitor {
     const helper = this.setupHelper(block, name);
 
     if (!helper.fn) {
-      const context = this.scopes[0];
-      value = this.container.hooks.blockHelperMissing!.call(context, value, helper.options);
+      value = this.container.hooks.blockHelperMissing!.call(this.context, value, helper.options);
     }
 
     return value;
@@ -665,7 +663,7 @@ class ElasticHandlebarsVisitor extends Handlebars.Visitor {
   } {
     return {
       fn: this.container.lookupProperty(this.container.helpers, helperName),
-      context: this.scopes[0],
+      context: this.context,
       params: this.resolveNodes(node.params),
       options: this.setupParams(node, helperName),
     };
@@ -679,9 +677,9 @@ class ElasticHandlebarsVisitor extends Handlebars.Visitor {
     if (decorator.params.length > 0) {
       if (!this.processedRootDecorators) {
         // When processing the root decorators, temporarily remove the root context so it's not accessible to the decorator
-        const context = this.scopes.shift();
+        const context = this.contexts.shift();
         options.args = this.resolveNodes(decorator.params);
-        this.scopes.unshift(context);
+        this.contexts.unshift(context);
       } else {
         options.args = this.resolveNodes(decorator.params);
       }
@@ -735,16 +733,16 @@ class ElasticHandlebarsVisitor extends Handlebars.Visitor {
       // stash parent program data
       const tmpRuntimeOptions = this.runtimeOptions;
       this.runtimeOptions = runtimeOptions;
-      const shiftContext = nextContext !== this.scopes[0];
-      if (shiftContext) this.scopes.unshift(nextContext);
+      const shiftContext = nextContext !== this.context;
+      if (shiftContext) this.contexts.unshift(nextContext);
       this.blockParamValues.unshift(runtimeOptions.blockParams || []);
 
       // execute child program
       const result = this.resolveNodes(program).join('');
 
       // unstash parent program data
       this.blockParamValues.shift();
-      if (shiftContext) this.scopes.shift();
+      if (shiftContext) this.contexts.shift();
       this.runtimeOptions = tmpRuntimeOptions;
 
       // return result of child program
@@ -809,6 +807,10 @@ class ElasticHandlebarsVisitor extends Handlebars.Visitor {
 
     return result;
   }
+
+  private get context() {
+    return this.contexts[0];
+  }
 }
 
 // ********************************************** //