Add underscore to js evaluation to allow for use of get() (#83)

This will help with nested fields that could potentially be `null`

---------

Co-authored-by: Liam Hegarty <liam.hegarty@incident.io>

docs/backstage/catalog-info.yaml

@@ -3,7 +3,6 @@ apiVersion: backstage.io/v1alpha1
 kind: API
 metadata:
   description: The incident.io public API
-  name: api-incident-io
   namespace: default
 spec:
   definition:

docs/backstage/pipelines/entries.jsonnet

@@ -52,18 +52,18 @@
       type_name: 'Custom["BackstageAPI"]',
       source: {
         filter: '$.apiVersion == "backstage.io/v1alpha1" && $.kind == "API"',
-        name: '$.metadata.name',
-        external_id: '$.metadata.namespace + "/" + $.metadata.name',
+        name: '_.get($.metadata, "name", "default API name")',
+        external_id: '_.get($.metadata, "namespace", "default description") + "/" + _.get($.metadata, "name", "default API name")',
         aliases: [
-          '$.metadata.name',
+          '_.get($.name, "name", "default alias")',
         ],
       },
       attributes: [
         {
           id: 'description',
           name: 'Description',
           type: 'Text',
-          source: '$.metadata.description',
+          source: '_.get($.metadata, "description", "default description")',
         },
         {
           id: 'api_type',

docs/expressions.md

@@ -86,6 +86,17 @@ The following expressions would evaluate to:
 - `$.runbooks` → `["https://github.com/incident-io/runbooks/blob/main/website.md"]`
 - `$.runbooks[0]` → `https://github.com/incident-io/runbooks/blob/main/website.md`
 
+
+You can also use `get` to evaluate nested fields that may be null.
+For the example entry above, the following expressions would evaluate to:
+
+- `_.get($.metadata, "name")` → `null`
+- `_.get($.metadata, "name", "default name")` → `default name`
+- `_.get($.details, "alias")` → `null`
+- `_.get($.details, "alias", "default alias")` → `default alias`
+- `_.get($.details, "description")` → `Marketing website`
+
+
 ## Migrating from CEL
 
 As shown above, the main difference between CEL and JavaScript is that your

expr/js_eval.go

@@ -11,8 +11,23 @@ import (
 	"github.com/go-kit/log/level"
 	"github.com/pkg/errors"
 	"github.com/robertkrimen/otto"
+	underscore "github.com/robertkrimen/otto/underscore"
 )
 
+var vm *otto.Otto
+
+func init() {
+
+	underscore.Enable()
+
+	// Create a Javascript virtual machine that we'll use for evaluating the source
+	// expression. We must be very careful: this is executing code on behalf of others, so
+	// comes with all normal warnings.
+	vm = otto.New()
+	vm.Interrupt = make(chan func(), 1)
+
+}
+
 // EvaluateJavascript can evaluate a source Javascript program having set the given
 // subject into the `$` variable.
 func EvaluateJavascript(ctx context.Context, source string, subject any, logger kitlog.Logger) (result otto.Value, err error) {
@@ -27,11 +42,9 @@ func EvaluateJavascript(ctx context.Context, source string, subject any, logger
 		}
 	}()
 
-	// Create a Javascript virtual machine that we'll use for evaluating the source
-	// expression. We must be very careful: this is executing code on behalf of others, so
-	// comes with all normal warnings.
-	vm := otto.New()
-	vm.Interrupt = make(chan func(), 1)
+	if vm == nil {
+		panic("Javascript virtual machine not initialised")
+	}
 
 	// Start a new function bounded context.
 	ctx, cancel := context.WithCancel(ctx)

expr/js_eval_test.go

@@ -98,6 +98,13 @@ var _ = Describe("Javascript evaluation", func() {
 		Expect(evaluatedResult).To(Equal(sourceEntry["metadata"].(map[string]any)["namespace"]))
 	})
 
+	It("handles possible null values with _.get", func() {
+		nestedSrc := "_.get($.metadata, \"badKey\", \"default value\")"
+		evaluatedResult, err := EvaluateSingleValue[string](ctx, nestedSrc, sourceEntry)
+		Expect(err).NotTo(HaveOccurred())
+		Expect(evaluatedResult).To(Equal("default value"))
+	})
+
 	When("parsing array values", func() {
 		It("returns an error if the input is not an array", func() {
 			topLevelSrc := "$.name"

reconcile/entries.go

@@ -248,7 +248,7 @@ func Entries(ctx context.Context, logger kitlog.Logger, cl EntriesClient, catalo
 			}
 		}
 
-		logger.Log("msg", fmt.Sprintf("found %d entries that need updated", len(toUpdate)))
+		logger.Log("msg", fmt.Sprintf("found %d entries that need updating", len(toUpdate)))
 
 		g, ctx := errgroup.WithContext(ctx)
 		g.SetLimit(10)