fix: add the ability to provide a random source

engine.go

@@ -113,6 +113,13 @@ func WithDebug() Opt {
 	}
 }
 
+// WithRandSource sets the random source to use in the engine.
+func WithRandSource(randSource func() float64) Opt {
+	return func(e *Engine) {
+		e.randSource = randSource
+	}
+}
+
 // Engine provides the templating engine.
 type Engine struct {
 	searchLocations []string
@@ -125,6 +132,8 @@ type Engine struct {
 
 	tracer trace.Tracer
 
+	randSource vm.RandSource
+
 	vm *vm.VM
 }
 
@@ -248,7 +257,7 @@ func (e *Engine) init(ctx context.Context, data any) (*vm.VM, error) {
 		return nil, ErrAlreadyInitialized
 	}
 
-	v, err := vm.New()
+	v, err := vm.New(e.randSource)
 	if err != nil {
 		return nil, fmt.Errorf("failed to create vm: %w", err)
 	}

internal/vm/vm.go

@@ -50,6 +50,9 @@ type Options struct {
 // Option represents an option for running a script.
 type Option func(*Options)
 
+// RandSource is a function that returns a seeded float64 value.
+type RandSource func() float64
+
 // WithStartingLineNumber sets the starting line number for the script.
 func WithStartingLineNumber(lineNumber int) Option {
 	return func(o *Options) {
@@ -63,7 +66,7 @@ type program struct {
 }
 
 // New creates a new VM.
-func New() (*VM, error) {
+func New(randSource RandSource) (*VM, error) {
 	g := goja.New()
 	_, err := g.RunString(underscore.JS)
 	if err != nil {
@@ -73,6 +76,12 @@ func New() (*VM, error) {
 	new(require.Registry).Enable(g)
 	console.Enable(g)
 
+	if randSource != nil {
+		g.SetRandSource(func() float64 {
+			return randSource()
+		})
+	}
+
 	return &VM{Runtime: g, globalSourceMapCache: make(map[string]*sourcemap.Consumer)}, nil
 }
 

internal/vm/vm_test.go

@@ -9,8 +9,30 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
+func TestVM_Run_Runtime_Success(t *testing.T) {
+	v, err := vm.New(nil)
+	require.NoError(t, err)
+
+	typeScript := `console.log("hello world");`
+
+	_, err = v.Run(context.Background(), "test", typeScript)
+	assert.NoError(t, err)
+}
+
+func TestVM_Run_Runtime_WithRandSource_Success(t *testing.T) {
+	v, err := vm.New(func() float64 {
+		return 0
+	})
+	require.NoError(t, err)
+
+	typeScript := `console.log("hello world");`
+
+	_, err = v.Run(context.Background(), "test", typeScript)
+	assert.NoError(t, err)
+}
+
 func TestVM_Run_Runtime_Errors(t *testing.T) {
-	v, err := vm.New()
+	v, err := vm.New(nil)
 	require.NoError(t, err)
 
 	typeScript := `type Test = {