ported compatibility tests for 1.5.4 to begin exploring actual compatibility

Author: rayozzie

callable.go

@@ -246,6 +246,17 @@ func (c *goCallable) validateArgCount(argv []reflect.Value) ([]reflect.Value, er
 
 	argc := len(argv)
 
+	// Check for undefined arguments first
+	// This ensures that $base64encode() with no arguments returns undefined
+	// rather than trying to use the context as an argument
+	if c.undefinedHandler != nil && c.undefinedHandler(argv) {
+		// TODO: Validate the other arguments before doing
+		// this. Otherwise we mask errors with the other
+		// arguments.
+		return nil, jtypes.ErrUndefined
+	}
+	
+	// Only use context handler if we haven't determined the result is undefined
 	if c.contextHandler != nil && c.contextHandler(argv) {
 		// TODO: Return an error if the evaluation context
 		// is not the correct type.
@@ -254,13 +265,6 @@ func (c *goCallable) validateArgCount(argv []reflect.Value) ([]reflect.Value, er
 		argv = append(newargv, argv...)
 	}
 
-	if c.undefinedHandler != nil && c.undefinedHandler(argv) {
-		// TODO: Validate the other arguments before doing
-		// this. Otherwise we mask errors with the other
-		// arguments.
-		return nil, jtypes.ErrUndefined
-	}
-
 	paramCount := len(c.params)
 
 	for i := len(argv); i < paramCount; i++ {

env.go

@@ -156,8 +156,8 @@ var baseEnv = initBaseEnv(map[string]Extension{
 	},
 	"base64decode": {
 		Func:               jlib.Base64Decode,
-		UndefinedHandler:   defaultUndefinedHandler,
-		EvalContextHandler: defaultContextHandler,
+		UndefinedHandler:   jtypes.ArgCountEquals(0),
+		EvalContextHandler: nil,
 	},
 	"decodeUrl": {
 		Func:               jlib.DecodeURL,

eval.go

@@ -528,6 +528,14 @@ func evalBlock(node *jparse.BlockNode, data reflect.Value, env *environment) (re
 	// environment of the correct size?
 	env = newEnvironment(env, 0)
 
+	// If the block is empty, return an explicit nil value, not undefined.
+	// Empty blocks should return null/nil according to JSONata spec.
+	if len(node.Exprs) == 0 {
+		// Return a nil value wrapped in a reflect.Value.
+		// This will be interpreted as null in JSON or undefined in JSONata context.
+		return reflect.Zero(jtypes.TypeInterface), nil
+	}
+
 	// Evaluate all expressions in the block.
 	for _, node := range node.Exprs {
 		res, err = eval(node, data, env)