Add a virtual method "length" on slices, arrays, strings to match JS engines

eval.go

@@ -364,6 +364,14 @@ func (v *evalVisitor) evalMethod(ctx reflect.Value, name string, exprRoot bool)
 	}
 
 	if !method.IsValid() {
+		// Special case for "length" for slices, arrays to provide the same behaviour as JavaScript engines
+		// where the "length" method is implicit.
+		if name == "length" {
+			switch ctx.Kind() {
+			case reflect.Slice, reflect.Array, reflect.String:
+				return reflect.ValueOf(ctx.Len()), true
+			}
+		}
 		return zero, false
 	}
 

eval_test.go

@@ -72,6 +72,27 @@ var evalTests = []Test{
 		nil, nil, nil,
 		"C",
 	},
+	{
+		"length method on a slice, like with JS engines",
+		"Length: {{arr.length}}",
+		map[string]interface{}{"arr": []int{0, 1, 2}},
+		nil, nil, nil,
+		`Length: 3`,
+	},
+	{
+		"length method on an array, like with JS engines",
+		"Length: {{arr.length}}",
+		map[string]interface{}{"arr": [...]int{0, 1, 2, 3}},
+		nil, nil, nil,
+		`Length: 4`,
+	},
+	{
+		"length method on a string, like with JS engines",
+		"Length: {{str.length}}",
+		map[string]interface{}{"str": "abcde"},
+		nil, nil, nil,
+		`Length: 5`,
+	},
 
 	// @todo Test with a "../../path" (depth 2 path) while context is only depth 1
 }