feat(stdlib): Implement ifSGNodeField.hasField() (#368)

The implementation is simpler than the existing _getField_ method, which
returns a value, while this simply returns a boolean if the field is
present.  

There is at least one bug in hasField in the official implementation
that I'm aware of, where calling hasField() with specific strings will
actually create the field, but this is a bug and I don't think we should
match that for compatibility sake.  

resolves #367

src/brsTypes/components/RoSGNode.ts

@@ -107,6 +107,7 @@ export class RoSGNode extends BrsComponent implements BrsValue, BrsIterable {
                 this.addfield,
                 this.addfields,
                 this.getfield,
+                this.hasfield,
                 this.observefield,
                 this.removefield,
                 this.setfield,
@@ -498,6 +499,19 @@ export class RoSGNode extends BrsComponent implements BrsValue, BrsIterable {
         },
     });
 
+    /** Returns true if the field exists */
+    private hasfield = new Callable("hasfield", {
+        signature: {
+            args: [new StdlibArgument("fieldname", ValueKind.String)],
+            returns: ValueKind.Boolean,
+        },
+        impl: (interpreter: Interpreter, fieldname: BrsString) => {
+            return this.fields.has(fieldname.value.toLowerCase())
+                ? BrsBoolean.True
+                : BrsBoolean.False;
+        },
+    });
+
     /** Registers a callback to be executed when the value of the field changes */
     private observefield = new Callable("observefield", {
         signature: {

test/brsTypes/components/RoSGNode.test.js

@@ -441,6 +441,19 @@ describe("RoSGNode", () => {
             });
         });
 
+        describe("hasfield", () => {
+            it("returns presence of a field", () => {
+                let node = new RoSGNode([{ name: new BrsString("foo"), value: new Int32(17) }]);
+
+                let hasField = node.getMethod("hasfield");
+                let result1 = hasField.call(interpreter, new BrsString("foo"));
+                expect(result1).toEqual(new BrsBoolean(true));
+
+                let result2 = hasField.call(interpreter, new BrsString("bar"));
+                expect(result2).toEqual(new BrsBoolean(false));
+            });
+        });
+
         describe("observefield", () => {
             it("adds an observer", () => {
                 let node = new RoSGNode([

test/e2e/BrsComponents.test.js

@@ -139,6 +139,10 @@ describe("end to end brightscript functions", () => {
             "hello",
             "field3 in node now is: ",
             "false",
+            "field3 present? ",
+            "true",
+            "fieldほ present? ",
+            "false",
             "callback 1 called",
             "callback 2 called",
             "field 3 updated",

test/e2e/resources/components/roSGNode.brs

@@ -35,6 +35,9 @@ sub main()
     print "field1 in node now is: " node1.getField("field1")      ' => hello
     print "field3 in node now is: " node1.getField("field3")      ' => false
 
+    print "field3 present? " node1.hasField("field3")
+    print "fieldほ present? " node1.hasField("fieldほ")
+
     node1.observeField("field1", "onCB1Called")
     node1.observeField("field1", "onCB2Called")
     node1.observeField("field2", "onField2Cb") ' This doesn't get called since field was removed