fix `NaN` is lexed as identifier, not as a number (#397)

close #393

boa/src/syntax/lexer/mod.rs

@@ -592,6 +592,7 @@ impl<'a> Lexer<'a> {
                         "true" => TokenKind::BooleanLiteral(true),
                         "false" => TokenKind::BooleanLiteral(false),
                         "null" => TokenKind::NullLiteral,
+                        "NaN" => TokenKind::NumericLiteral(NumericLiteral::Rational(f64::NAN)),
                         slice => {
                             if let Ok(keyword) = FromStr::from_str(slice) {
                                 TokenKind::Keyword(keyword)

boa/src/syntax/lexer/tests.rs

@@ -369,6 +369,19 @@ fn check_decrement_advances_lexer_2_places() {
     );
 }
 
+#[test]
+fn check_nan() {
+    let mut lexer = Lexer::new("let a = NaN;");
+    lexer.lex().expect("failed to lex");
+
+    match lexer.tokens[3].kind {
+        TokenKind::NumericLiteral(NumericLiteral::Rational(a)) => {
+            assert_eq!(a.is_nan(), true);
+        }
+        _ => assert!(false),
+    }
+}
+
 #[test]
 fn numbers() {
     let mut lexer = Lexer::new(