parser: simplify parsing of null, boolean and enum values (#2432)

Author: IvanGoncharov

src/language/parser.js

@@ -554,26 +554,21 @@ class Parser {
       case TokenKind.BLOCK_STRING:
         return this.parseStringLiteral();
       case TokenKind.NAME:
-        if (token.value === 'true' || token.value === 'false') {
-          this._lexer.advance();
-          return {
-            kind: Kind.BOOLEAN,
-            value: token.value === 'true',
-            loc: this.loc(token),
-          };
-        } else if (token.value === 'null') {
-          this._lexer.advance();
-          return {
-            kind: Kind.NULL,
-            loc: this.loc(token),
-          };
-        }
         this._lexer.advance();
-        return {
-          kind: Kind.ENUM,
-          value: ((token.value: any): string),
-          loc: this.loc(token),
-        };
+        switch (token.value) {
+          case 'true':
+            return { kind: Kind.BOOLEAN, value: true, loc: this.loc(token) };
+          case 'false':
+            return { kind: Kind.BOOLEAN, value: false, loc: this.loc(token) };
+          case 'null':
+            return { kind: Kind.NULL, loc: this.loc(token) };
+          default:
+            return {
+              kind: Kind.ENUM,
+              value: ((token.value: any): string),
+              loc: this.loc(token),
+            };
+        }
       case TokenKind.DOLLAR:
         if (!isConst) {
           return this.parseVariable();