Parser: Add 'skipKeyword' function (#1545)

based on 'expectOptionalKeyword' from #1541

Author: IvanGoncharov

src/language/parser.js

@@ -465,22 +465,19 @@ function parseFragment(
 ): FragmentSpreadNode | InlineFragmentNode {
   const start = lexer.token;
   expect(lexer, TokenKind.SPREAD);
-  if (peek(lexer, TokenKind.NAME) && lexer.token.value !== 'on') {
+
+  const hasTypeCondition = skipKeyword(lexer, 'on');
+  if (!hasTypeCondition && peek(lexer, TokenKind.NAME)) {
     return {
       kind: Kind.FRAGMENT_SPREAD,
       name: parseFragmentName(lexer),
       directives: parseDirectives(lexer, false),
       loc: loc(lexer, start),
     };
   }
-  let typeCondition;
-  if (lexer.token.value === 'on') {
-    lexer.advance();
-    typeCondition = parseNamedType(lexer);
-  }
   return {
     kind: Kind.INLINE_FRAGMENT,
-    typeCondition,
+    typeCondition: hasTypeCondition ? parseNamedType(lexer) : undefined,
     directives: parseDirectives(lexer, false),
     selectionSet: parseSelectionSet(lexer),
     loc: loc(lexer, start),
@@ -889,8 +886,7 @@ function parseObjectTypeDefinition(lexer: Lexer<*>): ObjectTypeDefinitionNode {
  */
 function parseImplementsInterfaces(lexer: Lexer<*>): Array<NamedTypeNode> {
   const types = [];
-  if (lexer.token.value === 'implements') {
-    lexer.advance();
+  if (skipKeyword(lexer, 'implements')) {
     // Optional leading ampersand
     skip(lexer, TokenKind.AMP);
     do {
@@ -1464,11 +1460,11 @@ function peek(lexer: Lexer<*>, kind: TokenKindEnum): boolean {
  * the lexer. Otherwise, do not change the parser state and return false.
  */
 function skip(lexer: Lexer<*>, kind: TokenKindEnum): boolean {
-  const match = lexer.token.kind === kind;
-  if (match) {
+  if (lexer.token.kind === kind) {
     lexer.advance();
+    return true;
   }
-  return match;
+  return false;
 }
 
 /**
@@ -1489,21 +1485,31 @@ function expect(lexer: Lexer<*>, kind: TokenKindEnum): Token {
 }
 
 /**
- * If the next token is a keyword with the given value, return that token after
- * advancing the lexer. Otherwise, do not change the parser state and return
- * false.
+ * If the next token is a keyword with the given value, return true after advancing
+ * the lexer. Otherwise, do not change the parser state and return false.
  */
-function expectKeyword(lexer: Lexer<*>, value: string): Token {
+function skipKeyword(lexer: Lexer<*>, value: string): boolean {
   const token = lexer.token;
   if (token.kind === TokenKind.NAME && token.value === value) {
     lexer.advance();
-    return token;
+    return true;
+  }
+  return false;
+}
+
+/**
+ * If the next token is a keyword with the given value, return that token after
+ * advancing the lexer. Otherwise, do not change the parser state and throw
+ * an error.
+ */
+function expectKeyword(lexer: Lexer<*>, value: string): void {
+  if (!skipKeyword(lexer, value)) {
+    throw syntaxError(
+      lexer.source,
+      lexer.token.start,
+      `Expected "${value}", found ${getTokenDesc(lexer.token)}`,
+    );
   }
-  throw syntaxError(
-    lexer.source,
-    token.start,
-    `Expected "${value}", found ${getTokenDesc(token)}`,
-  );
 }
 
 /**