diff --git a/src/parser/util.js b/src/parser/util.js
index 532e3fe7a4..891a193c25 100644
--- a/src/parser/util.js
+++ b/src/parser/util.js
@@ -1,5 +1,8 @@
-import { types as tt } from "../tokenizer/types";
+// @flow
+
+import { types as tt, type TokenType } from "../tokenizer/types";
 import Tokenizer from "../tokenizer";
+import type { Node } from "../types";
 import { lineBreak } from "../util/whitespace";
 
 // ## Parser utilities
@@ -7,7 +10,7 @@ import { lineBreak } from "../util/whitespace";
 export default class UtilParser extends Tokenizer {
   // TODO
 
-  addExtra(node, key, val) {
+  addExtra(node: Node, key: string, val: any): void {
     if (!node) return;
 
     const extra = node.extra = node.extra || {};
@@ -16,13 +19,13 @@ export default class UtilParser extends Tokenizer {
 
   // TODO
 
-  isRelational(op) {
+  isRelational(op: "<" | ">"): boolean {
     return this.match(tt.relational) && this.state.value === op;
   }
 
   // TODO
 
-  expectRelational(op) {
+  expectRelational(op: "<" | ">"): void {
     if (this.isRelational(op)) {
       this.next();
     } else {
@@ -32,25 +35,25 @@ export default class UtilParser extends Tokenizer {
 
   // Tests whether parsed token is a contextual keyword.
 
-  isContextual(name) {
+  isContextual(name: string): boolean {
     return this.match(tt.name) && this.state.value === name;
   }
 
   // Consumes contextual keyword if possible.
 
-  eatContextual(name) {
+  eatContextual(name: string): boolean {
     return this.state.value === name && this.eat(tt.name);
   }
 
   // Asserts that following token is given contextual keyword.
 
-  expectContextual(name, message) {
+  expectContextual(name: string, message?: string): void {
     if (!this.eatContextual(name)) this.unexpected(null, message);
   }
 
   // Test whether a semicolon can be inserted at the current position.
 
-  canInsertSemicolon() {
+  canInsertSemicolon(): boolean {
     return this.match(tt.eof) ||
       this.match(tt.braceR) ||
       lineBreak.test(this.input.slice(this.state.lastTokEnd, this.state.start));
@@ -58,31 +61,31 @@ export default class UtilParser extends Tokenizer {
 
   // TODO
 
-  isLineTerminator() {
+  isLineTerminator(): boolean {
     return this.eat(tt.semi) || this.canInsertSemicolon();
   }
 
   // Consume a semicolon, or, failing that, see if we are allowed to
   // pretend that there is a semicolon at this position.
 
-  semicolon() {
+  semicolon(): void {
     if (!this.isLineTerminator()) this.unexpected(null, tt.semi);
   }
 
   // Expect a token of a given type. If found, consume it, otherwise,
   // raise an unexpected token error at given pos.
 
-  expect(type, pos) {
-    return this.eat(type) || this.unexpected(pos, type);
+  expect(type: TokenType, pos?: ?number): void {
+    this.eat(type) || this.unexpected(pos, type);
   }
 
   // Raise an unexpected token error. Can take the expected token type
   // instead of a message string.
 
-  unexpected(pos, messageOrType = "Unexpected token") {
+  unexpected(pos: ?number, messageOrType: string | TokenType = "Unexpected token"): empty {
     if (messageOrType && typeof messageOrType === "object" && messageOrType.label) {
       messageOrType = `Unexpected token, expected ${messageOrType.label}`;
     }
-    this.raise(pos != null ? pos : this.state.start, messageOrType);
+    throw this.raise(pos != null ? pos : this.state.start, messageOrType);
   }
 }