Function expect

packages/babel-parser/src/parser/util.ts:155–159

(type: TokenType, loc?: Position | null)

  // raise an unexpected token error at given pos.

  expect(type: TokenType, loc?: Position | null): void {
    if (!this.eat(type)) {
      this.unexpected(loc, type);
    }
  }

  // tryParse will clone parser state.
  // It is expensive and should be used with cautions