From the Alaska Airlines blog:
Alaska Airlines and Japan Airlines (JAL) are expanding their partnership with additional codeshare flights that include former Virgin America routes, such as San Francisco to Boston, to Denver and to Newark.
Japan Airlines will start selling tickets for the new codeshare flights on Feb. 6 for travel after May 20, 2018.
Alaska and JAL originally teamed up in June 2016 with a codeshare agreement and frequent-flyer partnership, providing seamless travel and mileage earning opportunities between Alaska’s gateways in North America, Japan and throughout Asia. For members of Alaska’s Mileage Plan, base miles earned and class of service bonuses on Japan Airlines count toward Mileage Plan elite status.
JAL operates a modern fleet of Boeing 777-300ERs and 787s between Japan and North America.