The 31-year-old was walking home from shops in Livingston when he approached the youths, thought to be aged around 17 or 18, and asked them to be quiet.
They set upon him, leaving him lying unconscious at the roadside with a broken elbow and bruising.
The gang fled, leaving their victim lying unconscious until he was found by passers-by. They called an ambulance to take him to St John's Hospital.
Police are appealing for witnesses to the attack, which took place between 7pm and 7:30pm on Wednesday at the underpass between Morrisons at Carmondean and Fells Rigg.
Detective Constable Mark Forrest of Livingston CID said: "It was a horrific attack on an innocent member of the public who's just been walking past, and spoken to these youths and has been assaulted.
"One had a bat and one a golf club, and that's the last he remembers until regaining consciousness when a passer-by was coming from the shops, came across him and phoned the ambulance.
"Two of them assaulted him, but there were a few youths there at the time, although we don't know exactly how many at the moment. If there's any information from anyone who's been in the area we need them to come forward."
DC Forrest said there was no CCTV in the area, but officers hoped they would find witnesses who can help them.
The victim, who does not want to be named, was yesterday transferred to the ERI for surgery to his broken elbow, and was said to be badly shaken by the attack. Police were waiting to interview him as he recovered from surgery yesterday evening.
Livingston North Councillor Bruce Ferrie said: "People in the area are appalled at such a vicious, mindless attack. Youths carrying baseball bats and golf clubs can't be tolerated.
"I've spoken to a number of people in the area and said to them, 'If you speak to someone who witnessed the attack, get them to go to the police. You can remain anonymous'.
"The whole community in this area is just appalled that they would knock a man unconscious and leave him.
"It was an appalling, mindless act and it can't be tolerated."
Anyone with information should call Lothian and Borders Police on 0131-311 3131 or Crimestoppers in confidence and complete anonymity on 0800 555 111.