The 19-year-old winger has appeared three times for the West London side since breaking into the first team towards the end of last season.
"Josh has made significant progress in a short period of time," Bees manager Mark Warburton told the club website.
Brentford earned promotion to the second tier by finishing second in League One in 2013-14.
Restore Data
| Team | P | W | D | L | F | A | GD | PTS | |
| 1 | Liverpool | 14 | 11 | 2 | 1 | 29 | 11 | 18 | 35 |
| 2 | Chelsea | 15 | 9 | 4 | 2 | 35 | 18 | 17 | 31 |
| 3 | Arsenal | 15 | 8 | 5 | 2 | 29 | 15 | 14 | 29 |
| 4 | Manchester CityMan City | 15 | 8 | 3 | 4 | 27 | 21 | 6 | 27 |
| 5 | Nottingham ForestNott'm Forest | 15 | 7 | 4 | 4 | 19 | 18 | 1 | 25 |
| 6 | Aston Villa | 15 | 7 | 4 | 4 | 23 | 23 | 0 | 25 |
| 7 | Brighton & Hove AlbionBrighton | 15 | 6 | 6 | 3 | 25 | 22 | 3 | 24 |
| 8 | Bournemouth | 15 | 7 | 3 | 5 | 23 | 20 | 3 | 24 |
| 9 | Brentford | 15 | 7 | 2 | 6 | 31 | 28 | 3 | 23 |
| 10 | Fulham | 15 | 6 | 5 | 4 | 22 | 20 | 2 | 23 |
| 11 | Tottenham HotspurSpurs | 15 | 6 | 2 | 7 | 31 | 19 | 12 | 20 |
| 12 | Newcastle UnitedNewcastle | 15 | 5 | 5 | 5 | 19 | 21 | -2 | 20 |
| 13 | Manchester UnitedMan Utd | 15 | 5 | 4 | 6 | 19 | 18 | 1 | 19 |
| 14 | West Ham UnitedWest Ham | 15 | 5 | 3 | 7 | 20 | 28 | -8 | 18 |
| 15 | Everton | 14 | 3 | 5 | 6 | 14 | 21 | -7 | 14 |
| 16 | Leicester CityLeicester | 15 | 3 | 5 | 7 | 21 | 30 | -9 | 14 |
| 17 | Crystal Palace | 15 | 2 | 7 | 6 | 14 | 20 | -6 | 13 |
| 18 | Ipswich TownIpswich | 15 | 1 | 6 | 8 | 14 | 27 | -13 | 9 |
| 19 | Wolverhampton WanderersWolves | 15 | 2 | 3 | 10 | 23 | 38 | -15 | 9 |
| 20 | Southampton | 15 | 1 | 2 | 12 | 11 | 31 | -20 | 5 |
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