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 | 7 | 6 | 0 | 1 | 13 | 2 | 11 | 18 |
| 2 | Manchester CityMan City | 7 | 5 | 2 | 0 | 17 | 8 | 9 | 17 |
| 3 | Arsenal | 7 | 5 | 2 | 0 | 15 | 6 | 9 | 17 |
| 4 | Chelsea | 7 | 4 | 2 | 1 | 16 | 8 | 8 | 14 |
| 5 | Aston Villa | 7 | 4 | 2 | 1 | 12 | 9 | 3 | 14 |
| 6 | Brighton & Hove AlbionBrighton | 7 | 3 | 3 | 1 | 13 | 10 | 3 | 12 |
| 7 | Newcastle UnitedNewcastle | 7 | 3 | 3 | 1 | 8 | 7 | 1 | 12 |
| 8 | Fulham | 7 | 3 | 2 | 2 | 10 | 8 | 2 | 11 |
| 9 | Tottenham HotspurSpurs | 7 | 3 | 1 | 3 | 14 | 8 | 6 | 10 |
| 10 | Nottingham ForestNott'm Forest | 7 | 2 | 4 | 1 | 7 | 6 | 1 | 10 |
| 11 | Brentford | 7 | 3 | 1 | 3 | 13 | 13 | 0 | 10 |
| 12 | West Ham UnitedWest Ham | 7 | 2 | 2 | 3 | 10 | 11 | -1 | 8 |
| 13 | Bournemouth | 7 | 2 | 2 | 3 | 8 | 10 | -2 | 8 |
| 14 | Manchester UnitedMan Utd | 7 | 2 | 2 | 3 | 5 | 8 | -3 | 8 |
| 15 | Leicester CityLeicester | 7 | 1 | 3 | 3 | 9 | 12 | -3 | 6 |
| 16 | Everton | 7 | 1 | 2 | 4 | 7 | 15 | -8 | 5 |
| 17 | Ipswich TownIpswich | 7 | 0 | 4 | 3 | 6 | 14 | -8 | 4 |
| 18 | Crystal Palace | 7 | 0 | 3 | 4 | 5 | 10 | -5 | 3 |
| 19 | Southampton | 7 | 0 | 1 | 6 | 4 | 15 | -11 | 1 |
| 20 | Wolverhampton WanderersWolves | 7 | 0 | 1 | 6 | 9 | 21 | -12 | 1 |
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