The west face of Junction Pass is easily approached from the vicinity of Forester Pass; the only barrier to this easy route is the long approach to Forester Pass. Unless you’re already planning to be crossing Forester Pass on the JMT, consider using one of the routes on the eastern or southern side of Junction that, although harder, are much closer. However, Junction Peak makes an excellent bonus climb (~600 feet higher) if you’re on Forester Pass anyway. The closest trailheads to the JMT in the vicinity of Forester Pass are Onion Valley (via Kearsarge Pass) from the north or Symmes Creek (over Shepherd Pass) from the south, both accessed from the town of Independence. A crosscountry route over University Pass is shorter but not necessarily faster.
The easiest approach to the west face is from the top of Forester Pass simply because most of the elevation gain is accomplished via the JMT. You can either climb the intermediate peak that lies between Forester Pass and Junction Peak or traverse its south face. As an alternative, consider taking your backpack with you on the traverse if you are (1) crossing the pass from north to south, (2) have either experienced the excellent south side of the JMT trail descending from the pass or don’t care, and (3) feel comfortable crossing or climbing a class 2 route (the intermediate peak) with your pack. The traverse ends and the Junction Peak west face route begins at the top of Ski Mountaineer's Pass, which sports an easy class 1 scree south face. By carrying your backpack here and dropping it before the climb, you can save at least 30 minutes of the traverse time back and the descent of the JMT by descending the scree slope.
The entire route from Ski Mountaineer's Pass to the summit
From the top of Ski Mountaineer's Pass, the route is straight forward and quick. Climb into the first chute and cross over the adjoining chutes until you find yourself in the main chute that leads to the west side of the peak. Stay left while you ascend this chute to avoid the very steep glaciated rock in the center and then scramble to the top. The round trip should take you about an hour from Ski Mountaineer's and maybe double that if starting at and returning to Forester Pass.
Early in the season, ice ax and crampons may be needed for the route and the approach.
External LinksDay 15 on my JMT trip
Several Climber.org trip reports on the west face