Override baseBranch via env or as a CLI flag

[grikomsn]

Allowing to override baseBranch via environment variables or as CLI flags could be beneficial for various edge cases.

One example is a Next.js app deployed on Vercel with two branches assigned to different domains and environment variables (e.g. example.com for main, and dev.example.com for dev). If baseBranch is set to origin/main and remote caching is enabled, dev branch builds will use the main branch artifacts/caches, which it should not.

I may misunderstood how baseBranch works on the documentation, but from my understanding "it defaults to the main branch" which I don't know if it automatically assumes origin/main or resolving the current project git branch.

The hacky solution that I can think of is to rewrite the baseBranch property on package.json before Turborepo runs:

// update-branch.js

const fs = require("fs");
const path = require("path");
const packageJson = require("../package.json");

packageJson.turbo.baseBranch = process.env.VERCEL_GIT_COMMIT_REF || process.env.TURBO_BASE_BRANCH || "origin/main";
fs.writeFileSync(path.join(__dirname, "../package.json"), `${JSON.stringify(packageJson, null, 2)}\n`);

// then run node update-branch && yarn turbo run build

If theoretically using an environment variable or as a CLI flag is possible, it might look something like this:

# using cli flag
turbo run build --baseBranch="origin/dev"

# using environment variable
TURBO_BASE_BRANCH="origin/dev" turbo run build

I think it could be more than this, but as for the example use case it does fit and currently being deployed on production (and dev). What do y'all think? 😄