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DCE + Edge

Use this skill when changing conditional require() paths, Node-only imports, or edge/runtime branching.

DCE-Safe require() Pattern

Webpack only DCEs a require() when it sits inside the dead branch of an if/else whose condition DefinePlugin can evaluate at compile time.

// CORRECT - webpack can eliminate the dead branch
if (process.env.__NEXT_USE_NODE_STREAMS) {
  require('node:stream')
} else {
  // web path
}

What does NOT work:

• Early-return/throw guards : webpack doesn't do control-flow analysis for throws/returns, so the require() is still traced.
• Bareif without else: works for inline node:* specifiers but NOT for require('./some-module') that pulls a new file into the module graph.

Always test edge changes with pnpm test-start-webpack on test/e2e/app-dir/app/standalone.test.ts (has edge routes), not with NEXT_SKIP_ISOLATE=1 which skips the full webpack compilation.

TypeScript + DCE Interaction

Use if/else (not two independent if blocks) when assigning a variable conditionally on process.env.X. TypeScript cannot prove exhaustiveness across if (flag) { x = a }; if (!flag) { x = b } and will error with "variable used before being assigned". The if/else pattern satisfies both TypeScript (definite assignment) and webpack DCE.

Compile-Time Switcher Pattern

Platform-specific code (node vs web) can use a single .ts switcher module that conditionally require()s either .node.ts or .web.ts into a typed variable, then re-exports the shared runtime API as named exports. Keep the branch as if/else so DefinePlugin can dead-code-eliminate the unused require(). Keep shared types canonical in .node.ts, with .web.ts importing them via import type and the switcher re-exporting types as needed. Examples: stream-ops.ts and debug-channel-server.ts.

NEXT_RUNTIME Is Not a Feature Flag

In user-project webpack server compilers, process.env.NEXT_RUNTIME is inlined to 'nodejs'. Guarding Node-only require('node:*') paths with NEXT_RUNTIME === 'nodejs' does not prune anything. For feature-gated codepaths, guard on the real feature define (e.g. process.env.__NEXT_USE_NODE_STREAMS).

Edge Runtime Constraints

Edge routes do NOT use pre-compiled runtime bundles. They are compiled by the user's webpack/Turbopack, so define-env.ts controls DCE. Feature flags that gate node:* imports must be forced to false for edge builds in define-env.ts (isEdgeServer ? false : flagValue), otherwise webpack will try to resolve node:stream etc. and fail.

app-page.ts Template Gotchas

• app-page.ts is a build template compiled by the user's bundler. Any require() in this file is traced by webpack/turbopack at next build time. You cannot require internal modules with relative paths because they won't be resolvable from the user's project. Instead, export new helpers from entry-base.ts and access them via entryBase.* in the template.
• Template helpers should stay out of RenderResult. If app-page.ts needs a Node-stream-only utility, prefer a small dedicated helper module in server/stream-utils/ (with DCE-safe if/else + require()).

Verification

• Validate edge bundling regressions with pnpm test-start-webpack test/e2e/app-dir/app/standalone.test.ts
• For module-resolution/build-graph fixes, verify without NEXT_SKIP_ISOLATE=1

Related Skills

• $flags - flag wiring (config/schema/define-env/runtime env)
• $react-vendoring - entry-base boundaries and vendored React
• $runtime-debug - reproduction and verification workflow

Weekly Installs

277

Repository

vercel/next.js

GitHub Stars

138.5K

First Seen

Feb 17, 2026

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