Bun 1.3.9: Parallel Script Execution and JavaScriptCore’s SIMD Regex Revolution

Bun’s latest release packs serious developer experience improvements alongside deep engine-level optimizations. The headliner is bun run --parallel—finally, a native way to run multiple package.json scripts concurrently without reaching for third-party tools.

The Core Insight

The new --parallel and --sequential flags transform how developers work with monorepos and multi-script workflows:

# Run build and test concurrently
bun run --parallel build test

# Glob-matched script names  
bun run --parallel "build:*"

# All workspace packages in parallel
bun run --parallel --filter '*' build

# Sequential with failure handling
bun run --sequential --no-exit-on-error --workspaces test

Each script gets color-coded, padded output prefixes so you can trace what came from where:

build | compiling...
test  | running suite...
lint  | checking files...

With --filter or --workspaces, package names appear too:

pkg-a:build | compiling...
pkg-b:build | compiling...

The key distinction from --filter: parallel mode doesn’t respect dependency order. If package B depends on package A, --filter waits for A to complete first. --parallel fires everything immediately. This makes it perfect for long-running watch scripts that would otherwise block the dependency chain.

Why This Matters

Beyond the ergonomic improvements, this release includes significant JavaScriptCore upgrades with real-world performance implications:

SIMD-Accelerated Regex: Regular expressions with alternatives like /aaaa|bbbb/ now use SIMD instructions to scan 16 bytes at a time, rejecting non-matching positions before falling back to scalar matching. This is implemented for both ARM64 and x86_64.

JIT-Compiled Fixed-Count Patterns: Non-capturing groups with quantifiers like (?:abc){3} previously fell back to the slower interpreter. Now they’re JIT-compiled, yielding ~3.9x speedups.

Optimized Built-ins:
– String.prototype.startsWith: Up to 5.76x faster with constant folding
– Set.size / Map.size: 2.24x – 2.74x faster via DFG/FTL intrinsics
– String.prototype.trim: 1.1x – 1.42x faster with direct pointer access
– String.prototype.replace: Returns rope structures instead of eager copies

Markdown Performance: Bun.Markdown now uses SIMD-accelerated scanning for HTML escaping, plus cached tag strings in the React renderer. Result: 7-28% faster rendering depending on document size.

The HTTP/2 connection upgrade fix enables patterns used by libraries like http2-wrapper and crawlee—forwarding raw TCP connections from a net.Server to an Http2SecureServer now works correctly.

Key Takeaways

• bun run --parallel: Run multiple scripts concurrently with prefixed output
• bun run --sequential: Run scripts in order with the same output formatting
• Workspace integration: Works seamlessly with --filter and --workspaces
• --no-exit-on-error: Continue running even if one script fails
• mock() / spyOn() Symbol.dispose: Use using keyword for automatic mock cleanup
• NO_PROXY now respected: Works even with explicit proxy options
• --cpu-prof-interval: Configure CPU profiler sampling in microseconds
• ESM bytecode in --compile: --bytecode now works with --format=esm
• ARM64 fix: No more SIGILL crashes on ARMv8.0 CPUs (Raspberry Pi 4, AWS a1)

Looking Ahead

The JavaScriptCore upgrades demonstrate Bun’s commitment to engine-level performance. While Node.js inherits V8’s optimizations, Bun is actively contributing upstream improvements to WebKit/JSC. The regex SIMD work in particular shows meaningful investment in making JavaScript faster at the foundation level.

The --parallel flag addresses a gap that has sent many developers to tools like concurrently or npm-run-all. Having it built into the runtime with first-class workspace support makes monorepo development significantly smoother.

For teams evaluating Bun for production workloads, this release adds stability (the HTTP/2 fix, ARM64 crash fix) alongside developer experience improvements. The TypeScript type fixes are also worth noting if you’ve hit confusing errors with Bun.Build.CompileTarget.

Based on analysis of Bun v1.3.9