v8-jit

V8 JIT Optimization

Use this skill when writing or optimizing performance-critical code paths in Next.js server internals — especially per-request hot paths like rendering, streaming, routing, and caching.

Background: V8's Tiered Compilation

V8 compiles JavaScript through multiple tiers:

1. Ignition (interpreter) — executes bytecode immediately.
2. Sparkplug — fast baseline compiler (no optimization).
3. Maglev — mid-tier optimizing compiler.
4. Turbofan — full optimizing compiler (speculative, type-feedback-driven).

Code starts in Ignition and is promoted to higher tiers based on execution frequency and collected type feedback. Turbofan produces the fastest machine code but bails out (deopts) when assumptions are violated at runtime.

The key principle: help V8 make correct speculative assumptions by keeping types, shapes, and control flow predictable.

Hidden Classes (Shapes / Maps)

Every JavaScript object has an internal "hidden class" (V8 calls it a Map, the spec calls it a Shape). Objects that share the same property names, added in the same order, share the same hidden class. This enables fast property access via inline caches.

Initialize All Properties in Constructors

// GOOD — consistent shape, single hidden class transition chain
class RequestContext {
  url: string
  method: string
  headers

Rules:

• Assign every property in the constructor, in the same order, for every instance. Use null / undefined / false as default values rather than omitting the property.
• Prefer factory functions when constructing hot-path objects. A single factory makes it harder to accidentally fork shapes in different call sites.
• Never delete a property on a hot object — it forces a transition to dictionary mode (slow properties).
• Avoid adding properties after construction (obj.newProp = x) on objects used in hot paths.
• Object literals that flow into the same function should have keys in the same order:
• Use tuples for very small fixed-size records when names are not needed. Tuples avoid key-order pitfalls entirely.

Real Codebase Example

Span in src/trace/trace.ts initializes all fields in the constructor in a fixed order — name, parentId, attrs, status, id, _start, now. This ensures all Span instances share one hidden class.

Monomorphic vs Polymorphic vs Megamorphic

V8's inline caches (ICs) track the types/shapes seen at each call site or property access:

Once an IC goes megamorphic it does NOT recover (until the function is re-compiled). Megamorphic ICs also prevent Turbofan from inlining the function.

Keep Hot Call Sites Monomorphic

Practical strategies:

• Normalize inputs at the boundary (e.g. convert Buffer → Uint8Array once) and keep internal functions monomorphic.
• Avoid passing both null and undefined for the same parameter — pick one sentinel value.
• When a function must handle multiple types, split into separate specialized functions and dispatch once at the entry point:

This is the pattern used in stream-ops.ts and throughout the stream-utils code (Node.js vs Web stream split via compile-time switcher).

Closure and Allocation Pressure

Every closure captures its enclosing scope. Creating closures in hot loops or per-request paths generates GC pressure and can prevent escape analysis.

Hoist Closures Out of Hot Paths

Avoid Allocations in Tight Loops

Real Codebase Example

node-stream-helpers.ts hoists encoder, BUFFER_TAGS, and tag constants to module scope to avoid re-creating them on every request. The bufferIndexOf helper uses Buffer.indexOf (C++ native) instead of a per-call JS loop, eliminating per-chunk allocation.

Array Optimizations

V8 tracks array "element kinds" — an internal type tag that determines how elements are stored in memory:

Transitions are one-way — once an array becomes HOLEY or PACKED_ELEMENTS, it never goes back.

Rules

• Pre-allocate arrays with known size: new Array(n) creates a holey array. Prefer [] and push(), or use Array.from({ length: n }, initFn).
• Don't create holes: arr[100] = x on an empty array creates 100 holes.
• Don't mix types: [1, 'two', {}] immediately becomes PACKED_ELEMENTS.
• Prefer typed arrays only when you need binary interop/contiguous memory or have profiling evidence that they help. For small/short-lived collections, normal arrays can be faster and allocate less.

Real Codebase Example

accumulateStreamChunks in app-render.tsx uses const staticChunks: Array<Uint8Array> = [] with push() — keeping a packed array of a single type throughout its lifetime.

Function Optimization and Deopts

Hot-Path Deopt Footguns

• arguments object: using arguments in non-trivial ways (e.g. arguments[i] with variable i, leaking arguments). Use rest params instead.
• Type instability at one call site: same operation sees both numbers and strings (or many object shapes) and becomes polymorphic/megamorphic.
• eval / with: prevents optimization entirely.
• Highly dynamic object iteration: avoid for...in on hot objects; prefer Object.keys() / Object.entries() when possible.

Favor Predictable Control Flow

Watch Shape Diversity in switch Dispatch

This pattern is not always bad. Often the main pressure is at the shared dispatch site (node.type), while properties used only in one branch stay monomorphic within that branch. Reach for normalization/splitting only when profiles show this site is hot and polymorphic.

String Operations

• String concatenation in loops is usually fine in modern V8 (ropes make many concatenations cheap). For binary data, use Buffer.concat().
• Template literals vs concatenation: equivalent performance in modern V8, but template literals are clearer.
• string.indexOf() > regex for simple substring checks.
• Reuse RegExp objects: don't create a new RegExp() inside a hot function — hoist it to module scope.

Map and Set vs Plain Objects

• Map is faster than plain objects for frequent additions/deletions (avoids hidden class transitions and dictionary mode).
• Set is faster than obj[key] = true for membership checks with dynamic keys.
• For static lookups (known keys at module load), plain objects or Object.freeze({...}) are fine — V8 optimizes them as constant.
• Never use an object as a map if keys come from user input (prototype pollution risk + megamorphic shapes).

Profiling and Verification

V8 Flags for Diagnosing JIT Issues

Targeted Profiling in Next.js

Using % Natives (Development/Testing Only)

With --allow-natives-syntax:

Checklist for Hot Path Code Reviews

• All object properties initialized in constructor/literal, same order
• No delete on hot objects
• No post-construction property additions on hot objects
• Functions receive consistent types (monomorphic call sites)
• Type dispatch happens at boundaries, not deep in hot loops
• No closures allocated inside tight loops
• Module-scope constants for regex, encoders, tag buffers
• Arrays are packed (no holes, no mixed types)
• Map/Set used for dynamic key collections
• No arguments object — use rest params
• try/catch at function boundary, not inside tight loops
• String building via array + join() or Buffer.concat()
• Return types are consistent (no string | null | undefined mixes)

Related Skills

• $dce-edge — DCE-safe require patterns (compile-time dead code)
• $runtime-debug — runtime bundle debugging and profiling workflow