Node.js 27 Halves Default Heap Limit, Counts Buffers in Heap

Heap Limit Calculation Changed

Starting with Node.js 27, if --max-old-space-size is not explicitly set, the default heap limit equals half of the system's physical memory, clamped between 256 MB and 4 GB. For example:

4 GB machine → 2 GB limit

2 GB machine → 1 GB limit

64 GB server → 4 GB limit (capped)

The intent is to prevent Node from consuming all memory on small machines and from growing unbounded on large ones. The trade‑off is that deployments that never set this flag will see their usable heap capacity change silently.

Official migration advice: applications that relied on the old default should explicitly set --max-old-space-size. To verify the active limit at runtime, print v8.getHeapStatistics().heap_size_limit (value in bytes).

# Don't rely on the default value
node --max-old-space-size=4096 server.js

Buffer Memory Now Counted in Heap

More critically, Buffer and ArrayBuffer instance memory is now included in the heap limit. Previously, a service holding 500 MB of JavaScript objects and 3 GB of Buffer data would only report ~500 MB heapUsed; the 3 GB was off‑heap and invisible to monitoring. After upgrading to Node.js 27, that 3 GB counts toward the heap limit, so the same service may immediately hit the ceiling and throw JavaScript heap out of memory.

Workloads that process files, transcode images, or buffer large logs are especially affected. The article recommends checking peak Buffer usage before upgrading.

Other Changes

V8 15.6 – enables Math.sumPrecise(), Iterator.zip(), Iterator.prototype.join() and three other features by default; import defer and immutable ArrayBuffer still require flags.

npm 12.1.0 bundled.

fs.glob – rewritten as a core built‑in, no longer depends on the external glob package; bulk file matching in large repositories is faster.

http.keepAliveTimeout default raised from 5 seconds to 65 seconds. Longer connection reuse increases idle‑connection resource usage; if a front‑end load balancer uses a 60‑second timeout, the backend may still be waiting on a connection the front end has already closed.

Node.js 27 is currently in alpha; the stable release is expected around April next year.