Understanding Hermes V1

This Callstack live stream brought together Łukasz Chludziński, Szymon Chmal, and Matt Hargett to discuss the long-awaited Hermes V1 release. While the name might sound like a major rewrite, Hermes V1 isn’t “static Hermes.” Instead, it represents a significant improvement to the existing engine, bringing stronger language support, performance optimizations, and experimental JIT compilation capabilities.

The evolution of Hermes

Matt shared his early experience integrating Hermes at PlayStation years before it became open source. Designed to improve startup times and efficiency, Hermes focused on executing precompiled bytecode rather than interpreting JavaScript text at runtime. This shift, comparable to .NET’s early optimizations, meant faster app startup and smaller memory footprints, which became critical for React Native apps on constrained devices.

What’s new in V1

Hermes V1 introduces native support for more modern JavaScript features (let, const, async, ES classes, maps, and sets), reducing the need for polyfills and leading to smaller, faster apps. It also removes property count limits on objects and adds an experimental JIT engine. While JIT can boost performance, the team discussed how it’s not universally beneficial, particularly for battery-sensitive or lower-end devices.

Performance, JIT, and real-world trade-offs

Matt outlined the nuanced reality of runtime performance. While JIT can accelerate computation-heavy workloads, it may harm startup times and energy efficiency on mobile and TV platforms. Hermes’ design allows developers to balance speed, memory, and energy, turning features like JIT on or off depending on device class. This approach reflects the engine’s adaptability across a global ecosystem where users’ hardware varies widely, from flagship phones to low-power Fire TV sticks.

The road ahead

Static Hermes remains in development, but V1 lays the groundwork for it. With the community already benefiting from ongoing improvements in interpreter performance and memory efficiency, the team anticipates Hermes’ next phase to further unify developer experience across devices.

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