Comment by olliej

No.

The runtime is the combination of the execution engine, the garbage collector, and the environment.

V8, JSC, SpiderMonkey, LibJS, etc all provide all of those.

They all have APIs that let you (the embedder) a few new objects and functions, as it would be a useless embedding API otherwise. But that's all you're doing: adding some glorified callbacks to an existing runtime. You are in no way "rolling your own JS runtime".

To break it down:

Execution Engine: the part of the runtime that evaluates JS, an interpreter, jit, or some combination. That would be "Ignition" and "TurboFan" in V8, "LLInt" and "FTL" in JSC, "WarpMonkey" in SpiderMonkey.

Garbage Collector: the part of the runtime that supports object allocation and reclamation. "Orinoco" in V8, but not sure if given a separate marketing name in other runtimes.

The environment: this is all the builtin objects and functionality, things like the global object, the regex engine (note it's not a regex runtime because all it does find the start and stop sections of matches, the embedding environment is responsible for everything else), all those core things like the Object, Array, Math, Number, etc types, and all of their implementations and runtime functions. The Execution engine does not need any of those to be implemented, as to the engine there is essentially no distinction between those builtin things and anything else written in JS.

When you embed V8, JSC, LibJS, etc you are getting a full runtime, that can do a huge amount. You _might_ choose to use there APIs to add some new objects or or functions, but what you are doing is negligible, and certainly not "a runtime".