Add atob, btoa, structuredClone, queueMicrotask and performance

[bnoordhuis]

Continuing from saghul/txiki.js#418:

To avoid duplication across embedders, implement atob, btoa, structuredClone¹, queueMicrotask and performance as quickjs-ng C APIs and/or JS globals. We also have to add DOMException but that seems okay to me. We can make them opt-in when the JSContext is created.

¹ TBD how to handle transferables

• globalThis.atob()
• globalThis.btoa()
• globalThis.performance.now()
• globalThis.performance.timeOrigin
• globalThis.queueMicrotask()
• globalThis.structuredClone()

[saghul]

Doesn't the performance.now put us into a position where we need to start branching out to platform specific code?

Is there a minimal implementation we could support and then decide to use it at context creation time with flags perhaps?

[M0n7y5]

Perhaps you can fetch counter from processor? Maybe RDTSC on X86 would help? That would be compiler specific which is not that bad imho.

[M0n7y5]

Nwm it needs TSC which is platform specific. That means it belongs to std runtime implementation.

[bnoordhuis]

Node.js uses uv_hrtime() and that's basically a wrapper around clock_gettime(CLOCK_MONOTONIC) or QueryPerformanceCounter(). Seems okay to me to use that here as well.

[saghul]

Yep I had that in mind too. Does also work on macOS and BSDs? I haven't checked the implementation in a while :-)

If it's simple let's do it !

[bnoordhuis]

Even macOS has clock_gettime as of 10.12 so I don't think there are any blockers.

[saghul]

Awesome!

[M0n7y5]

Yeah but clock_gettime is not everywhere just like QueryPerformanceCounter(). I suggest to not make performance or any other platform specific things to be part of the core library. Sort of extension to standard lib is more useful. With proper defines which allows you to opt-out of these things if needed.

[saghul]

Why not? Implementing Atomics already requires some if-defing around, and if all we need is to support 2 implementations of performance.now, that's a drop in a bucket compared to the size of quickjs.c.

[bnoordhuis]

clock_gettime is not everywhere

Where is it missing? Linux, macOS, (Free|Net|Open|Dragonfly)BSD, Solaris, Fuchsia, QNX and Emscripten all have it.

[saghul]

Brain dump, since I've been thinking about these a bit the past week.

Both atob/btoa and structuredClone depend on DOMException for error reporting, and if we are to do it we might as well make it compliant, so we should have DOMException I guess.

I noticed no other engine provides structuredClone, it seems to always be implemented by the embedder for some reason. If we can do it, then why not? (IMHO).

Last, atob/btoa require a base64 encoder/decoder, which we will also need to add for https://github.com/tc39/proposal-arraybuffer-base64 so I guess it ties it up nicely...