It'd be great if we could js_set an async function, not just sync

[dstromberg]

Is your feature request related to a problem? Please describe

A clear and concise description of what the problem is.
Yes. If I try to crypto.subtle.digest("SHA-512", msgUint8), that means async is required. But if I try to call it from js_set, then sync is required. TTBOMK, that's an inherent conflict that keeps js_set from being used with with crypto.subtle.digest.

Describe the solution you'd like

js_set'ing an async function should "do the right thing", instead of returning a string that vaguely describes the Promise.

Describe alternatives you've considered

A clear and concise description of any alternative solutions or features you've considered.
I tried a few ways of turning the async crypto.subtle.digest into something synchronous, but none of them worked. The general Javascript community seems hostile to the idea.
I also tried to find a synchronous SHA-512 module, but found that none of the ones I tried were "compatible enough" with njs.

Additional context

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[xeioex]

Hi @dstromberg,

See r.setReturnValue() for this.

For example:

http {
    js_import http.js;

    js_set $hash    http.hash;

    server {
        listen 8000;

        location = /crypto {
            add_header Hash $hash;
            return     200;
        }
    }
}

async function hash(r) {
    let hash = await crypto.subtle.digest('SHA-512', r.headersIn.host);
    r.setReturnValue(Buffer.from(hash).toString('hex'));
}

export default {hash};

[dstromberg]

r.setReturnValue() helped in one case, and didn't in another. I'm going to open a new ticket about the one that doesn't work.