Does Vite block old browsers, like Safari 12, by using Promise.allSettled?

[kevinrenskers]

I have a SvelteKit website, which uses Vite 5. I am getting error reports in Sentry about the use of Promise.allSettled, all coming from Safari 12 / Mobile Safari 12 users, which indeed has no support for Promise.allSettled. I was surprised by this usage, as I don't use it, and Svelte and SvelteKit also don't use it.

Digging in a bit deeper it seems that this code is coming from Vite itself. Here is a screenshot of my build chunk where sure enough Promise.allSettled is being used:

Put through https://beautifier.io, the code looks like this:

const Rs = "modulepreload",
    As = function(t, e) {
        return new URL(t, e).href
    },
    qn = {},
    p = function(e, n, r) {
        let o = Promise.resolve();
        if (n && n.length > 0) {
            const i = document.getElementsByTagName("link"),
                a = document.querySelector("meta[property=csp-nonce]"),
                c = (a == null ? void 0 : a.nonce) || (a == null ? void 0 : a.getAttribute("nonce"));
            o = Promise.allSettled(n.map(u => {
                if (u = As(u, r), u in qn) return;
                qn[u] = !0;
                const l = u.endsWith(".css"),
                    d = l ? '[rel="stylesheet"]' : "";
                if (!!r)
                    for (let f = i.length - 1; f >= 0; f--) {
                        const g = i[f];
                        if (g.href === u && (!l || g.rel === "stylesheet")) return
                    } else if (document.querySelector(`link[href="${u}"]${d}`)) return;
                const m = document.createElement("link");
                if (m.rel = l ? "stylesheet" : Rs, l || (m.as = "script"), m.crossOrigin = "", m.href = u, c && m.setAttribute("nonce", c), document.head.appendChild(m), l) return new Promise((f, g) => {
                    m.addEventListener("load", f), m.addEventListener("error", () => g(new Error(`Unable to preload CSS for ${u}`)))
                })
            }))
        }

        function s(i) {
            const a = new Event("vite:preloadError", {
                cancelable: !0
            });
            if (a.payload = i, window.dispatchEvent(a), !a.defaultPrevented) throw i
        }
        return o.then(i => {
            for (const a of i || []) a.status === "rejected" && s(a.reason);
            return e().catch(s)
        })
    };

And this seems to come from Vite's client.mjs. So is Vite breaking my site in Safari 12? What can I do about this?

Sadly I cannot use plugin-legacy, as it doesn't seem to work with SvelteKit :(