Prefetch returns `undefined` on slow connections

[sarahdayan]

Describe the bug

I'm running an app that uses prefetch while throttling the network ("Slow 3G" mode on Chrome). When doing so, prefetch(url) returns undefined instead of a Promise.

When the network is throttled on "Slow 3G", the output of navigator.connection is as follows:

navigator.connection.effectiveType // "2g"
navigator.connection.saveData // false

Given the precondition in the prefetch function, the function should return a rejected promise. However, it looks like in the production bundle, both Promise.reject statements are gone.

Here's a (beautified) snapshot of the relevant code chunk in the shipped quicklink.js from my node_modules:

function i(e, r, i) {
  if (!(i = navigator.connection) || !i.saveData && !/2g/.test(i.effectiveType)) return Promise.all([].concat(e).map(function(e) {
    if (!o.has(e)) return o.add(e), (r ? function(e) {
      return window.fetch ? fetch(e, {
        credentials: "include"
      }) : n(e)
    } : t)(new URL(e, location.href).toString())
  }))
}

So, in my case described above, we're not going into that if statement, and the function implicitly returns undefined. When chaining prefetch(url) with catch, this raises a runtime error.

I saw that the library uses microbundle, which I'm not familiar with, so I'm not aware of stripping behavior. At first glance, no apparent options seem to remove such code.

To Reproduce

1. Go to this sandbox on Chrome (or a browser that supports the Network Information API and lets you throttle the network).
2. Switch the connection to "Slow 3G" and refresh.
3. See the logged undefined in the browser console.

Expected behavior

I'd expect prefetch always to return a Promise, as documented.

Version:

• macOS Catalina 10.15.7
• Chrome 85.0.4183.121 with throttled network ("Slow 3G")

Additional context, screenshots, screencasts

Here's a screenshot of the linked sandbox.

Once the root cause of the missing code is identified, it could be a nice improvement to also run tests in a slow environment. Since the project already uses Puppeteer, it should be able to interact with Chrome DevTool's Network.emulateNetworkConditions and change the latency and network throughput.

[akhmadshin]

If you throttle network to slow 3g, window.navigator.connection.effectiveType === '2g' And because of it prefetch function returns undefind (return; === return undefind;). Probably they should return Promise.resolve(undefind) https://github.com/GoogleChromeLabs/quicklink/blob/453a661fa1fa940e2d2e044452398e38c67a98fb/src/index.mjs#L117