date-fns | How do I format to UTC

I would suggest using the built-in Date util:

const date = new Date("2019-10-25T08:10:00Z");
const isoDate = date.toISOString();

console.log(`${isoDate.substr(0, 10)} ${isoDate.substr(11, 8)}`);

Outputs:

2019-10-25 08:10:00

Not a general solution for any format, but no external libraries required.

You were almost there. This works for me:

import { parseISO } from "date-fns";
import { format, utcToZonedTime } from "date-fns-tz";

const time = "2019-10-25T08:10:00Z";

const parsedTime = parseISO(time);
console.log(parsedTime); // 2019-10-25T08:10:00.000Z

const formatInTimeZone = (date, fmt, tz) =>
  format(utcToZonedTime(date, tz), 
         fmt, 
         { timeZone: tz });

const formattedTime = formatInTimeZone(parsedTime, "yyyy-MM-dd kk:mm:ss xxx", "UTC");
console.log(formattedTime); // 2019-10-25 08:10:00 +00:00

Behind the scenes

The date-fns[-tz] libraries stick to the built-in Date data type that carries no TZ info.
Some functions treat it as a moment-in-time, but some like format treat it more like a struct of calendaric components — year 2019, ..., day 25, hour 08, ....

Now the trouble is a Date is internally only a moment in time. Its methods provide a mapping to/from calendaric components in local time zone.

So to represent a different time zone, date-fns-tz/utcToZonedTime temporarily produces Date instances which represent the wrong moment in time — just to get its calendaric components in local time to be what we want!

And the date-fns-tz/format function's timeZone input affects only the template chars that print the time zone (XX..X, xx..x, zz..z, OO..O).

See https://github.com/marnusw/date-fns-tz/issues/36 for some discussion of this "shifting" technique (and of real use cases that motivated them)...
It's a bit low-level & risky, but the specific way I composed them above — formatInTimeZone() — is I believe a safe recipe.

Note
The following solution will not work for all time zones, so if timezone accuracy is critical for your application you might want to try something like the answer from Beni. See this link for more info

I had the exact same question today and did some research to see if anyone has come up with anything better since this question was posed. I came across this solution which fit my needs and stylistic preference:

import { format, addMinutes } from 'date-fns';

function formatDate(date) {
  return format(addMinutes(date, date.getTimezoneOffset()), 'yyyy-MM-dd HH:mm:ss');
}

Explanation

getTimezoneOffset returns the number of minutes needed to convert that date to UTC. In PST (-0800 hours) it would return 480 whereas for somebody on CST (+0800 hours) it would return -480.

I had the same problem. What I do is remove the timezone from the ISO string and then use that time with date-fns:

let time = "2019-10-25T08:10:00Z".slice(0, -1)

The above is a time with no time zone, and because there is no timezone date-fns assumes the local timezone, so when you do:

format(parseISO(time), 'h:mm a')

you get: 8:10 AM, or whatever format you prefer. You just have to be careful with the string that you are slicing. If its always the same format then it should work.