const url = myAction();
    console.log(url);

<img src={getImagorImageUrl(imagorSecret, baseUrl, imageSrc)} />

export function GET() {
  const url = getImagorImageUrl(...)
  return fetch(url)
}

Good to know: Route Handlers are only available inside the app directory. They are the equivalent of API Routes inside the pages directory meaning you do not need to use API Routes and Route Handlers together.

import { NextApiRequest, NextApiResponse } from "next";

export default async function handler(
  req: NextApiRequest,
  res: NextApiResponse
) {

  const url = getImagorImageUrl(...)
  const result = await fetch(url);
  const buffer = Buffer.from(await result.arrayBuffer());

  res.setHeader(
    "Content-Type",
    result.headers.get("Content-Type") ?? "image/jpg"
  );
  
  return res.send(buffer);
}

async function getImagorSrc(width) {
    const response = await fetch("/api/images?src=someUrl&width=1400");
    return response.json();
}

getImagorSrc(mainWidth)
    .then((image) => {
        // how to use my result as string here?
    })

useEffect(() => {
    const mainWidth = 580;
    const srcsetWidths = [
        1440,
        1150,
        900,
        580
    ];

    async function getImagorSrc(width) {
        const response = await fetch("/api/images?src=" + node.properties.image.src + "&width=" + width);
        return response.json();
    }

    getImagorSrc(mainWidth)
        .then((image) => {
            setImgMainSrc(image)
        })

    // here comes the loop for my srcset sizes
    for (let i = 0; i < srcsetWidths.length; i++) {
        getImagorSrc(srcsetWidths[i])
            .then((image) => {
                setImgSrcset(imgSrcset + image + ' ' + srcsetWidths[i] + 'w,')
            })
        }
    }, [])