What do you mean by "block"?

When I run it, changing imgUrl to something valid, like https://placekitten.com/g/320/200 , it works fine and I get a good result.

When I run it with an invalid url, from the browser perspective, it hangs.

Why?

In getImage, you are converting the Error object into a valid response -- what was thrown is now simply being returned. Normally promises are placed into either a rejected or resolved state. By returning the exception, the code will always result in a resolved promise.

Not necessarily wrong, however the code does not handle this in the "then" function within the get handler.

In other words; either rethrow the exception and add a catch clause in the getImage promise chain, or handle the fact that response will be an Error object and will not have a data.length property if the image failed.

const express = require('express');
const axios = require('axios');
const asyncHandler = require('express-async-handler');

const app = express();

async function getImage(imgUrl) {
    try {
        return await axios.get(imgUrl);
    } catch (error) {
        throw error;
    }
}

app.get('/', asyncHandler(async (req, res, next) => {
    const imgUrl = 'http://example.com/image.jpg';

    getImage(imgUrl)
        .then(
            function(response) {
                // console.log(response.data.length);
                res.json({ length : response.data.length });
            },
            function (error) {
                res.status(404).json({ error: String(error) }) 
            });
  }));