Ajax - Get size of file before downloading

Question

Basically, I want to figure out whether I should download a file using AJAX, depending on how large the filesize is.

I guess this question could also be rephrased as

Answer 1

Sometimes HEAD can act differently than GET so I suggest something like this that aborts the request after getting Content-Length header:

new Promise(resolve => {
    var xhr = new XMLHttpRequest();
    xhr.open('GET', '/a.bin', true);
    xhr.onreadystatechange = () => {
        resolve(+xhr.getResponseHeader("Content-Length"));
        xhr.abort();
    };
    xhr.send();
}).then(console.log);