Read local file

Select text or image

function fileSelect(t) { //target is the button
    console.assert(t == button);
    const f = t.files[0];
    const size = " "+f.size+" bytes ";
    msg.innerText = f.type+size;
    if (f.type.startsWith("image")) {
        reader.onload = function(evt) {
            const a = evt.target.result; //image source
            out.innerHTML = "<img class=thumb "
                +"src='"+a+"' title='"+f.name+"' />";
            console.log(f.name+size+f.type);
        };
        reader.readAsDataURL(f);
    } else if (isText(f)) {
        reader.onload = function(evt) { //text
            const a = evt.target.result.replace(/</g, "&lt;");
            out.innerHTML = "<pre>"+a+"</pre>";
            console.log(f.name+size+f.type);
        };
        reader.readAsText(f);
    } else {
        msg.innerText += "Unknown ";
        out.innerText = "";
    }
}

Ref: File selection