generic-file-reader

(function() {
    'use strict';

    function fileCallback(fileData) {
        console.log(fileData);
    }

    let background = document.createElement('div');
    background.style.position = 'fixed';
    background.style.left = '0';
    background.style.top = '0';
    background.style.width = '100%';
    background.style.height = '100%';
    background.style.zIndex = '99';
    background.style.background = 'rgba(0,0,0,0.4)';
    let dialFrame = document.createElement('div');
    dialFrame.style.position = 'absolute';
    dialFrame.style.padding = '12px';
    dialFrame.style.left = '50%';
    dialFrame.style.top = '50%';
    dialFrame.style.transform = 'translate(-50%, -50%)';
    dialFrame.style.background = 'white';
    let promptButton = document.createElement('input');
    promptButton.setAttribute('type', 'file');
    promptButton.setAttribute('accept', '.txt');
    promptButton.style.display = 'block';
    let cancelButton = document.createElement('button');
    cancelButton.setAttribute('type', 'button');
    cancelButton.innerText = 'Cancel';
    cancelButton.style.display = 'block';

    document.body.appendChild(background);
    background.appendChild(dialFrame);
    dialFrame.appendChild(promptButton);
    dialFrame.appendChild(cancelButton);

    promptButton.addEventListener('change', buttonChange);
    cancelButton.addEventListener('click', cancelClick);

    function removeListeners() {
        promptButton.removeEventListener('change', buttonChange);
        cancelButton.removeEventListener('click', cancelClick);
    }

    function buttonChange(evt) {
        let fread = new FileReader();
        fread.addEventListener('load', () => {
            // return found value to callback for processing
            fileCallback(fread.result);
        }, {once: true});

        // here you can read as something else if needed
        fread.readAsText(evt.target.files[0]);
        console.log('file was read');
        removeListeners();
        background.remove();
    }

    function cancelClick(evt) {
        console.log('dialog was canceled');
        removeListeners();
        background.remove();
    }
}());