Get size of the selected file

<input type="file" id="upload" />
<div id="size"></div>

// Query the elements
const fileEle = document.getElementById('upload');
const sizeEle = document.getElementById('size');

fileEle.addEventListener('change', function (e) {
    const files = e.target.files;
    if (files.length === 0) {
        // Hide the size element if user doesn't choose any file
        sizeEle.innerHTML = '';
        sizeEle.style.display = 'none';
    } else {
        // File size in bytes
        sizeEle.innerHTML = `${files[0].size} B`;

        // Display it
        sizeEle.style.display = 'block';
    }
});

// Convert the file size to a readable format
const formatFileSize = function (bytes) {
    const sufixes = ['B', 'kB', 'MB', 'GB', 'TB'];
    const i = Math.floor(Math.log(bytes) / Math.log(1024));
    return `${(bytes / Math.pow(1024, i)).toFixed(2)} ${sufixes[i]}`;
};

// Display the file size
sizeEle.innerHTML = formatFileSize(files[0].size);