js tool function-format file size

ES6 code:

function formatFileSize(fileSize, idx = 0) {  
    const units = ["B", "KB", "MB", "GB"];  
    if (fileSize < 1024 || idx === units.length - 1) {  
        return fileSize.toFixed(1) + units[idx];  
    }  
    return formatFileSize(fileSize / 1024, ++idx);  
}

Old version code:

function formatFileSize(fileSize, idx) {  
    var units = ["B", "KB", "MB", "GB"];  
    idx = idx || 0;  
    if (fileSize < 1024 || idx === units.length - 1) {  
        return fileSize.toFixed(1) + units[idx];  
    }  
    return formatFileSize(fileSize / 1024, ++idx);  
}