上传文件时，怎样返回上传进度给前台？

我通过formidable中间件，获取文件上传进度，怎样返回给前台呢？

form.on('progress', function(bytesReceived, bytesExpected) {
            console.log(`bytesReceived:${Math.round(bytesReceived/bytesExpected*100)}`)
        });