hash(data,salt,progress,cb)中，progress这个参数怎么用？
这是我找到的hash参数的解释，可还是不会用，知道的同学请帮我解释一下，谢谢！
hash(data, salt, progress, cb)
error - First parameter to the callback detailing any errors.
result - Second parameter to the callback providing the encrypted form.
data - [REQUIRED] - the data to be encrypted.
salt - [REQUIRED] - the salt to be used to hash the password.
progress - a callback to be called during the hash calculation to signify progress
callback - [REQUIRED] - a callback to be fired once the data has been encrypted.
