Add support for custom getTime

[barhun]

This support for the optional field timeOffset allows consumers of the API to avoid unnecessary calls to the /api/v1/time endpoint for each private call. One can call the time method only once at startup and then calculate an offset between the local time and the returned value which, if provided as an option, will be used for any future private call.

[coveralls]

Coverage decreased (-26.4%) to 64.0% when pulling 2174ba2 on barhun:master into aa54224 on HyperCubeProject:master.

[balthazar]

Thanks! Rather than having a timeOffset, I think it would be better to instead allow a custom function to be passed, returning the time integer. If not provided it would simply be: () => Date.now() and we resolve the promise with this value. This would allow for more complex operations to be made if we need it

What do you think?

[barhun]

Well, indeed, it would be a more generic solution. Yet, since both the server time and the local time are monotonically increasing and the offset between them is always a constant value, this one also appears to solve almost all cases.

Now, I think providing a function returning a promise would be the best choice as it would also let us later remove the option useServerTime at some point in future. One could say Binance({apiKey, apiSecret, timePromise: Binance().time}) instead of using useServerTime: true, but we need to keep it for now 'cause backward-compatibility must be conserved. For the ones who want to make use of an offset like me, it would be something similar to the following: Binance({apiKey, apiSecret, timePromise: () => Promise.resolve(Date.now() - timeOffset)})

This promise-based solution will allow people to query their own time server if they ever need to, thus seems a great solution. Deal? (:

[balthazar]

useServerTime is a convenient way for people to get the time added to the request without having to do it on their side, I don't really see it going away.

Because the underlying code is using Promise.resolve, we do not need to force using a promise with the library, although it is possible for example:

getTime: () => Date.now() + TIME_OFFSET

getTime: async () => {
  const time = await fetch('mycustom-ntpserver.com')
  return time + TIME_OFFSET
}

This gives the most customization capabilities while keeping it simple

[balthazar]

Looks good! Do you mind putting the default getTime function out of the privateCall so it's not being recreated everytime? Merging after this!

Also about typings, will it be an issue if the provided getTime returns a promise?

[balthazar]

Almost perfect :D Rather than having the ternary checking the typeof, why not keep it a default like you were doing? getTime = defaultGetTime?

[barhun]

'cause NOT everyone uses TypeScript. (: Just kidding, pushing it now one last time! Thanks for all your feedbacks.

[balthazar]

❤️