Can I execute another M function within Json.FromValue() for a web API call ?

Hi Anonymous ,
in general, you can get rid of some transformations, like so, as the Json.FromValue transforms that data into the needed binary form already:

let
    url = "baseURL"
    header = [#"Content-Type"="application/json",#"Accept"="application/json"],
    body = Json.FromValue([metrics = {"NEW_REVIEWS"}, dimensions = {"DAYS"}, filters = [ startDate = #date(Date.Year(Date.From(Date.AddDays(DateTime.LocalNow(),-1))),Date.Month(Date.From(Date.AddDays(DateTime.LocalNow(),-1))),Date.Day(Date.From(Date.AddDays(DateTime.LocalNow(),-1)))), endDate = #date(Date.Year(Date.From(Date.AddDays(DateTime.LocalNow(),-8))),Date.Month(Date.From(Date.AddDays(DateTime.LocalNow(),-8))),Date.Day(Date.From(Date.AddDays(DateTime.LocalNow(),-8))))]]),
    Source = Json.Document(Web.Contents(url,[Headers = header,Content=body]))
   in
    Source

Then, it could be that you need to transform the dates into text for the API to work correctly. The Date.ToText function gives you some options to apply the correct format (XXXXXXX):

let
    url = "baseURL"
    header = [#"Content-Type"="application/json",#"Accept"="application/json"],
    body = Json.FromValue([metrics = {"NEW_REVIEWS"}, dimensions = {"DAYS"}, filters = [ startDate = Date.ToText(#date(Date.Year(Date.From(Date.AddDays(DateTime.LocalNow(),-1))),Date.Month(Date.From(Date.AddDays(DateTime.LocalNow(),-1))),Date.Day(Date.From(Date.AddDays(DateTime.LocalNow(),-1)))), XXXXXXXX), Date.ToText(endDate = #date(Date.Year(Date.From(Date.AddDays(DateTime.LocalNow(),-8))),Date.Month(Date.From(Date.AddDays(DateTime.LocalNow(),-8))),Date.Day(Date.From(Date.AddDays(DateTime.LocalNow(),-8)))), XXXXXXXX)]]),
    Source = Json.Document(Web.Contents(url,[Headers = header,Content=body]))
   in
    Source