使用R.groupWith（ramda）

Question

I'm trying to group the following array acc to dateValue , but can't seem to get it to work. What am I missing here?

 const dates = [ {"id":83,"dateValue":"2017-05-24"}, {"id":89,"dateValue":"2017-05-28"}, {"id":91,"dateValue":"2017-05-25"}, {"id":78,"dateValue":"2017-05-24"}, {"id":84,"dateValue":"2017-05-25"} ] const groups = R.groupWith(R.eqProps('dateValue'),dates) console.log(groups) 

 <script src="//cdn.jsdelivr.net/ramda/latest/ramda.min.js"></script> 

I've included a link to ramda repl with this code loaded

expected result:

[ [ { dateValue: "2017-05-24", id: 83 },
    { dateValue: "2017-05-24", id: 78 } ],
  [ { dateValue: "2017-05-28", id: 89 } ],
  [ { dateValue: "2017-05-25", id: 91 } ],
  [ { dateValue: "2017-05-25", id: 84 } ] ]

Answer 1

There are several differences between groupBy and groupWith .

• groupBy accepts a function which yields a grouping key for a single item. groupWith accepts a binary predicate that says whether two items match.
• groupBy collects all items into single collections. groupWith only collects matching itemss which are consecutive .
• groupBy returns an object that maps those keys to the list of matching items. groupWith returns a list of lists of (again consecutive) matching items.

It's that second point which is tripping you up. You probably want groupBy :

R.groupBy(R.prop('dateValue'))(dates)
// or
R.compose(R.values, R.groupBy(R.prop('dateValue')))(dates);

You can see this in action on the Ramda REPL .