Group items in Typescript with lodash

Question

I need to group a collection of objects by date:

var meetings =
[
   {date:"2001-01-01 13:00", place:"park"},
   {date:"2001-01-01 14:00", place:"school"},
   {date:"2001-01-02 11:00", place:"house"}
];

so it becomes:

var groupedMeetings = 
[
   {
      day:"2001-01-01", 
      meetings: 
      [
         {date:"2001-01-01 13:00", place:"park"},
         {date:"2001-01-01 14:00", place:"school"}
      ]
   },
   {
      day:"2001-01-02", 
      meetings: 
      [
         {date:"2001-01-02 11:00", place:"house"}
      ]
   }
]

I was able to group them with _.groupBy and _.map

var g = _.groupBy(meetings, function(i){return getDatePart(i.date);});
var groupedMeetings  = _.map(g, function(items,key){return {day:key, meetings:items};});

But I have problems with doing the same in Typescript with https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/lodash/lodash.d.ts

First, I declared my classes:

class Meeting
    {
       date: Date,
       place: Meeting[]
    }
class GroupedMeeting
    {
      constructor(private day: Date, private meetings: Meeting[]) {}
    }

Now I dont know which overload of groupBy to use? And how to map it to list of grouped items?

var grouped: _.Dictionary<Meeting[]> = _.groupBy(meetings, (i: Meeting) => { return this.getDatePart(i.date); });

var groupedMeetings : GroupedMeeting[] = _.map(grouped, (items: Meeting[], key: Date) => { return new GroupedMeeting (key, items); });

Answer 1

You can make use of the _.groupBy() method and add a callback that returns the value you need to group it with, which is the year, month, day part of the date. You can then set the values needed by using _.map().

The example below uses ES6:

var meetings =
[
   {date:"2001-01-01 13:00", place:"park"},
   {date:"2001-01-01 14:00", place:"school"},
   {date:"2001-01-02 11:00", place:"house"}
];

var result = _(meetings)
  .groupBy(meeting => meeting.date.split(' ').shift())
  .map((meetings, day) => ({ day, meetings }))
  .value();

document.write('<pre>' + JSON.stringify(result, 0, 4) + '</pre>');

<script src="https://cdn.jsdelivr.net/lodash/4.11.2/lodash.min.js"></script>

Here is the ES5 version:

var meetings =
[
   {date:"2001-01-01 13:00", place:"park"},
   {date:"2001-01-01 14:00", place:"school"},
   {date:"2001-01-02 11:00", place:"house"}
];

var result = _(meetings)
  .groupBy(function(meeting) { return meeting.date.split(' ').shift(); })
  .map(function(meetings, day) { return { day: day, meetings: meetings }; })
  .value();

document.write('<pre>' + JSON.stringify(result, 0, 4) + '</pre>');

<script src="https://cdn.jsdelivr.net/lodash/4.11.2/lodash.min.js"></script>

Answer 2

If you have the typings for lodash (for newer versions of typescript do npm i --save-dev @types/lodash), methods like _.groupBy or _.keyBy accept a generics type.

Eg: _.groupBy<TypeEachGroupCarries>(expression) will return an array of arrays that contain items of the type TypeEachGroupCarries.

Answer 3

Why use a library when you already have all the tools ? :)

const dateMap  = meetings
    .reduce(
    (m, v) => {
        const day = v.date.substring(0,10)
        const entry = m[day]
        if (typeof entry === 'undefined') {
            m[day] = [v] 
        }
        else {
            entry.push(v)
        }
        return m
    },
    {}    
)

const groupedMeetings = Object.keys(dateMap).map(d => {
    return {
        day: d,
        meetings: dateMap[d]
    }
})    

See it running in the Typescript playground