问题
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); });
回答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>
回答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
.
回答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
来源:https://stackoverflow.com/questions/36955906/group-items-in-typescript-with-lodash