@kingjs/linq.group-by

Generates a sequence of groups composed of elements of another sequence which share a common key.

Usage

Group 0, 1, 2, 3 by even/odd numbers like this:

var groupBy = require('@kingjs/linq.group-by');
var sequence = require('@kingjs/enumerable.create');
var toArray = require('@kingjs/linq.to-array');

var evenOdd = groupBy.call(
  sequence(0, 1, 2, 3), 
  function(x) { return x % 2; }
);

var groups = toArray.call(evenOdd);
var even = groups[0];
var odd = groups[1];

var result = {
  evenKey: even.key,
  even: toArray.call(even),
  oddKey: odd.key,
  odd: toArray.call(odd),
};

result;

result:

{
  evenKey: 0,
  even: [ 0, 2 ],
  oddKey: 0,
  odd: [ 1, 3 ],
}

Group people by their age and project their names like this:

var groupBy = require('@kingjs/linq.group-by');
var sequence = require('@kingjs/enumerable.create');
var toArray = require('@kingjs/linq.to-array');

var people = groupBy.call(
  sequence(
    { name: 'Alice', age: 17 },
    { name: 'Bob', age: 16 },
    { name: 'Chris', age: 30 }
  ), 
  function(x) { return x.age <= 18; }, // key selector
  function(x) { return x.name; }, // element selector
  function(group) {  // result selector
    return 'Under 18: ' + group.key + '; ' + toArray.call(group).join(', ');
  },
);

toArray.call(people);

result:

[
  'Under 18: true; Alice, Bob',
  'Under 18: false; Chris'
]

API

declare function groupBy(
  this: Enumerable,
  keySelector: (x) => any,
  elementSelector?: (x) => any,
  resultSelector?: (group) => any
)

Interfaces

• Enumerable: See @kingjs/enumerable.define.

Parameters

• this: The sequence to partition.
• keySelector: Selects key common to each group.
• elementSelector: Maps an element before inclusion in a group.
• resultSelector: Maps a group before returned.
  • group: Sequence of elements in the group.

Return Value

Sequence of sub-sequences where each sub-sequence contains elements forming a group. Each group has a key property whose value was selected by keySelector and is shared by elements of the group.

Install

With npm installed, run

$ npm install @kingjs/linq.group-by

Acknowledgments

Like Enumerable.GroupBy.

License

MIT