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Cheat sheet: from basic LINQ to JavaScript

3rd April

2014

written by

As a developer in the .NET world where LINQ is first class citizen, when going to JavaScript it seems that some methods are missing. There is even a few libraries that tries to remedy this, but if you are just looking to get the job done the most often used methods are right at hand. (Looking for features such as Lazy-evaluation or observables, linq.js and RxJS offers these).

In the following I’ll list the most used operations and their JavaScript equivalents cheat sheet style. Notice that I’m using Lambda expressions in JavaScript to get the code a bit more concise, if they aren’t available just replace expressions like:

(data) => {}

with

function(data) {}

All operations are executed against this example:

var persons = [
    { firstname: 'Peter', lastname: 'Jensen', type: 'Person', age: 30 },
    { firstname: 'Anne', lastname: 'Jensen', type: 'Person', age: 50  },
    { firstname: 'Kurt', lastname: 'Hansen', type: 'Person', age: 40  }
];

Notice that some of the operations modifies the source array, if you don’t want that just clone it with:

persons.slice(0);

Every operation is named after the method’s name in LINQ:

All

	var result = persons.All(person => person.type == "Person");

JavaScript

	var result = persons.filter(person => person.type == 'Person').length == persons.length;

Concat

	var result = persons.Concat(persons);

JavaScript

	var result = persons.concat(persons);

Count

	var result = persons.Count();

JavaScript

	var result = persons.length;

Distinct

	var lastnames = persons.Select(person => person.lastname);
    var result = lastnames.Distinct();

JavaScript

	var lastnames = persons.map(person => person.lastname);
    var result = lastnames.filter((value, index) => lastnames.indexOf(value) == index);

Empty

	var result = Enumerable.Empty<dynamic>();

JavaScript

	var result = [];

First

	var result = persons.First();

JavaScript

	var result = persons[0];
    if (!result) throw new Error('Expected at least one element to take first')

FirstOrDefault

	var result = persons.FirstOrDefault();

JavaScript

	var result = persons[0];

ForEach

	var fullnames = new List<string>();
    persons.ForEach(person => fullnames.Add(person.firstname + " " + person.lastname));

JavaScript

var fullnames = [];
persons.forEach(person => fullnames.push(person.firstname + ' ' + person.lastname))

GroupBy

	var result = persons.GroupBy(person => person.lastname);

JavaScript

	var result = persons.reduce((previous, person) => {
		(previous[person.lastname] = previous[person.lastname] || []).push(person);
    	return previous;
	}, []);

IndexOf

	var result = persons.IndexOf(persons[2]);

JavaScript

	var result = persons.indexOf(persons[2]);

Last

	var result = persons.Last();

JavaScript

	var result = persons[persons.length-1];
    if (!result) throw new Error('Expected at least one element to take last')

LastOrDefault

	var result = persons.LastOrDefault();

JavaScript

	var result = persons[persons.length-1];

OrderBy

	var result = persons.OrderBy(person => person.firstname);

JavaScript

	persons.sort((person1, person2) => person1.firstname.localeCompare(person2.firstname));

OrderByDescending

	var result = persons.OrderByDescending(person => person.firstname);

JavaScript

	persons.sort((person1, person2) => person2.firstname.localeCompare(person1.firstname));

Reverse

	persons.Reverse();

JavaScript

	var result = persons.reverse();

Select

	var result = persons.Select(person => new {fullname = person.firstname + " " + person.lastname});

JavaScript

	var result = persons.map(person => ({ fullname: person.firstname + ' ' + person.lastname }) );

Single

	var result = persons.Single(person => person.firstname == "Peter");

JavaScript

	var onePerson = persons.filter(person => person.firstname == "Peter"); 
    if (onePerson.length != 1) throw new Error('Expected at excactly one element to take single')
    var result = onePerson[0];

Skip

	var result = persons.Skip(2);

JavaScript

	var result = persons.slice(2, persons.length);

Take

	var result = persons.Take(2);

JavaScript

	var result = persons.slice(0, 2);

Where

	var result = persons.Where(person => person.lastname == "Jensen");

JavaScript

	var result = persons.filter(person => person.lastname == 'Jensen');

So clearly you don’t need a library if you only need the basic operations. The code above with tests is available at GitHub.

This post is also available in Danish at QED.dk.

C#, Geeky, JavaScript

8 Comments

LINQ kenderens guide til JavaScript | Poul Foged

03/04/2014

[…] This post is also available in English at Complexitymaze.com. […]
Kevin Reed

03/04/2014

You should use [].every instead of the unnecessary [].filter combined with [].length when doing the LINQ equivalent of .All()

persons.every(person => person.type == ‘Person’)

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/every
Poul

04/04/2014

@Kevin Reed, I did not know about every(). Thanks for the feedback
Lino Silva

27/03/2015

Did not know about Array.prototype.every as well. Cheers Kevin.
JavaScriptLinq

18/08/2015

You can use JavaScriptLinq http://www.javascriptlinq.com/ It havs all linq functions
Liazy

10/02/2016

persons.Any(p=>p.Type == “person”)
persons.some(p=>p.type == “person”)

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/some
Jorma

02/06/2016

Here is one missing function

Select
C#
var result = persons.Select(person => new {fullname = person.firstname + ” ” + person.lastname});

JavaScript
var result = persons.Select(function(person) {return {fullname = person.firstname + ” ” + person.lastname});
Siderite

06/01/2020

Check out an implementation of LINQ in Javascript using modern ES6 language features: https://siderite.dev/blog/linq-in-javascript-linqer/