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TypeScript vs. C#: LINQ

TypeScript

TypeScript has no equivalent for the language-integrated-natural-query aspect of LINQ. (hey, isn't that literally the whole acronym?)

True, you can't write the following LINQ statement in TypeScript

var adultUserNames =  from u in users  where u.Age >= 18  select u.Name;

However, the IEnumerable<T> extension methods, which are at the heart of LINQ, have equivalents in TypeScript (or can be emulated).

Aggregate

// C#var leftToRight = users.Aggregate(initialValue, (a, u) => /* ... */);
// TypeScriptconst leftToRight = users.reduce((a, u) => /* ... */, initialValue);const rightToLeft = users.reduceRight((a, u) => /* ... */, initialValue);

All

// C#var allReady = users.All(u => u.IsReady);
// TypeScriptconst allReady = users.every(u => u.isReady);

Any

// C#var isDirty = users.Any(u => u.IsDirty);
// TypeScriptconst isDirty = users.some(u => u.isDirty);

Append

// C#var allUsers = users.Append(oneMoreUser);
// TypeScriptconst allUsers = [ ...users, oneMoreUser ];

Average

// C#var avgAge = users.Average(u => u.Age);
// TypeScriptif (users.length < 1) {  throw new Error('source contains no elements');}const avgAge = users.reduce((a, u) => a + u.age, 0) / users.length;

Cast

// C#var people = users.Cast<Person>();
// TypeScriptconst people = users as Person[];// Note: not semantically the same. The C# version throws an exception// if any of the users can't be cast to type Person.

Concat

// C#var allUsers = users.Concat(moreUsers);
// TypeScriptconst allUsers = [ ...users, ...moreUsers ];

Contains

// C#var hasAdmin = users.Contains(admin);
// TypeScriptconst hasAdmin = users.includes(admin); // Use a polyfill for IE support

Count

// C#var n = users.Count();
// TypeScriptconst n = users.length;

DefaultIfEmpty

// C#var nonEmptyUsers = Enumerable.DefaultIfEmpty(users);
// TypeScriptconst nonEmptyUsers = users.length ? users : [ null ];

Distinct

// C#var uniqueNames = users.Select(u => u.Name).Distinct();
// TypeScriptconst uniqueNames = Object.keys(  users.map(u => u.name).reduce(    (un, u) => ({ ...un, n }),    {}  ));

ElementAt

// C#var nth = users.ElementAt(n);
// TypeScriptif (n < 0 || n > users.length) {  throw new Error('Index was out of range');}const nth = users[n];

ElementAtOrDefault

// C#var nth = users.ElementAtOrDefault(n);
// TypeScriptconst nth = users[n];

Empty

// C#var noUsers = IEnumerable.Empty<User>();
// TypeScriptconst noUsers: User[] = [];const noUsers = [] as User[];

Except

// C#var maleUsers = users.Except(femaleUsers);
// TypeScriptconst maleUsers = users.filter(u =>  !femaleUsers.includes(u) // Use a polyfill for IE support);

First

// C#var first = users.First();
// TypeScriptif (users.length < 1) {  throw new Error('Sequence contains no elements');}const first = users[0];

FirstOrDefault

// C#var first = users.FirstOrDefault();
// TypeScriptconst first = users[0];

List.ForEach

// C#users.ToList().ForEach(u => /* ... */);
// TypeScriptusers.forEach(u => /* ... */);

GroupBy

// C#var usersByCountry = users.GroupBy(u => u.Country);
// TypeScriptconst usersByCountry = users.reduce((ubc, u) => ({  ...ubc,  [u.country]: [ ...(ubc[u.country] || []), u ],}), {});

Intersect

// C#var targetUsers = usersWhoClicked.Intersect(usersBetween25And45);
// TypeScriptconst targetUsers = usersWhoClicked.filter(u =>  usersBetween25And45.includes(u) // Use a polyfill for IE support);

Last

// C#var last = users.Last();
// TypeScriptif (users.length < 1) {  throw new Error('Sequence contains no elements');}const last = users[users.length - 1];

LastOrDefault

// C#var last = users.LastOrDefault();
// TypeScriptconst last = users[users.length - 1];

Max

// C#var oldestAge = users.Max(u => u.Age);
// TypeScriptif (users.length < 1) {  throw new Error('source contains no elements');}const oldestAge = users.reduce((oa, u) => Math.max(oa, u.age), 0);

Min

// C#var youngestAge = users.Min(u => u.Age);
// TypeScriptif (users.length < 1) {  throw new Error('source contains no elements');}const youngestAge = users.reduce((ya, u) => Math.min(ya, u.age), Number.MAX_VALUE);

OfType

// C#var bots = users.OfType<Bot>();
// TypeScript// No equivalent

OrderBy / ThenBy

// C#var sorted = users.OrderBy(u => u.Age).ThenBy(u => u.Name);
// TypeScriptconst sorted = users.sort((a, b) => {  const ageDiff = b.age - a.age;  if (ageDiff) return ageDiff;  return a.name.localeCompare(b.name); // Use a polyfill for IE support});

Reverse

// C#var backwards = users.Reverse();
// TypeScriptconst backwards = users.reverse();// Caution: users is also reversed!

Select

// C#var names = users.Select(u => u.Name);
// TypeScriptconst names = users.map(u => u.name);

SelectMany

// C#var phoneNumbers = users.SelectMany(u => u.PhoneNumbers);
// TypeScriptconst phoneNumbers = users.reduce((pn, u) => [ ...pn, ...u.phoneNumbers ], []);

Single

// C#var user = users.Single();
// TypeScriptif (users.length > 1) {  throw new Error('The input sequence contains more than one element');}else if (!users.length) {  throw new Error('The input sequence is empty');}const user = users[0];

SingleOrDefault

// C#var user = users.Single();
// TypeScriptconst user = users[0];

Skip

// C#var otherUsers = users.Skip(n);
// TypeScriptconst otherUsers = users.filter((u, i) => i >= n);

SkipWhile

// C#var otherUsers = users.SkipWhile(predicate);
// TypeScriptlet i = 0;while (i < users.length && predicate(users[i++]));const otherUsers = users.slice(i - 1);

Sum

// C#var totalYears = users.Sum(u => u.Age);
// TypeScriptif (users.length < 1) {  throw new Error('source contains no elements');}const totalYears = users.reduce((ty, u) => ty + u, 0);

Take

// C#var otherUsers = users.Take(n);
// TypeScriptconst otherUsers = users.filter((u, i) => i < n);

TakeWhile

// C#var otherUsers = users.TakeWhile(predicate);
// TypeScriptlet i = 0;while (i < users.length && predicate(users[i++]));const otherUsers = users.slice(0, i - 1);

Union

// C#var allUsers = someUser.Union(otherUsers);
// TypeScriptconst allUsers = otherUsers.reduce((au, u) =>   au.includes(u) // Use a polyfill for IE support    ? au    : [ ...au, u ]}), someUsers));

Where

// C#var adults = users.Where(u => u.Age >= 18);
// TypeScriptconst adults = users.filter(u => u.age >= 18);

Zip

// C#var matches = buyers.Zip(sellers, (b, s) => new { Buyer = b, Seller = s });
// TypeScriptconst matches = [];for (let i = 0; i < buyers.length && i < sellers.length; i++) {  matches.push({    buyer: buyers[i],    seller: sellers[i],  });}

More in the TypeScript vs. C# Series

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