The Array.prototype functions have been available for a long time but it’s since Arrow functions that “Linq lambda syntax”-like functionality is available in JavaScript.

This blog post explains the most common functions by comparing them to their C# equivalents.

A basic example:

// JavaScript
const result = [0, 1, 2, 3, null]
    .filter(x => x !== null)
    .map(x => x * 10)
    .sort((a, b) => b - a);

expect(result).toEqual([30, 20, 10, 0]);

// C#
var result = new int?[] {0, 1, 2, 3, null}
    .Where(x => x != null)
    .Select(x => x * 10)
    .OrderByDescending(x => x)
    .ToArray();

Whenever you want to write a for to loop over an array, the Array.prototype functions probably allow you to achieve the same result but in more functional and succinct manner.

Do note that the deferred execution we know from Linq does not apply to Array.prototype!

Comparison Table

Commonly used methods

Select = map

// C#
var result = enumerable.Select(x => x);

// JavaScript
let result = array.map(x => x);

// Using spread to avoid mutation
const input = [{k: 1, v: true}, {k: 2, v: false}];
const result = input.map(x => ({...x, v: !x.v}))

When mapping to an object without code block, you need to wrap your object between extra parentheses like

[0, 1].map(x => ({value: x}));

// Because without the extra parentheses
[0, 1].map(x => {value: x});
// --> [undefined, undefined]

// it is the same as writing:
[0, 1].map(x => {
  value: x; // No error, because JavaScript?
  return undefined;
});

Where, Distinct = filter

Where and filter behave pretty much exactly alike. Linqs Distinct we’ll need to implement ourselves.

const input = [0, 0, 1, 5, 5];
// Equals true when the first occurrence of the value is the current value
const result = input.filter((element, index, array) => array.indexOf(element) === index);
// Or: [...new Set(input)];
expect(result).toEqual([0, 1, 5]);

Aggregate, GroupBy, … = reduce

Linq has Sum, Min, Max, Average, GroupBy, etc. While JavaScript doesn’t have them, they can all be achieved trivially with reduce

Sum, Min, Max, Average:

const input = [0, 1];
const sum = input.reduce((total, cur) => total + cur, 0);
const average = sum / input.length;
const min = Math.min.apply(Math, input); // Old school
const min = Math.min(...input); // Using spread

Aggregate, GroupBy:

const input = [0, 1, 2, 3];

const result = input.reduce((acc, cur) => {
  if (cur % 2 === 0) {
    acc.even.push(cur);
  } else {
    acc.odd.push(cur);
  }
  return acc;
}, {even: [], odd: []});

expect(result).toEqual({even: [0, 2], odd: [1, 3]});

slice

Linq has First, Last, Skip, SkipLast, SkipWhile, Take, TakeLast, TakeWhile. JavaScript has slice.

// Shallow copy
const input = [0, 1, 2, 3];
expect(input.slice()).toEqual([...input]);

// Signature
slice(startIndex = 0, endIndex = length-1);

with

Since 2023, there is also with which returns a new array where one item is replaced with a new value by index.

const newArr = arr.with(index, newValue);

Mutations

These functions operate in place.
Since ECMAScript 2023, some have non-mutating alternatives!

OrderBy = sort and toSorted

• Use toSorted if you need a new array for the sorted result.
• Without compareFn the array is sorted according to each character’s Unicode code point value.
• The compareFn should return a number:
  • -1 (or any negative number): a comes before b
  • 0: Equal
  • 1 (or any positive number): a comes after b

// Numbers
[10, 5].sort(); // [10, 5]: each element is converted to a string
[10, 5].sort((a, b) => a - b); // [5, 10]

// Strings
['z', 'e', 'é'].sort(); // ['e', 'z', 'é']
['z', 'e', 'é'].sort((a, b) => a.localeCompare(b)); // ['e', 'é', 'z']

// Dates
[d1, d2, d3].sort((a, b) => a.getTime() - b.getTime());

C#

• The signature is a bit different: OrderBy(Func<TSource, TKey> keySelector, IComparer<TKey> comparer)
• OrderBy vs OrderByDescending: switch a and b…
• JS doesn’t have ThenBy chaining possibilities, you’ll have to do this yourself.

forEach

While forEach is not really doing any mutation by itself, it is often what it’s used for. Mutations are especially dangerous in for example a Redux environment where UI changes might lag. The same can usually be achieved with map.

const input = [{value: 1, visited: 0}, {value: 2, visited: 0}];

// Potentially dangerous
input.forEach(el => {
    el.visited++;
});

// ...el will create a shallow copy only!
const result = input.map(el => ({...el, visited: el.visited + 1}));

// Or use the good old loop?
for (let itm of input) {
    console.log(itm.value);
}

Other mutators

• reverse(): There is now also toReversed() (2023)
• push(el1, [el2, ...]): Add element(s) at the end. Returns the new array length.
• shift(): Remove the first element at the start. Returns the removed element.
• unshift(el, [el2, ...]): Add element(s) at the start. Returns the new array length.
• splice(start, [deleteCount, [el1, [el2, ...]]])
  • The swiss army knife: add and/or remove element(s)
  • Returns array of removed elements (or empty array)
  • There is now also toSpliced (2023)