Manipulating Arrays in Elm and JavaScript: An Overview of Map, Filter, and their Combination

ELM ?

Elm is a functional programming language for building web applications. It is designed to be statically typed, fast, and easy to use. Elm compiles to JavaScript and can be used to build front-end web applications with a focus on user interface and user experience. Elm is known for its simplicity, reliability, and performance, making it a popular choice for building robust and scalable web applications.

Elm uses a functional programming style, which means that it emphasizes immutability, pure functions, and declarative programming. This makes Elm programs easy to reason about, test, and maintain. Elm also provides a strong type system, which helps catch errors early in the development process and reduces the likelihood of bugs in production.

In addition to its functional programming features, Elm also provides a rich standard library and a powerful toolset for building user interfaces. Elm is often compared to React, a popular JavaScript library for building user interfaces, as it provides similar functionality with a more functional approach.

In Elm and JavaScript, the map and filter functions are commonly used to manipulate arrays of data.

Here's an example in Elm of using map and filter to transform a list of numbers:

elm

list = [1, 2, 3, 4, 5]

doubled = List.map (\x -> x * 2) list

evens = List.filter (\x -> x % 2 == 0) list

In this example, list is an array of numbers. The List.map function is used to apply a function to each element of the list, resulting in a new list doubled where each element is twice the original value. The List.filter function is used to select only the even numbers from the list, resulting in a new list evens.

Here's an equivalent example in JavaScript:

javascript

let list = [1, 2, 3, 4, 5];

let doubled = list.map(x => x * 2);

let evens = list.filter(x => x % 2 == 0);

In this example, the Array.prototype.map and Array.prototype.filter methods are used to achieve the same result as the Elm example. The map method applies a function to each element of the array and returns a new array, while the filter method selects only the elements that satisfy a condition and returns a new array.

In both Elm and JavaScript, map and filter are powerful functions that can be used to manipulate arrays in a concise and readable way. They are widely used in functional programming and are essential tools for working with collections of data.

•    How to filter in map?

In JavaScript, you can use the Array.prototype.filter method in combination with the Array.prototype.map method to filter and transform elements in an array.

Here's an example of how to use filter and map together:

javascript

let numbers = [1, 2, 3, 4, 5];

let evenSquared = numbers

  .filter(x => x % 2 == 0)

  .map(x => x * x);

console.log(evenSquared); // [4, 16]

In this example, the Array.prototype.filter method is used to select only the even numbers from the numbers array. The Array.prototype.map method is then used to square each of the even numbers. The result is a new array evenSquared containing the squares of only the even numbers in numbers.

Note that the order of the filter and map methods is important. If you were to reverse the order, the filter method would be applied to the result of the map method, which would not produce the desired result.

The combination of filter and map is a common pattern in functional programming and is a powerful way to manipulate arrays in a concise and readable way.