Stop using for loops, here are other cool options
Tags: javascript, webdev, functional
Published: 2020-02-27 00:09:12

If you are someone who always resorts to the for loop and forEach loop to iterate over items in an array, then this is for you. We will explore a few array methods that you can employ as an alternative to a simple “for” loops.

There are a lot of advantages of using specific methods over generic `for` or `forEach` version.

* It’s easy to write and others can interpret it easily
* It’s easy to maintain, extend, and test
* You can write pure functions without any side effects
* Helps you thinking in terms of functional programming
* If you ever plan to use libraries like RxJS, it will surely help

## **Using Map**
The below code looks pretty familiar, right? You see that the for loop is taking each element from the array, performing some operation (here it is multiplying numbers), and putting that into a new array.

```javascript
find square of numbers
var numberArray = [1,2,3,4,5,6,7,8,9,10];
//for Version
var squareNumbers = [];
for (var counter=0; counter < numberArray.length; counter++){
   squareNumbers.push(numberArray[counter] * numberArray[counter])
}
console.log(squareNumbers);
```

`Array.map` is a builtin function that yields a new array by transforming each element of the source array to a new value based on the supplied method for transformation.

It loops over all the elements in order, calling a supplied transformation function for each element, and internally pushes the results into a new array. We just need to supply the transformation function, and the rest of the work will be done by the map function. For example:

```javascript
find the square of numbers
var numberArray = [1,2,3,4,5,6,7,8,9,10];
//Map version
var squareNumbers = numberArray.map(number => number*number);
console.log(squareNumbers);
```

In simple terms, `Array.map()` transforms a given array into a new one after performing the transformation function on each element in the array.

***

## **Using Filter**

You see that the below for loop is taking each element from the array, checking some condition (here it is checking number is even or not), and if the condition is true then it puts that into a new array.

```javascript
Filter even numbers
var numberArray = [1,2,3,4,5,6,7,8,9,10];
//for Version
var evenNumbers = [];
for (var counter=0; counter < numberArray.length; counter++){
    if (numberArray[counter] %2 === 0){
        evenNumbers.push(numberArray[counter])
    }}
console.log(evenNumbers);
```

`Array.filter` is another handy function that builds a new array of elements based on the given “validator” functions. It loops over all the elements of the source array, calling the “validator” function for each item, if the “validator” function returns true, the element will be internally added to a new array. With the filter function, we can just supply the core “validation” logic of the function, leaving the rest of the work to the `filter` which makes it easy to write and understand.

```javascript
Filter even numbers
var numberArray = [1,2,3,4,5,6,7,8,9,10];
//filter version
var evenNumbers2 = numberArray.filter(number => number%2===0);
console.log(evenNumbers2);
```

Looking at the same function implemented with `filter()`, it becomes clear immediately that it's filtering the original array based on the condition used inside the “validator” function.

With the `for` and `forEach` version, we need to analyze that there is an empty array and then elements are added to it based on the condition. With the `filter` function, we only need to think of the validation function logic and leaving the rest to the `filter`, and that makes the code looks sleek and natural.

***

## **Using reduce**

Another familiar `for` loop is where we take each element and do some kind of accumulation operation (here it is adding all the elements), returning the accumulated value.

```javascript
find sum of elements in the array
var numberArray = [1,2,3,4,5,6,7,8,9,10];
//for version
var sum = 0;
for (var counter=0; counter < numberArray.length; counter++){
     sum += numberArray[counter]
}
console.log(sum);
```

`Array.reduce()` is used when you want to process all the elements of an array to get a single value out of it. Reduce is a little tricky one to understand in beginning but once you understand it’s really simple to use.

It is important to note that `Array.reduce()` doesn't execute the function when there is no item in the array and also this method doesn’t make any change to the original array.

Array.reduce takes two arguments, one is the **reduce** function and second is the initial value which is called an **accumulator**. It calls the reduce function for each element by giving the accumulator value. The reduce function processes the current element and updates the accumulator value and passes it to the next iteration. At the end of the last loop, accumulator becomes the final result. 

Let's explore with example

```javascript
find sum of all elements in the array
var numberArray = [1,2,3,4,5,6,7,8,9,10];
//Reduce version
var sum = numberArray.reduce(((acc, num) => acc + num), 0);
console.log(sum);
```

***

## **Function composition**
There are other array utility methods like every, slice, splice, some, concat, sort which everyone should be aware of. Using the right kind of function not only makes the code cleaner, but it also makes it easy to test and extend. Plus you are writing futuristic code by using these functions. These functions are native JavaScript functions that are supported in all the browsers and are getting faster day by day. It also helps to compose smaller functions to create a broader use case.

```javascript
using evenNumbers and sum, we can easily fund sum of even numbers
var numberArray = [1,2,3,4,5,6,7,8,9,10];
var FilterFn = (number => number%2===0);
var squareMapFn = (number => number*number);
var sumFn = ((sum, number) => sum + number);
var sumOfSquareOfEvenNums = numberArray
                               .filter(FilterFn)
                               .map(squareMapFn)
                               .reduce(sumFn,0);
console.log(sumOfSquareOfEvenNumbers)
```

Writing the above example with a conventional for loop will take more lines of code, that ultimately makes it less clear.

***

## **Note on performance**
Array.filter, map, some have the same performance as forEach. These are all marginally slower than for/while loop. Unless you are working on performance-critical functionalities, it should be fine using the above methods. With JIT, JavaScript execution engines are very fast and it's getting even faster day by day. So start taking advantage of these methods in your application.

***
Thank you for reading my article. ✌🏻👨🏼‍✈️