Arrays are wonderful and a very particular type in JavaScript. There are many useful built-in properties and methods that will help you resolve any task which involves arrays. Today we are going to discuss 15 array methods every developer should know.

• some()
• every()
• reduce()
• map()
• flat()
• filter()
• forEach()
• findIndex()
• find()
• sort()
• concat()
• fill()
• includes()
• reverse()
• flatMap()

Notice the list is not enumerated as I don’t believe one method is more important than the other, each of them will resolve a different problem, and thus is important we are familiar with all.

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some()

The some() tests whether at least one element in the array passes the test implemented by the callback function. The callback function will receive 3 arguments, the item, the index, and the full array. Additionally, is possible to assign a value for this when executing the callback by using the argument thisArg.

Definition:

arr.some(callback(element[, index[, array]])[, thisArg])

Examples:

const a = [1, 2, 3, 5, 8].some(item => item > 5)
const b = [1, 2, 3, 4, 5].some(item => item > 5)

console.log(a)
console.log(b)

---------
Output
---------
> true
> false

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every()

The every() method is in a way similar to the some() method, but it tests whether all the elements in the array pass the test implemented by the callback function.

Definition:

arr.every(callback(element[, index[, array]])[, thisArg])

Examples:

const a = [10, 9, 8, 7, 6].every(item => item > 5)
const b = [7, 6, 5].every(item => item > 5)

console.log(a)
console.log(b)

---------
Output
---------
> true
> false

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reduce()

The reduce() method executes a callback function once for each assigned value present in the array, taking 4 arguments:

1. accumulator
2. currentValue
3. currentIndex
4. array

The first time the callback is called, accumulator and currentValue can be either the initialValue if provided, and the first value in the array if not.

Definition:

arr.reduce(callback( accumulator, currentValue[, index[, array]] )[, initialValue])

How reduce() works

Let’s see with an example how reduce() works:

[0, 1, 2, 3, 4].reduce((accumulator, currentValue, currentIndex, array) => accumulator + currentValue)

If we go step by step and put in a table all the parameters plus the resulting value of the callback, we would get the following:

And the final result would be 10. In our particular case I did not provide an initial value, let’s try that next

const initialValue = 10
[0, 1, 2, 3, 4].reduce((accumulator, currentValue, currentIndex, array) => accumulator + currentValue, initialValue)

With this new scenario our table would like:

And the final resulting value is 20.

The reduce() function is great and it has several uses like sum all the values of an array or in an object array, counting for particular items in the array, grouping objects, merging arrays contained in array of objects, removing duplicates, etc.

#arrays #java #code #javascript