How to get first N number of elements from an array

Question

I am working with Javascript(ES6) /FaceBook react and trying to get the first 3 elements of an array that varies in size. I would like do the equivalent of Linq take(n).

In my Jsx file I have the following:

var items = list.map(i => {
  return (
    <myview item={i} key={i.id} />
  );
});

Then to get the first 3 items I tried

  var map = new Map(list);
    map.size = 3;
    var items = map(i => {
      return (<SpotlightLandingGlobalInboxItem item={i} key={i.id} />);
    });

This didn't work as map doesn't have a set function.

Can you please help?

Solution 1

I believe what you're looking for is:

// ...inside the render() function

var size = 3;
var items = list.slice(0, size).map(i => {
    return <myview item={i} key={i.id} />
});                       
return (
  <div>
    {items}
  </div>   
)

Solution 2

To get the first n elements of an array, use

const slicedArray = array.slice(0, n);

Solution 3

arr.length = n

This might be surprising but length property of an array is not only used to get number of array elements but it's also writable and can be used to set array's length MDN link. This will mutate the array.

If you don't care about immutability or don't want to allocate memory i.e. for a game this will be the fastest way.

to empty an array

arr.length = 0

Solution 4

You can filter using index of array.

var months = ['Jan', 'March', 'April', 'June'];
months = months.filter((month,idx) => idx < 2)
console.log(months);

Solution 5

Use Slice Method

The javascript slice() method returns a portion of an array into a new array object selected from start to end where start and end represent the index of items in that array. The original array will not be modified.

syntax : slice(start, end)

Let us say we have an array with 7 items [5,10,15,20,25,30,35] and we want the first 5 elements from that array:

let array = [5,10,15,20,25,30,35]
let newArray = array.slice(0,5)

console.log(newArray)

View more solutions

Comments

[Chris]

You're correct overall, but semantically you should use filter to first filter down the set of elements, then, map the filtered down set if you are taking this approach.

[elQueFaltaba]

The filter function would go through all the elements in the array, while the slice would not, so it's better performant-wise to use slice, right?

[Danny Harding]

Note that the slice function on arrays returns a shallow copy of the array, and does not modify the original array. developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/…

[Veslav]

In the first example you use slice but in the second you use splice.

[MikeBeaton]

.filter on it's own is not a great choice, at least not if the input array might be long. .filter goes through every element of the array checking its condition. .slice would not do this, but would just extract the first n elements and then stop processing - which would definitely be what you want for a long list. (As @elQueFaltaba already said in comments to another answer.)

[Angel Joseph Piscola]

This is also wrong. You will get ["Apple", "Mango"] from this. The first part of slice is not "where to start deleting", it's where to start the slice from. It doesn't modify the original array and won't delete anything.

[Oldrich Svec]

This will also expand the array if it is smaller than N

[Rishabh876]

Does this throw any error if n is larger than size?

[denski]

This is not correct. Slice returns a new array of the sliced items. Should be fruits.slice(0,2), where 0 is the starting index and 2 is the number to take. developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/…

[Morgoth]

@Rishabh876 No it does not. For array.slice(0, n); it returns [0, min(n, array.length)).

[Manuel de la Iglesia Campos]

Pawel's answer seems the best option in resource-critical environments, and when the remaining elements can be discarded. Considering the case when the array is already smaller, this is a little improvement: if (arr.length > n) arr.length = n

[charlie6411]

I echo @Rishabh876 Another upped answer which is just incorrect and can be hard to debug at a later point.

[ashleedawg]

@Morgoth - I'm confused here -- are you saying this is incorrect because, in cases when the array has less than 3 elements, this method won't return 3 elements?

[Morgoth]

@ashleedawg Depends on your definition of correct. If array has less than 3 elements and you slice it for 3, then it will return all the elements in the array (i.e. less than 3). That is a very sensible behaviour, but if that does not make sense for your application you might want to check the length of the array first.

[alex]

please just pay attention you'll receive the values in the array beginning with the one at index start untill the one at index end - 1. this is good to allow looping.