How to get the index of first element in array without loop?

Question

I have an array of size 5010. For example, a=[0...5009] and my first element is placed at index 5002 which is dynamic and may change. So, how do I get directly the first element of the array without looping through array in javascript.

What do you mean first element? The first element is at `a[0]` — nick zoum, Jun 13 '19 at 13:14
your first element is at index 0. if you want the item at 5002 use `myArray[5002]`... — I wrestled a bear once., Jun 13 '19 at 13:15
If you want to avoid writing a for loop, you can use `Array.filter()` for example. Which loops internally. Avoiding a loop at all isn't possible. Besides, I guess you are referring to your first element as a object with e.g id: 1 at position 5002 — r3dst0rm, Jun 13 '19 at 13:15
If your object has an unique id you may use the `.find` method. `arr.find( item => item.id === YOUR_ID )` — darklightcode, Jun 13 '19 at 13:16

Nina Scholz · Answer 1 · 2019-06-13T13:31:40.143

You need some kind of looping.

You could use Array#some with a sparse array and return true for the first nonsparsed item, after assigning the value to a variable.

This approach works for any value, even for undefined or null or any other falsy value.

var array = [],
    index;

array.length = 5010;
array[5002] = null;

array.some((_, i) => (index = i, true));
console.log(index);

Array#findIndex does not work with unknown value, because it visits every element, even the sparse elements.

var array = [],
    index;

array.length = 5010;
array[5002] = 'some value'

index = array.findIndex((v, i) => (console.log(i, v), true));
console.log(index);

score 0 · Answer 2 · answered Jun 13 '19 at 13:17

0

If the previous elements are undefined, you may need to filter the array first to get only defined values in the array.

const definedValues = array.filter(item => item); definedValues[0] // this will gives you the first defined item in the array.

answered Jun 13 '19 at 13:17

Jules Pollender

38
7

nick zoum · Answer 3 · 2019-06-13T13:22:54.650

If you want to get the index of an element whose value you already have, use Array#indexOf.

var list = [1,2,3];
console.log(list.indexOf(2)); // 1

If you want to find the index using a function, use Array#findIndex.

var list = [1,2,3];
console.log(list.findIndex(item => item === 2)); // 1

So if you want to find the first element that is a troothy do:

var list = [null,2,3];
console.log(list.findIndex(item => item)); // 1

score 0 · Answer 4 · edited Jun 13 '19 at 13:51

0

You can try :

var array1 = [1, 2, 3];

var firstElement = array1.shift();

console.log(array1); // expected output: Array [2, 3]

console.log(firstElement); // expected output: 1

Here you can find some other examples Get first element in array

edited Jun 13 '19 at 13:51

Clemsang

5,053
3
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answered Jun 13 '19 at 13:28

NikoSch

1
1

How to get the index of first element in array without loop?

4 Answers4