How can I return all previous items in a JavaScript array than a current value?

Question

Let's say I have an array:

var myArr = new Array('alpha','beta','gamma','delta');

And that I want a function to return an array of all items before a given item:

function getAllBefore(current) {
    var myArr = new Array('alpha','beta','gamma','delta');
    var newArr = ???
    return newArr;
}

getAllBefore('beta'); // returns Array('alpha');
getAllBefore('delta'); // returns Array('alpha','beta','gamma');

What's the fastest way to get this? Can I split an array on a value? Do I have to loop each one and build a new array on the fly? What do you recommend?

What about if I wanted the opposite, i.e. getAllAfter()?

Use indexOf to get the position of the first item that matches what you're looking for. Then use slice from 0 to that index — Ian
– Ian, Commented Jul 2, 2013 at 22:05
Use .indexOf() to find the match and .slice() to make a copy of part of the array. — jfriend00
– jfriend00, Commented Jul 2, 2013 at 22:05
No need for slice, just set the length to the index + 1 (as long as modifying the original is OK). :-) Also, Array.prototype.indexOf is ES5 so may need extra support for older browsers. — RobG
– RobG, Commented Jul 2, 2013 at 22:19

nnnnnn · Accepted Answer · 2013-07-02 22:06:15Z

22

function getAllBefore(current) {
    var myArr = new Array('alpha','beta','gamma','delta');
    var i = myArr.indexOf(current);
    return i > -1 ? myArr.slice(0, i) : [];
}

Get the index of the specified item. If found, .slice() from 0 to that index. If not found, return an empty array (or whatever other default value you like).

Note that .indexOf() is not supported (for arrays) in IE8 and older, but there is a shim you can use, or you could just use a simple for loop instead.

answered Jul 2, 2013 at 22:06

nnnnnn

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6 Comments

Ryan Over a year ago

Thanks! What would I change to get getAllAfter(current)?

Ian Over a year ago

@Ryan myArr.slice(i + 1)

nnnnnn Over a year ago

All after? return i > -1 ? myArr.slice(i+1) : myArr.slice(0); - that is, return from 1 after the index if the item is found, otherwise return a copy of the whole array if not found (or whatever other default you want for when not found; you could use return i > -1 ? myArr.slice(i+1) : []; to default to an empty array).

EmmyS Over a year ago

+1 for using ternary operators... I rarely see them in javascript and although some people hate then, for a simple either/or, I find it much easier to read.

nnnnnn Over a year ago

@RobG - Yes, that requirement hasn't been spelled out, but I covered those options in my previous comment and in my answer. I don't really like using .slice(0, i<0 ? 0 : i) because it seems kind of a waste to use .slice() to return an empty array. Doing it as shown in my answer looks clearer to me, and also allows an easy change to return null or undefined should the requirement change...

|

user16938897 · Accepted Answer · 2022-02-14 17:26:43Z

2

javascript slice array

// array.slice(start, end)
const FRUITS = ["Banana", "Orange", "Lemon", "Apple", "Mango"];
var citrus = FRUITS.slice(1, 3);
// citrus => [ 'Orange', 'Lemon' ]

// Negative values slice in the opposite direction
var fromTheEnd = FRUITS.slice(-3, -1);
// fromTheEnd => [ 'Lemon', 'Apple' ]

array cut only last 5 element

 arr.slice(Math.max(arr.length - 5, 0))

answered Feb 14, 2022 at 17:26

user16938897

Comments

McGarnagle · Accepted Answer · 2013-07-02 22:05:57Z

1

Use indexOf and slice:

newArr = myArr.slice(0, myArr.indexOf(current));

answered Jul 2, 2013 at 22:05

McGarnagle

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3 Comments

Pilgerstorfer Franz Over a year ago

what abount index of returning -1?

McGarnagle Over a year ago

@PilgerstorferFranz no worries there, it would just return an empty array.

Bergi Over a year ago

@McGarnagle: No, it returns everything but the last element then.

Lukho Mdingi · Accepted Answer · 2022-12-13 14:45:15Z

1

I recently had to do something like this for an array of objects. This is what I went with:

const myArr = [
    { controlId: 1, value: 'alpha'},
    { controlId: 2, value: 'beta' },
    { controlId: 3, value: 'gamma' },
    { controlId: 4, value: 'delta'}
];

function getAllBefore(id) {
    const index = myArr.findIndex( ({ controlId }) => controlId === id);
    return myArr.filter((_, i) => i < index);
}

answered Dec 13, 2022 at 14:45

Lukho Mdingi

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Comments

MikeD · Accepted Answer · 2013-07-02 22:10:58Z

0

Try something like this

var index = myArr.indexOf('beta');
var before = myArray.slice(0, index);

answered Jul 2, 2013 at 22:10

MikeD

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