I have an array

var arr = ["1", "3", "2", "4"];

I need a function that returns the next or the previous array key based on a given key value:

function closestTo(arr, key, direction) { 
 // do stuff here and return the next or previous id
}

So to find next of 4, I call the function; closestTo(arr, 4, 'next' ) this should return 1

And closestTo(arr, 4, 'prev' ) should return 2

Any ideas of this could also achieved with underscore?

I have an array

var arr = ["1", "3", "2", "4"];

I need a function that returns the next or the previous array key based on a given key value:

function closestTo(arr, key, direction) { 
 // do stuff here and return the next or previous id
}

So to find next of 4, I call the function; closestTo(arr, 4, 'next' ) this should return 1

And closestTo(arr, 4, 'prev' ) should return 2

Any ideas of this could also achieved with underscore?

• javascript
• arrays
• iteration
• underscore.js

• For 1 and "prev" it has to be ...? And if the key isn't found? – Andreas Commented Feb 8, 2015 at 0:07
• it has to be 4 :) .. It is like a loop; so always returns something except when the supplied key is not found; inthis case it returns null or false.. .. – numediaweb Commented Feb 8, 2015 at 0:08

3 Answers 3

Maybe something like this?

function closestTo(arr, key, direction) {
    var offset_index = (direction === 'prev') ? -1 : 1;

    // Convert to integers
    var intarr = arr.map(function(x) {
        return parseInt(x, 10);
    });

    return intarr[(intarr.length + intarr.indexOf(key) + offset_index) % intarr.length];
}

I have wrote script for you:)
http://jsfiddle/maxim_mazurok/6s7z6zwt/
But array should be like var arr = [1, 2, 3, 4]; if you want to call function with number as a second param.

var arr = [1, 2, 3, 4];

function closestTo(arr, key, direction) {
    var last = arr.length - 1;
    var first = 0;
    var keyIndex = arr.indexOf(key);
    switch (direction) {
        case ('next'):
            if (keyIndex != last) {
                return arr[keyIndex + 1];
            } else {
                return arr[first];
            }
            break;
        case ('prev'):
            if (keyIndex != first) {
                return arr[keyIndex - 1];
            } else {
                return arr[last];
            }
    }
}
alert(closestTo(arr, 4, 'next' ));
alert(closestTo(arr, 4, 'prev' ));

You only need pure JavaScript for this:

function closestTo(arr, key, direction) {
    var keyIndex = arr.indexOf(key),
        indexToReturn;

    if (direction === 'prev') {
        indexToReturn = keyIndex > 0 ? keyIndex - 1 : arr.length -1;
    } else if (direction === 'next') {
        indexToReturn = keyIndex < arr.length - 1 ? keyIndex + 1 : 0;
    }

    return arr[indexToReturn];
}