Previous or next element in array based on string “direction”

Question

I'm trying to make function that will return next or previous item in array, based on parameter "direction". For example I have array = ['ferrari', 'bmw', 'merc', 'bugatti'] and I want my my function to return 'bugatti' IF currentPointer = 'ferrari' and direction = 'left'

nextPrev(array,direction,currentPointer)

In php we have function next() which moves the internal pointer ... but I don't know how to do it in javascript ...

Answer 1

The basic idea would be:

function nextPrev(array, direction, currentPointer) {
    var index = array.indexOf(currentPointer);
    if (direction=="left") {
        return array[index == 0 ? array.length-1 : index-1];
    } else if (direction=="right") {
        return array[index == array.length-1 ? 0 : index+1];
    } else {
        // default action or throw error
    }
}

You can reorganize this a bit:

function nextPrev(array, direction, currentPointer) {
    var index = array.indexOf(currentPointer);
    var len = array.length;
    if (direction=="left") {
        index--;
    } else if (direction=="right") {
        index++
    } else {
        // default action or throw error
    }
        return array[(index + len) % len];
}

You might want to add a check that array.indexOf returns a valid index (in case currentPointer contains something not in array ).

Answer 2

You can write something like this:

function makeDirectionalIterator(array){
    var currIndex = 0;

    return {
       nextPrev: function(direction){
           if(direction === 'left') {
              return currIndex < array.length-1 ?
                 {value: array[++currIndex], done: false} :
                 {done: true};
           }
           else {
              return currIndex > 0 ?
                 {value: array[--currIndex], done: false} :
                 {done: true};
           }
       },

       current: function() {
           return { value: array[currIndex] };
       }
    }
}

Then you can use it like the following

var itr = makeDirectionalIterator(array);
itr.current().value;
itr.nextPrev('left').value;
itr.nextPrev('left').done;

Answer 3

Try Something like this.

Using the numerical locations of the array and conditionally cycle through:

 var array = ['ferrari', 'bmw', 'merc', 'bugatti']; var returnedElement = nextPrev(array, "left", "ferrari"); // Show Returned Value (Console) console.log(returnedElement); function nextPrev(array, direction, currentPointer) { var arraySize = array.length - 1; var currentIndex = array.indexOf(currentPointer); if (direction === "left") { // Decrease array by one if (currentIndex == 0) { // Return Previous (Max Array) return array[arraySize] } else { return array[currentIndex - 1] } } else if (direction === "right") { // Increase array by one if (currentIndex == arraySize) { // Go to zero position return array[0] } else { return array[currentIndex + 1] } } else { console.log("Use either 'left' or 'right'"); } }