Array separator

separator.js

function separator(array, delimiter) {

    let len = array.length;

    const arr = new Array(len * 2 - 1).fill(0);

    return arr.reduceRight((p, _, i) => 
        [ 
            i % 2 === 0 
                ? array[--len] 
                :  delimiter, 
            ...p
        ]
    , [])
}