when to use reduce and reduceRight?

Question

Can you describe this for me?

var arr, total;
arr = [1, 2, 3, 4, 5];
total = arr.reduce(function(previous, current) {
return previous + current;
});
// total         


        

Answer 1

Array.reduceRight() is great when:

• you need to iterate over an Array of items to create HTML
• AND need a counter in HTML prior to the items

.

var bands = {
    Beatles: [
        {name: "John", instruments: "Guitar"},
        {name: "Paul", instruments: "Guitar"},
        {name: "George", instruments: "Guitar"},
        {name: "Ringo", instruments: "Drums"}]
};
function listBandplayers(bandname, instrument) {
    var bandmembers = bands[bandname];
    var arr = [  "" , 0 , ` of ${bandmembers.length} ${bandname} play ` , instrument , "",
                "\n
" , ...bandmembers , "\n
" ];
    var countidx = 1;
    return arr.reduceRight((html, item, idx, _array) => {
            if (typeof item === 'object') {
                if (item.instruments.contains(instrument)) _array[countidx]++;
                item = `\n\t
` + item.name + "
";
            }
            return item + html;
    });
}
console.log( listBandplayers('Beatles', 'Drums') );
/*
1 of 4 Beatles play Drums

    
John
    
Paul
    
George
    
Ringo

*/
console.log( listBandplayers('Beatles', 'Guitar') );
/*
3 of 4 Beatles play Guitar

    
John
    
Paul
    
George
    
Ringo

*/