JSFiddle - React, Tailwind, and code Playground

HTML

<div id="output"></div>

JavaScript

var outputArea = document.getElementById('output');
var output = function(text) {
    outputArea.innerHTML += '<div>' + text + '</div>';
};

var input = [1, 2, 3];

output('"reduce" demo');
output('<br\>Input: ' + input);

var reduce = function(cb, seq) {
    var ret = seq[0];
    
    for(var i = 1, len = seq.length; i < len; i++) {
        ret = cb(ret, seq[i]);
    }
    
    return ret;
}

output('<br\>Reduce items (mul)');
var result = reduce(
    function(a, b) {
        return a * b;
    },
    input
);

output('<br\>Result: ' + result);