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 = [3, 4, 23, 5];

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

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

output('<br\>Multiply by two');
var result = map(
    function(v) {
        return v * 2;
    },
    input
);

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