Javascript like-module definition.

index.html

<script src="math.js"></script>
<script src="main.js"></script>

main.js

if (typeof Math !== 'undefined') {
    if (typeof Math.sum === 'function') {
        console.log(Math.sum(1, 2));
    } else {
        throw new Error('Math.sum function is not defined.');
    }
} else {
    throw new Error('A module `Math` is undefined.');
}

math.js

var sum = function() {
    var total = 0;
    for (var idx in arguments) {
        total += arguments[idx];
    }
    return total;
};

window.Math = {
    sum: sum
};