postage_calculator.js

function match(conditions) {
    for (let i = 0; i < conditions.length; i++) {
        if (conditions[i][0]()) {
            return conditions[i][1]();
        }
    }
    return undefined;
}

function postage(order) {
    return match([
        [() => order.location === 'UK',
            () => match([
                [() => totalPrice(order) >= 100,                            () => 0],
                [() => totalWeight(order) < 1,                              () => 2.99],
                [() => totalWeight(order) >= 1 && totalWeight(order) < 3,   () => 4.99],
                [() => totalWeight(order) >= 3,                             () => 6.99]
            ])
        ],
        [() => order.location === 'EU',
            () => match([
                [() => totalWeight(order) < 1,                              () => 4.99],
                [() => totalWeight(order) >= 1 && totalWeight(order) < 3,   () => 6.99],
                [() => totalWeight(order) >= 3,                             () => 8.99]
            ])
        ]
    ]);
}