let equipment = {
    head: {},
    cape: {},
    neck: {},
    ammunition: {},
    weapon: {},
    body: {},
    shield: {},
    legs: {},
    hand: {},
    feet: {},
    ring: {}
};

function createShortName(name) {
    return name.toLowerCase().replace(/s/g, '');
}

function addItem(slot, name, cost, img, income=0, atk_bonus=0, str_bonus=0, def_bonus=0, rngd_bonus=0, mage_bonus=0) {

    let newItem = {
        slot: slot,
        name: name,
        cost: cost,
        img: img,
        income: income,
        atk_bonus: atk_bonus,
        str_bonus: str_bonus,
        def_bonus: def_bonus,
        rngd_bonus: rngd_bonus,
        mage_bonus: mage_bonus
    }

    equipment[slot][createShortName(name)] = newItem;
}

let currentSlot = 'head';
addItem(currentSlot, 'head_item_1', 100, 'img');
addItem(currentSlot, 'head_item_2', 200, 'img');

currentSlot = 'cape';
addItem(currentSlot, 'cape_item_1', 100, 'img');
addItem(currentSlot, 'cape_item_2', 200, 'img');

currentSlot = 'neck';
addItem(currentSlot, 'neck_item_1', 100, 'img');
addItem(currentSlot, 'neck_item_2', 200, 'img');

// etc. etc. etc.

function bindFirst(func, firstArgument) {
    return function(...arguments) {
        return func(firstArgument, ...arguments);
    }
}

const addToHead = bindFirst(addItem, 'head');

addToHead('head_item_1', 100, 'img');
...

let newItems = new Map([
    [
        'head',
        [{
            name: 'head_item_1',
            cost: 100,
            img: 'img'
        },
        {
            name: 'head_item_1',
            cost: 100,
            img: 'img'
        }]
    ], [
        'cape',
        [{
            name: 'cape_item_1',
            cost: 100,
            img: 'img'
        },
        {
            name: 'cape_item_1',
            cost: 200,
            img: 'img'
        }]
    ]
]);

items.forEach((items, slot) => {
    const addToSlot = bindFirst(addItem, slot);
    items.forEach(item => {
        addToSlot(item.name, item.cost, item.img);
    });
});