'use strict';

const checkArray = val => {
    if (!Array.isArray(val)) {
        throw Error('value must be an array');
    }
};

module.exports = {
    toArray: arraylike => [...arraylike],

    ensureArray: val => Array.isArray(val) ? val : [val],

    head (ar) {
        checkArray(ar);

        const [head, ...rest] = ar;
        return head;
    },

    tail (ar) {
        checkArray(ar);

        const [ tail ] = ar.slice(-1);
        return tail;
    },

    unique (ar) {
        checkArray(ar);
        return ar.filter((val, i, input) => i === input.indexOf(val));
    },

    flatten: function flatten (ar) {
        checkArray(ar);

        return ar.reduce((acc, item) => {
            if (Array.isArray(item)) {
                acc.push(...flatten(item));
            } else {
                acc.push(item);
            }

            return acc;
        }, []);
    }
};