How many array types are in javascript?

Question

I wish I can create a function that will help me to get the last item from an array, NOT just from a normal array, but from other array types such as NodeList, (the only two types I know are NodeLists and Arrays)

my function:

       const getLastItem = (list) => {
            if (list instanceof Array || OR NODELIST OR ANYTHING ELSE) {
                return list.slice(-1)[0];
            }
            if(typeof list == 'object'){
           // get the last key from an object, it's another thing which I wish to append to this
           // function
              return Object.keys(list).slice(-1)[0]
           //this will return the last key from an object
            }
        }

[EDIT]: and do you think that there is a better implementation than this one?

function isIterable(obj) { return typeof obj[Symbol.iterator] === 'function'; } — Roberto Zvjerković
– Roberto Zvjerković, Commented Aug 27, 2020 at 11:29
Does "last key from an Object" actually make sense? Is there a defined order that would mean that "last" had any useful meaning? — SpoonMeiser
– SpoonMeiser, Commented Aug 27, 2020 at 11:30
First convert your iterable to an array then return the last element. — pishpish
– pishpish, Commented Aug 27, 2020 at 11:33
I guess it shouldn't make any real difference, but I try to avoid additional function calls when possible. Too bad JS never got [-1] syntax. — pishpish
– pishpish, Commented Aug 27, 2020 at 11:39

Lionel Rowe · Accepted Answer · 2020-08-27 16:13:47Z

Something like this? The function grabs the last item from iterables and calls itself recursively on Object.entries of non-iterable objects. It returns undefined if it can't do either (or if the iterable is empty).

const getLastItem = obj => {
    if (obj && obj[Symbol.iterator]) {
        const arr = [...obj]

        return arr[arr.length - 1]
    } else if (obj && typeof obj === 'object') {
        // You could also use `Object.keys` or `Object.values`
        // depending what you want to happen.
        // Note that relying on the order of properties
        // in normal objects is typically a bad idea.
        return getLastItem(Object.entries(obj))
    }
    
    return undefined
}

;[
    // all of the following work
    getLastItem([1, 2]),
    getLastItem({a: 1, b: 2}),
    getLastItem(document.querySelectorAll('p')),
    getLastItem(new Set([1, 2])),
    getLastItem(new Map([['a', 1], ['b', 2]])),
    getLastItem(document.querySelector('p').classList),
    getLastItem('string!'),
    
    // all of the following are `undefined`
    getLastItem(null),
    getLastItem(undefined),
    getLastItem(0),
    getLastItem(1),
    getLastItem(document.querySelectorAll('fake-element')),
    getLastItem([]),
    getLastItem({}),
    getLastItem(''),
    getLastItem(new WeakMap([[{}, 1], [{}, 2]])), // not iterable
].forEach(x => console.log(x))

// Edge case: if `obj` is an infinite iterable, you'll get
// stuck in an infinite loop due to use of the spread operator...
// So don't do this 😉
// 
// const infiniteIterable = (n => ({
//     *[Symbol.iterator] () {
//         while(true) yield n++
//     },
// }))(0)
// 
// getLastItem(infiniteIterable)

<p class="class1 class2 class3">first</p>

<p>second</p>

<p>last</p>

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