Question 53192 in Javascript Question posted in
A very good W3school tutorial can be found here.

Getting the last item from a Map – a simple solution? – Javascript

I found some answers suggesting iterating through the Map, setting the iterated element to a variable and then reading it after the iteration. But isn’t there any better, more elegant view? I could not find a better solution so far.

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7

Answers


  1. 0

    Convert the Map to an array using the Array.from() method and then use the array index to access the last item.

    // Convert Map to an array and get the last item
const myArray = Array.from(myMap);
const lastItem = myArray[myArray.length - 1];

// Log the last item to the console
console.log(lastItem);

    you can get the last item by accessing the array index [myArray.length - 1]

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  2. 0

    You can convert the Map to an array of entries, then get the last element.

    const map = new Map([['a', 1], ['b', 2], ['c', 3]]);
let lastEntry = [...map].at(-1);
console.log(lastEntry);

// or only get last key/value
let lastKey = [...map.keys()].at(-1);
let lastValue = [...map.values()].at(-1);
console.log(lastKey, lastValue);

    However, it’s more efficient to just iterate over the entries and keep the last one.

    const map = new Map([['a', 1], ['b', 2], ['c', 3]]);
let entry; for (entry of map);
console.log(entry);
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  3. 0

    With the built-in Map there’s no way other than iterating the entries and return what comes last. You can, however, extend Map so that it can record its own "history", for example:

    class MapWithHistory extends Map {
    history = []

    set(key, val) {
        this.history.push(key)
        return super.set(key, val)
    }
}

m = new MapWithHistory()

m.set('foo', 123)
m.set('bar', 456)
m.set('baz', 789)

lastKey = m.history.at(-1)

console.log(lastKey)
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  4. 0 
    const inputMap = new Map([
  ['k1', 'v1'],
  ['k2', 'v2'],
  ['k3', 'v3']
]);

const lastEntry = [...inputMap.entries()].pop();
console.log(lastEntry);
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  5. 0

    convert a map to the array with spread operator and use .pop

    const inputMap = new Map([
  ['k1', 'v1'],
  ['k2', 'v2'],
  ['k3', 'v3']
]);

const lastEntry = [...inputMap].pop();
console.log(lastEntry);
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  6. 0

    No, there is not. Maps are not an ordered data structure in the sense that they support indexed access – all they have is a deterministic iteration order. If you care about controlling the order of elements or accessing them by index, use an array (possibly in addition to the map).

    In terms of elegance, I would recommend a helper function that takes an iterator:

    function last(iterator) {
  let element;
  for (element of iterator) {}
  return element;
}

    then call it as

    last(map.values())

    or

    last(map.entries())
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  7. 0

    You could iterate the map directly and use a counter from size and get the last pair.

    const
    map = new Map([['k1', 'v1'], ['k2', 'v2'], ['k3', 'v3'], ['k4', 'v4']]),
    last = [];

let s = map.size;
map.forEach((k, v) => {
    if (!--s) last.push(k, v);
});

console.log(last);
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