JavaScript - Map() increment value

Question

I have a map as follows:

let map = new Map();
map.set("a", 1);
//Map is now {'a' => 1}

I want to change the value of a to 2, or increment it: map.get("a")++;

Currently, I am using the following:

map.set("a", (map.get("a"))+1);

However, this does not feel right. Does anyone know a cleaner way of doing this? Is it possible?

It's looks okay. PS: it feels you're using `iterate` term while trying to express something else. — zerkms, Dec 02 '18 at 20:39
This is okay as JavaScript uses a single-threaded runloop, so there's no chance of `a` getting read within another event handler between the `.get` and `.set` (if this code were in an event handler using a shared map). — thinkski, Jul 29 '19 at 23:57

trincot · Answer 1 · 2020-08-30T07:57:29.137

30

The way you do it is fine. That is how you need to do it if you are working with primitive values. If you want to avoid the call to map.set, then you must revert to a reference to a value. In other words, then you need to store an object, not a primitive:

let map = new Map();
map.set("a", {val: 1});

And then incrementing becomes:

map.get("a").val++;

edited Aug 30 '20 at 07:57

answered Dec 02 '18 at 20:59

trincot

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score 16 · Answer 2 · answered Aug 08 '21 at 17:25

16

for modern js, you could do this:

map.set("a", (map.get("a") ?? 0) + 1)

answered Aug 08 '21 at 17:25

AsukaMinato

1,017
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score 4 · Answer 3 · answered Dec 02 '18 at 20:59

According to the ECMAScript® 2015 Language Specification states, Map manipulation is based prototypes and the prototype methods assigned to add or retrieve data to or from a Map are the set and get methods respectively.

Except for the unnecessary parenthesis around your map.get("a"), your code is perfectly okay. That's how the Map is meant to be used. If you are looking for something that "may" reduce the length of your code and if it does works for your specific requirement, you may use the JavaScript Object.

So dear, your code is just the same as this:

map.set("a", map.get("a")+1);

score 4 · Answer 4 · answered Oct 18 '20 at 03:48

Map does not have a built-in feature that increments a value. However, since JavaScript constructs can be modified you can actually do that yourself:

let map = new Map()
// Modify your map
map.constructor.prototype.increment = function (key) {
  this.has(key) && this.set(key, this.get(key) + 1)
}
map.constructor.prototype.decrement = function (key) {
  this.has(key) && this.set(key, this.get(key) - 1)
}

// Now you can use it right away
map.set('test', 1)
map.increment('test')
map.increment('test')
map.decrement('test')
console.log(map.get('test')) // returns 2

To make this easier in the future, you can abstract it into a utility:

class CyborgMap extends Map {
  constructor() {
    super()
    
    this.constructor.prototype.increment = function (key) {
      this.has(key) && this.set(key, this.get(key) + 1)
    }
    this.constructor.prototype.decrement = function (key) {
      this.has(key) && this.set(key, this.get(key) - 1)
    }
  }
}

Now import it and use it:

import { CyborgMap } from "./utils"

let map = new CyborgMap()
map.set('test', 1)
map.increment('test')
map.increment('test')
map.decrement('test')
console.log(map.get('test')) // returns 2

score 1 · Answer 5 · edited Nov 08 '21 at 17:13

1

In map, if you didn't find any element it will give undefined. So, we can do the increment with the below code:

map.set("a", map.get("a") == undefined ? 1 : map.get("a") + 1);

or just

map.set('a', map.get('a') + 1 || 1);

edited Nov 08 '21 at 17:13

rflw

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answered Jun 26 '21 at 06:18

Mr. Perfectionist

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score 0 · Answer 6 · answered Dec 02 '18 at 20:56

0

Map#get returns the value of the specified element. It is opposite of an object accessor (object['a']) and is not eligible for a left-hand side assignment.

The conclusion is to use always Map#set for setting a new value.

answered Dec 02 '18 at 20:56

Nina Scholz

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score -1 · Answer 7 · answered Dec 02 '18 at 20:52

I don't know any cleaner way to do that, nevertheless I think that everything depends from the context of your code.

If you are iterating an array or something else and you want to increase your variable, I suggest to use a local variable to do that and, at the end of the iteration, set the value in the map.

var i = map.get('a')

values.forEach( el => {
  i += el.someField
})

map.set('a', i)

JavaScript - Map() increment value

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