Is it possible to find or add an element in one step in a Javascript Map?

I would like to do the following in one step (to avoid looking twice for the right place of the key):

// get the value if the key exists, set a default value otherwise
let aValue = aMap.get(aKey)
if(aValue == null) {
    aMap.set(aKey, aDefaultValue)
}

Instead I would like to search for the key only once.

In c++, one can use std::map::insert() or std::map::lower_bound()

In javascript the code could look like this:

let iterator = aMap.getPosition(aKey)
let aValue = aMap.getValue(iterator)
if(aValue == null)
{
    aMap.setWithHint(aKey, aValue, iterator)
}

or

let aValue = aMap.getOrSet(aKey, aDefaultValue) 

I suppose that it is not possible, but I want to make sure I am correct. Also I am interested in knowing why it is not possible while it is an important feature.

Is it possible to find or add an element in one step in a Javascript Map?

I would like to do the following in one step (to avoid looking twice for the right place of the key):

// get the value if the key exists, set a default value otherwise
let aValue = aMap.get(aKey)
if(aValue == null) {
    aMap.set(aKey, aDefaultValue)
}

Instead I would like to search for the key only once.

In c++, one can use std::map::insert() or std::map::lower_bound()

In javascript the code could look like this:

let iterator = aMap.getPosition(aKey)
let aValue = aMap.getValue(iterator)
if(aValue == null)
{
    aMap.setWithHint(aKey, aValue, iterator)
}

or

let aValue = aMap.getOrSet(aKey, aDefaultValue) 

I suppose that it is not possible, but I want to make sure I am correct. Also I am interested in knowing why it is not possible while it is an important feature.

• javascript
• dictionary
• find
• addition

Share Improve this question Follow edited May 23, 2017 at 12:00 CommunityBot 111 silver badge asked May 10, 2016 at 9:47 arthur.swarthur.sw 11.7k99 gold badges5151 silver badges110110 bronze badges 3

• Do I understand you correctly that you are concerned about aKey having to be looked up twice in your code example? – nils Commented May 10, 2016 at 10:19
• Yes, this is the problem. – arthur.sw Commented May 10, 2016 at 12:36
• 1 In general, you don't have to worry about these kind of micro-optimizations in JavaScript. Modern JavaScript engines do a pretty good job at handling those. Also, as far as I know, there is no way to prevent this using maps. – nils Commented May 10, 2016 at 12:38

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2 Answers 2

Sorted by: Reset to default 5

The lookup has to happen anyway, it doesn't matter much if you avoid it, at least until the engines are optimized much more.

But Map.has is a nicer solution and should be a bit faster than Map.get(). For example:

myMap.has(myKey) ? true : myMap.set(myKey, myValue)

Performance should be irrelevant on this level unless you're google-scale. But if it's a serious bottleneck, an Array should still be faster than Map/Set for the forseeable future.

I personally ended up changing my Map to a simple Object. That allows to write a reduce (that groups entries into a Map of Sets) like this:

.reduce((a, [k, v]) => (a[k] = a[k] || new Set()).add(v) ? a : a, {})

With Map it should have bee

.reduce((a, [k, v]) => (a.has(k) ? a : a.set(k, new Set())).get(k).add(v) ? a : a, new Map())

That feels little too cumbersome for this purpose.

I agree that something like this would be ideal if ever supported:

.reduce((a, [k, v]) => a.getOrSet(k, new Set()).add(v) ? a : a, new Map())