Why item in Set is called value instead of key in JavaScript?

Question

Set.prototype.forEach which accepts a callback function in ES6 The function has 3 parameters to be consistent with Map.prototype.forEach. MDN says both the first 2 parameters for the callback are values because there is no keys in Set object. Why the item in Set is considered as value instead of key? No duplicated key in Set object would make more sense because there is also no duplicated key in Map.

Another aspect that value in Set looks like key in Map is that it makes Set.prototype.has(value) and Map.prototype.has(key) more consistent. Duplicated keys are NOT allowed in Array (key is index) and Map, but this cannot be extended to Set which doesn't allow duplicated values. Make it as duplicated keys are not allowed in all containers might be a more succinct rule.

A Map object has a key and a value which are separate entities. A Set object just has a value which you use to add the value to the Set and use to test if the value is in the Set. Essentially, a Set is indexed by the value so it's kind of like a Set is a Map where the key and value are the same. — jfriend00
– jfriend00, Commented Dec 3, 2016 at 20:22
Your last edit has driven me to a question to your own question: what's the question? :D — Matías Fidemraizer
– Matías Fidemraizer, Commented Dec 3, 2016 at 20:40
@MatíasFidemraizer My last edit added some evidence that calling item in Set as key instead of value might be more appropriate. So my question ist still why the stand calls item in Set as value, not as key. :) — Thomson
– Thomson, Commented Dec 3, 2016 at 22:24

Matías Fidemraizer · Accepted Answer · 2016-12-03 20:14:30Z

2

It's just because the semantics of set collections. A set stores values while a map stores keys and values.

For example, both motorbikes and bicycles own two wheels, but you still say that there're motorbikes and bicycles. Just because set values and map keys are unique doesn't mean that they're the same thing.

answered Dec 3, 2016 at 20:14

Matías Fidemraizer

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pato · Accepted Answer · 2016-12-03 20:24:56Z

2

From the MDN link which you have attached.

The are no keys in Set objects. However, the first two arguments are both values contained in the Set, so that the callback function is consistent with the forEach methods for Map and Array.

We can say that set is just an array which keeps distinct elements. There are no keys in sets.

edited Dec 3, 2016 at 20:24

answered Dec 3, 2016 at 20:18

pato

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2 Comments

Matías Fidemraizer Over a year ago

Well most collections have an underlying array storage ;D

pato Over a year ago

Well, we have to start from something :D

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