How do you run map on a Javascript Set? [duplicate]

Question

Javascript has an Array.prototype.map but there doesn't seem to be an equivalent for sets. What's the recommended way to run map on a set in Javascript?

EDIT: Adding an example as requested

users = new Set([1,2,3])

// throws an error: 
// Uncaugh TypeError: users.map is not a function
users.map(n => n*n)

see also stackoverflow.com/q/31084619/1048572 or stackoverflow.com/q/31232415/1048572 — Bergi
– Bergi, Commented Sep 4, 2016 at 11:51
what would you like to do with the result of the calculation? a new array, a new Set? a new Map? — Nina Scholz
– Nina Scholz, Commented Sep 4, 2016 at 11:55

Community · Accepted Answer · 2020-06-20 09:12:55Z

6

You can use the

Spread-Operator

The spread syntax allows an expression to be expanded in places where multiple arguments (for function calls) or multiple elements (for array literals) or multiple variables (for destructuring assignment) are expected.

on your Set to map over the Set like so.

const s = new Set([1,2,3,4]);

const a = [...s].map( n => n * 2 )

console.log(a)

edited Jun 20, 2020 at 9:12

CommunityBot

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answered Sep 4, 2016 at 11:45

DavidDomain

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1 Comment

user6445533 Over a year ago

You should wrap your map expression in a Set constructor, because mapping a Set should return a Set. The problem arises with duplicate return values though: new Set(Array.from(s).map(n => n % 2)) yields Set {1, 0} - but mapping shouldn't transform the structure of the Set.

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