DEV Community

loading...

Iota (from Golang) in JavaScript

dz4k profile image Deniz Akşimşek Originally published at denizaksimsek.com on ・2 min read

(Skip to code)

Enums in Javascript

Currently, the ways we create enums include

  • String literals (see addEventListener), which can be typed with TypeScript, but look a bit ugly

    type Direction = 'north' | 'east' | 'south' | 'west'
    
  • TypeScript enums

    enum Direction { north, east, south, west }
    
  • Integer constants, IMO your best option if you’re not using TypeScript

    const Directions = { north: 0, east: 1, south: 2, west: 3 }
    

Enums in Go [1]

Go doesn’t have enums, but an unusual keyword iota:

type Direction int
const (
    North Direction = iota
    East
    South
    West
)
Enter fullscreen mode Exit fullscreen mode

There’s something subtle going on here. The iota relies on a few Go features:

  • When multiple const declarations are grouped together, the right hand side is implicitly repeated
  • Iota is incremented every time it is evaluated, and reset with each const

My JavaScript shorthand is nowhere near as magical… but it does make use of proxies.

function iota(start = 0) {
    let count = start
    return new Proxy({}, {
        get(o, prop) {
            if (prop in o) return o[prop]
            else return o[prop] = count++
        }
    })
}

const { north, east, south, west } = iota()
console.log(north)
Enter fullscreen mode Exit fullscreen mode

So, is this function any good?

For one, it lacks some of Go’s iota capabilities — you can’t create bitmasks with this the way you would in Go with 1 << iota. We could augment it a bit by accepting a callback:

function iota(cb = (i => i)) {
    let count = 0
    return new Proxy({}, {
        get(o, prop) {
            if (prop in o) return o[prop]
            else return o[prop] = cb(count++)
        }
    })
}

// flag bits
const { hasPermissionFlag, userModeFlag, useLegacyProtocolFlag } = iota(i => 1 << i)
const hasPermission = options & hasPermissionFlag
Enter fullscreen mode Exit fullscreen mode

I don’t think bitmasks are very common at all in JavaScript code, though.

A more significant setback is that you can’t get a list of all the enum values — nothing we can’t fix:

function iota(start = 0) {
    let count = start
    let firstProp = true
    return new Proxy({}, {
        get(o, prop) {
            if (firstProp) {
                firstProp = false
                return { // Enum descriptor
                    get values() { return o }
                }
            }
            if (prop in o) return o[prop]
            else return o[prop] = count++
        }
    })
}

const { Direction, north, east, south, west } = iota()
console.log(Direction)
Enter fullscreen mode Exit fullscreen mode

This is open for extension — we could add more methods on the enum description such as converting the integer value of a Direction to its name, or validating a Direction that we parsed from a config file.

I might have a metaprogramming addiction.


  1. Initially I wasn’t going to bring Go into this at all. However, it turns out enum is a reserved word in JS, so I went with iota for the name of the function, and felt the need to explain it. ↩︎

Discussion (0)

pic
Editor guide