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Deniz Akşimşek
Deniz Akşimşek

Posted on • Originally published at denizaksimsek.com on

Iota (from Golang) in JavaScript

#javascript #metaprogramming

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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 )
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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)
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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
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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)
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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. ↩︎

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