No overload matches this call. Type 'string' is not assignable to type 'Signals'

Question

I am using typescript to build a microservice and handling signals as well. The code was working fine till a few days ago but recently it started throwing errors. Couldn\'t find

Answer 1

Solution 1: Keep numeric enum signals

Object.values(signals)
  // numeric enum includes reverse mapping, filter numbers out and keep "SIGHUP" etc.
  .filter((s): s is NodeJS.Signals => typeof s !== "number") 
  .forEach(signal => {
    process.on(signal, ...) // works now
  })

Solution 2: Use pure signal string literal types

// these string literal items are strongly typed by built-in NodeJS.Signals type
Object.values(["SIGHUP", "SIGINT", "SIGTERM"])
  .forEach(signal => {
    process.on(signal, ...) // works now
  })

Solution 3: Change to string enum (no reverse mapping)

enum signals2 {
  SIGHUP = "SIGHUP",
  SIGINT = "SIGINT",
  SIGTERM = "SIGTERM"
}

Object.values(signals2)
  .forEach(signal => {
    process.on(signal, ...) // works now
  })

Why does the error happen?

Numeric enums like signals include a reverse mapping. For example you can do the following:

const r1 = signals.SIGHUP // r1 value: 1
const r2 = signals[signals.SIGINT] // r2 value: "SIGINT"
const r3 = signals[15] // r3 value: "SIGTERM"

That is why you get (string | signals)[] back for Object.values(signals), where string stands for the enum keys and signals for the enum values.

Now, parameter signal in process.on(signal, ...) must be one of the predefined Node.JS string literal types. However we pass in string | signals item type, so TS yells at this point.