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

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

This sometimes happens when you have passed an incorrect number of arguments to an anonymous function:

    Object.keys(data).reduce((key: string) => {

    }, {}); 

will raise error:

No overload matches this call. Overload 1 of 3

Pass it the correct number of arguments:

    Object.keys(data).reduce((acc: any, key: string) => {
    }, {});

I also had this strange issue, but I worked around it using type assertions (in my case using a string enum):

(Object.values(someEnum) as string[]).concat(otherStringArray);

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

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<NodeJS.Signals>([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.