typescript does not detect the type was narrowed-CodePudding

function encodeToken(value: number | string | null | undefined) {
  if (value !== null && value !== undefined) {
    value = value.toString()
  }

  return encodeBase64(value) // typescript complains this line
}

function encodeBase64(value: string | null | undefined) { ... }

Typescript complains as below:

(parameter) value: string | number Argument of type 'string | number' is not assignable to parameter of type 'string'. Type 'number' is not assignable to type 'string'

The "value" which I passed to encodeBase64 would be string | null | undefined, any number value will transform to string,

Does anyone know how to fix it? Very appreciated

CodePudding user response：

This happens when "strictNullChecks" is false in tsconfig.

Without the 'if' statement, this is ok:

function encodeToken(value: number | string | null | undefined) {
  value = value.toString() //narrowed down from `number | string` to `string` after this line
  return encodeBase64(value) 
}

But this does not work:

function encodeToken(value: number | string | null | undefined) {
  if (value !== null && value !== undefined) {
    value = value.toString()
  }
  return encodeBase64(value) //value is still considered `number | string`, so it complains
}

Apparently, TS skipped some checking for the if (value !== null && value !== undefined) part when "strictNullChecks" is false.