I have 2 variables a & b;

const a:string | undefined = undefined; 
const b:string | undefined = undefined; 

Those variables get passed as arguments to 2 different functions. Function 1 checks to see if a & b are undefined and returns false if they are, if not it returns true.

const checkIfUndefined = (a:undefined | boolean,b:undefined | boolean):boolean =>{

  if((a && b) === undefined){
    return false; 
  }

  return true
}

I am using both of the functions with a turnery operator like this:

checkIfFalse(a,b) ? log(a,b) : null;

the second function (log) still throws an error because it still thinks a & b are undefined even though second function will only get run if checkIfFalse function will return true.

Is there a way to work around this in typescript?

This is the error im getting:

Argument of type 'undefined' is not assignable to parameter of type 'boolean'.(2345)

Typescript playground link https://www.typescriptlang.org/play?#code/MYewdgzgLgBAhgLmgJwJZgOYwD4wK5gAmApgGbrGEwC8 RZFhA3DAFCiSwBGSUamOOiXJhKNIQ1HM27cNBjAAFsWABrAJKkAYnAA2EYuIAUiAsMaCU6DABoeZyWNxXMASgRcQIXcThgaAHwA3qysMDCopEYmMABksTBcrjTUtA4ilK4h4eHIxFB4yP6kegYsYTAAvqG5 YX fHjErNWynDC6IFi0Jrz8tjwuGMnUwW0Q3sQAdJ0YJnauTC2hSioa2qXE80kwAPwdXdvJCDBgeLq6TEA

CodePudding user response：

The problem is that checkIfFalse(a,b) does not assert that a and b are indeed not undefined.

You can use a custom type guard to adjust your checkIfUndefined method:

const checkIfUndefined = (a_value: string | undefined): a_value is string => {
  return a_value !== undefined;
}

This function asserts that a_value is a string, so you'd be able to do this:

const log = (a:string) => {
  console.log(a);
}

checkIfUndefined(a) ? log(a) : null;

And combined:

const a:string | undefined = undefined; 
const b:string | undefined = undefined; 

const checkIfUndefined = (a_value: string | undefined): a_value is string =>{
  return a_value !== undefined;
}

const log = (a:string,b:string) =>{
console.log(a,b);
}

checkIfUndefined(a) && checkIfUndefined(b) ? log(a, b) : null;

Playground

CodePudding user response：

Variables a and b are of type string or undefined, in methods you are accepting values of a and b as boolean | undefined