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ViewsTypescript not recognizing checks from function
I have a function that validates input
export const isValidString = (value: string | null | undefined): boolean => {
return value != null && value.length > 0 && value.trim() != "";
};
const func = (input: string) =>{
// some code
}
const someFunction = (input : string | null | undefined) => {
if(isValidString(input)){
func(input); //This is the line that is complaining
}
};
the call to the func is throwing an error which I'm not sure why as the isValidString is making sure the input is not undefined. Any reason why ?
Argument of type 'string | undefined' is not assignable to parameter of type 'string'.
Type 'undefined' is not assignable to type 'string'.ts(2345)
However, if I just do
if(input != null) func(input);
Everything works
·
Juan Pablo Isaza
1 answers
Answer question0
When isValidString() only returns a boolean, typescript does not know that you have checked that your type is a string now.
E.g. when you hoover over the input variable in this typescript playground example, you can see that the type is still string | undefined | null
To change that, your function must be a type-guard
export const isValidString = (value: string | null | undefined): value is string => {
Note, the return type of the function is now value is string
When you hoover over input in the new example, you can see that the type is now string
·
Juan Pablo Isaza
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