Typescript - Argument of type 'string | (string | null)[]' is not assignable to parameter of type 'string'

Question

I am getting a typescript error from the following code:

if (this.$route?.query?.domainName) {
  this.setDomain(this.$route.query.domainName);
}

The above code throws the following error:

Typescript - Argument of type 'string | (string | null)[]' is not assignable to parameter of type 'string'

 if (this.$route?.query?.domainName) {
   this.setDomain(this.$route.query.domainName);
                  ^
 }

My setDomain function just accepts a parameter of type string like so:

setDomain(domain: string) {
  this.domainName = domain;
}

I don't understand how the parameter could ever be null as I am checking the object property exists using the nested ? after the object properties in the if statement. Why would it be throwing this error?

Your argument is still a nullable string, but your method only accepts a non-nullable string as parameter. You would need to cast your argument to a non-nullable string before passing it. — Tobias von Falkenhayn
– Tobias von Falkenhayn, Commented Mar 29, 2021 at 10:16

Cameron Little · Accepted Answer · 2021-03-29 10:22:40Z

2

In your code, domainName could still be something other than a string - an array ((string | null)[]). Your conditional guard is only verifying that it's not a falsey value, not that it's a string.

If you check that it's a string, it should work. Note that this example will allow an empty string, which your current code won't.

declare const $route: null | { query: null | { domainName: null | string | (string | null)[] } }
declare const setDomain: (domain: string) => void;

if ($route?.query?.domainName) {
    // reproduces your error
    setDomain($route.query.domainName);
}

if (typeof $route?.query?.domainName == "string") {
    // no error
    setDomain($route.query.domainName);
}

See in the typescript playground

answered Mar 29, 2021 at 10:22

Cameron Little

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