Typescript error: Type 'undefined[]' is not assignable to type 'void'

Question

In my Angular app I have a filter function that keeps track of both user inputed filter values, and whether or not those filter values are currently enabled/active. I am initializing these filters like so:

filters = { language: [], location: [], zipArray: [], firstName: [], lastName: [] };

I am running into a Typescript error with this section of code -- specifically this line: return this.filters.zipArray = [];

public onZipcodeEnabledChange(enabled): void {
    this.filters.zipArray = this.getZipcodeArray();
    if (!enabled || this.filters.zipArray && this.filters.zipArray[0] === ''){
        return this.filters.zipArray = [];
    }
}

The TypeScript error I'm getting is:

Type 'undefined[]' is not assignable to type 'void'.

I'm not understanding what the issue is here?

You have the return type of onZipcodeEnabledChange as void, but you're returning an empty array. Try writing return; on its own line after you set the zipArray filters. — Explosion Pills
– Explosion Pills, Commented Apr 18, 2018 at 16:49
void means it doesnt return anything and you have 2 issues 1 you're returning an assignment and two you're returning something if you want to resolve : string[] — Joe Warner
– Joe Warner, Commented Apr 18, 2018 at 16:50

Laxmikant Dange · Accepted Answer · 2018-04-18 16:59:29Z

8

You declared return type of onZipcodeEnabledChange as void it means that function will not return anything. And in if statement, you are returning assignment result of return this.filters.zipArray = []; i.e. [].

Solution 1

Just remove return keyword will work for you.

public onZipcodeEnabledChange(enabled): void {
    this.filters.zipArray = this.getZipcodeArray();
    if (!enabled || this.filters.zipArray && this.filters.zipArray[0] === ''){
       this.filters.zipArray = [];
    }
}

Solution 2

And if you want to return array from that function, you need to replace void with any[].

public onZipcodeEnabledChange(enabled): any[] {
    this.filters.zipArray = this.getZipcodeArray();
    if (!enabled || this.filters.zipArray && this.filters.zipArray[0] === ''){
       this.filters.zipArray = [];
       return this.filters.zipArray;
    }
}

edited Apr 18, 2018 at 16:59

answered Apr 18, 2018 at 16:50

Laxmikant Dange

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5 Comments

Joe Warner Over a year ago

and vis vera if you do want to return you'll have to change void to : any[] or string[] whatver the type the stuff in the array

Muirik Over a year ago

Great. Thanks. Will make this change.

Laxmikant Dange Over a year ago

@JoeWarner, Absolutely, I updated the answer. Thanks :)

Joe Warner Over a year ago

lol i was typing my answer out you beat me too it xD good stuff

David B Over a year ago

With Solution 2, like @JoeWarner mentioned, I would suggest typing the method return type instead of using any whilst also typing the properties here, you could initialise this something like zipArray: ['value1'] with the values, also I would recommend moving the return outside of the if block, so both paths return the same type for readability and this methods caller.

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