In Angular 8, you cannot use the 'greater than' operator with 'void' and 'number' types

Upon compiling my TypeScript file, I encountered an error message that states:

Error: Operator '>' cannot be applied to types 'void' and 'number'

Any suggestions on how to resolve this issue would be greatly appreciated.

Below is the section of code causing the error:

this.languages.forEach((o: any, i: any) => {
                const control1 = new FormControl(this.UserSelectedLanguages.indexOf(o.id) > -1); // if first item set to true, else false
                (this.languageform.controls.selectedlang as FormArray).push(control1);
});
javascriptangulartypescript

Answer №1

After some reflection, I realize that typescript focuses on static typing, making it contradictory to treat the return from indexOf as a boolean.

Additionally, it seems unlikely for indexOf to return void. Is UserSelectedLanguages a standard javascript array?

Although I'm not well-versed in angular or typescript, the expression 

this.UserSelectedLanguages.indexOf(o.id)
seems to yield void, leading to an invalid comparison between void and a number. In this scenario, there is no need to compare it with anything. Its presence alone, indicated by non-void status, confirms its existence:

// simply using this may result in true or false
!!this.UserSelectedLanguages.indexOf(o.id)

Answer №2

Double check to verify that the variable this.UserSelectedLanguages is indeed an array, as it appears to not be one.

Furthermore, a temporary solution could involve implementing the following code snippet:

const control1 = new FormControl(<number>this.UserSelectedLanguages.indexOf(o.id) > -1);

Answer №3

Appreciate all the helpful responses!

This method resolved my issue without any hitches.

I replaced 

this.UserSelectedLanguages.indexOf(o.id) > -1
with 
this.UserSelectedLanguages.includes(o.id)
and voilà, problem solved!

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