How to correctly type the "this" keyword referencing the Pinia-store itself

[DevKame]

This is a function within my piniaStore of my Vue+TS project:

My issue is with the word "this". It works but my ts compiler is crying.

    function validateRegisterInput(RegisterInput: RegisterInput): ValidationResult {
        let result: ValidationResult =
        {
            success: true,
            reason: null,
        };
        let key: keyof RegisterInput;
        for(key in RegisterInput)
        {
            if(key === "username")
            {
                switch(true)
                {
                    case RegisterInput[key].length < 3 || RegisterInput[key].length > 15:
                        result.success = false;
                        result.reason = "name-length";
                        break;
                    case RegisterInput[key].trim() === "":
                        result.success = false;
                        result.reason = "empty-name";
                        break;
                    case RegisterInput[key][0] === " " || RegisterInput[key][RegisterInput[key].length - 1] === " ":
                        result.success = false;
                        result.reason = "space-start-or-end";
                        break;
                    case RegisterInput[key].match(/^[^()<>{}[\]+*/%=>]*$/) === null:
                        result.success = false;
                        result.reason = "special-signs";
                        break;
                }
            }
            if(key === "email")
            {
                console.log(this.getAllUsers.value.find((user: VocabUser) => user.email === RegisterInput[key]));
                switch(true)
                {
                    case RegisterInput[key].trim() === "":
                        result.success = false;
                        result.reason = "empty-email";
                        break;
                    case RegisterInput[key][0] === " " || RegisterInput[key][RegisterInput[key].length - 1] === " ":
                        result.success = false;
                        result.reason = "space-start-or-end";
                        break;
                    case this.getAllUsers.value.find((user: VocabUser) => user.email === RegisterInput[key]) !== undefined:
                        result.success = false;
                        result.reason = "email-exists";
                        break;
                }
            }
            if(!result.success)
            {
                break;
            }
        }
        return result;
    }

getAllUsers is a computed property within my store and i want to access it using the this keyword.

Like i said, it works fine but, again, typescript is throwing errors:
When i hover over the this keyword it says this:

How do i type this correctly?