Error with array: his.xyz is undefined

Ionic Angular.
I have a reactive form where I want to push the results of formControls into into an String-Array. The push-command gives the error “Type error: this.wo1 is undefined”.

HTML: ` <form [formGroup]=“nxtForm”>
Montag

Dienstag

Mittwoch

TS:` public nxtForm: FormGroup;
wo1: Array;
mo: string;
mot: string;
di: string;
dit: string;
mi: string;
mit: string;

constructor(
public formBuilder: FormBuilder,
private router: Router) {this.nxtForm = this.formBuilder.group({
mo: ‘’,
di: ‘’,
mi: ‘’,
wo1: ,
});
}

ngOnInit() {
}

goNext() {
if (this.mo === ‘f’) {
this.mot = ‘MO: früh’;
} else if (this.mo === ‘s’) {
this.mot = ‘MO: spät’;
} else if (this.mo === ‘t’) {
this.mot = ‘MO: 24h’;
}
this.wo1.push(this.mot); ’

… The last line i the one causing the error.

Thanks for your help!

Edit: in the shown code, wo1 is defined as Array. In fact, I also defined it as a string. I don’t know why thjis is not shown in the published text?

I found the solution. The way I had built this in ionic 5, it worked. Now, IO have to define the variable inside the procedure Next() as let wo1: string[].

I cant see where mo1 is initialised. It has a type declaration but never gets a value assigned - so push cannot work

So something like this.wo=