How do I get the index of object in array using angular?

I need to have the index of an object in array so I can delete this part of the array. I tried using this:

var index = this.urenRegistratie.indexOf(newDatum);


But it keeps returning -1 and I don't know why this is happening.

this is the part of the code I have. it gets data out of a form in html and places that into my array, now I already have an if statement ready ( exisitingDatum ) , my code needs to be in there. Can someone please help me out a bit?

store(newValue:number, newDatum, newAanwezig, newComment){
const existingDatum = this.urenRegistratie.find(billable => {
return billable.datum === newDatum;
return
});

if (!existingDatum) {
let billable = new BillableHours();
billable.datum = newDatum;
billable.hours = +newValue;
billable.aanwezig = newAanwezig;
billable.comment = newComment;

if(billable.aanwezig == "Aanwezig" && billable.hours !== 0 && billable.datum !== null) {
this.urenRegistratie.push(billable);
}
}

if(existingDatum) {

}

}

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