Need for product_price to be a number

When I get the product_price it returns "Unsupported value type" as a result of the function below but I need for it to be a number. I tried this.product_price = parseFloat(this.product_price); but that returns the error: Argument of type 'number' is not assignable to parameter of type 'string'. How do I cast product_price as a number?

In my HTML:

<div>{{ totalPrice }}div>
<ul *ngFor="let product of products">
<li>
<div>{{product.price }}div>
<button type="submit" class="btnAddAction" (click)="onSubmit( [product.price] )">Add to Cartbutton>
li>
ul>

The HTML is getting the products from the database which was created with this JSON:

{
"products": [
{
"price": 20.50,
},
{
"price": 20.50,
},
{
"price": 12.75,
}
]
}

In my controller typescript file:

get totalPrice() {
if (typeof this.product_price === "string") {
return "string";
}
if (typeof this.product_price === "number") {
return "number";
} else {
throw Error("Unsupported value type")
}
}
product_price: number;
onSubmit(product_price){
product_price = parseFloat(product_price);
const data = {
product_price
};
this.items.push(data);
localStorage.setItem('items', JSON.stringify(this.items));
}