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 Cart</button>
  </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));
}

The solution to this problem was posted at https://groups.google.com/forum/#!topic/angular/fViPHF8Ir-k