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Chapter 20/End of Chapter/sportsstore/src/data/cart_models.ts

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+export interface CartLine {
+    productId: number;
+    quantity: number;
+}
+
+export interface Cart {
+    lines: CartLine[];
+}
+
+export const createCart = () : Cart => ({ lines: [] });
+
+export const addLine = (cart: Cart, productId: number, quantity: number) => {
+    const line = cart.lines.find(l => l.productId == productId);
+    if (line !== undefined) {
+        line.quantity += quantity;
+    } else {
+        cart.lines.push({ productId, quantity })
+    }
+}
+
+export const removeLine = (cart: Cart, productId: number) => {
+    cart.lines = cart.lines.filter(l => l.productId !== productId);
+}