This option makes the TypeScript compiler run indefinitely. Whenever a source file is changed, the compiling process is triggered automatically to generate the new version. This is a useful option to switch on during our developments:

1. Let's give this a try by entering the following in a terminal:

tsc orderDetail --watch

2. The compiler should run and, when completed, give the message Watching for file changes. Let's change the getTotal method in the OrderDetail class to handle situations when discount is undefined:

getTotal(discount: number): number {
  const priceWithoutDiscount = this.product.unitPrice * this.quantity;
  const discountAmount = priceWithoutDiscount * (discount || 0);
  return priceWithoutDiscount - discountAmount;
}

3. When we save orderDetail.ts, the compiler will say File change detected. Starting incremental compilation... and carry out the compilation.

To exit the watch mode, we can kill the terminal by clicking the bin icon in the Terminal.