You can have an Internet-ready smart TV with a Roku plugged into it, and at some point you'll want to watch something that neither of them support. (For me, it's password-protected Vimeo streams.) That's when you need to plug your laptop directly into your HDTV.
If your laptop lacks an HDMI port, connecting it to a TV isn't so easy. But it isn't incredibly difficult either. It's all a matter of figuring out which ports you do have and which adapters you need.
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If you have a very small laptop, you may have an HDMI port and not know it. Look for a micro HDMI port: It's about the same size as a micro USB port. You can buy an adapter or a cable that will let you connect it to the standard HDMI port on your TV.
If you don't have Micro HDMI, see if your laptop has a DisplayPort, which can handle the same digital video and audio signals as HDMI. You can buy a DisplayPort/HDMI adapter or cable cheaply and easily. (This is actually how I watch those password-protected Vimeo streams.)
Don't have either of those Maybe your laptop has a DVI port. You can't miss it; it's the silliest-looking port ever designed. And once again, adapters and cables are readily available to connect this port to your HDTV.
But there's a problem: DVI is a video-only format. It doesn't carry audio. So you have to find another way to get the sound out. Your television might have an analog audio input associated with one of the HDMI ports. If it does, you can use a 3.5mm male-to-male audio cable to connect your laptop's sound system to your TV's speakers. If it doesn't, you'll have to use other speakers. If you have a separate receiver in the room, connect the laptop's audio to that. Otherwise, consider buying some computer speakers.
If your laptop doesn't have any of these ports, it will almost certainly have good old VGA. The same goes for your HDTV. Check your TV's manual to see what resolution the laptop should be set to. And don't expect the same image quality you'd get with HDMI, DisplayPort, or DVI.
VGA, like DVI, is video-only. The solutions I suggested above should work here, as well. And you're far more likely to have an audio input associated with the VGA port.