Which term describes JavaScript running in the background without blocking the page UI?

Which term describes JavaScript running in the background without blocking the page UI?

• A. window.localStorage
• B. contenteditable
• C. web worker
• D. onblur and onfocus

Answer: (C)

Running JavaScript without freezing the page hinges on moving heavy work off the main thread into a background task. A web worker does exactly that by running in its own separate thread, so long computations don’t block the UI or make the page feel unresponsive. This lets you perform tasks like data processing, complex calculations, or image analysis while the user can still scroll, click, or interact with the page. A key detail is that a worker cannot touch the page’s DOM directly. It communicates with the main thread through message passing, using postMessage to send data back and forth, and it can use transferable objects to move large data efficiently. You start a worker with a separate script file, and you set up listeners to handle messages from the worker. Other options don’t provide background execution. Local storage (localStorage) is a synchronous API for persisting data on the client and runs on the main thread, not in the background. Contenteditable is a feature that lets users edit content directly in the page. Onblur and onfocus are events that respond to focus changes, not background processing. So, for keeping the UI responsive while doing heavy work, web workers are the right choice.

Running JavaScript without freezing the page hinges on moving heavy work off the main thread into a background task. A web worker does exactly that by running in its own separate thread, so long computations don’t block the UI or make the page feel unresponsive. This lets you perform tasks like data processing, complex calculations, or image analysis while the user can still scroll, click, or interact with the page.

A key detail is that a worker cannot touch the page’s DOM directly. It communicates with the main thread through message passing, using postMessage to send data back and forth, and it can use transferable objects to move large data efficiently. You start a worker with a separate script file, and you set up listeners to handle messages from the worker.

Other options don’t provide background execution. Local storage (localStorage) is a synchronous API for persisting data on the client and runs on the main thread, not in the background. Contenteditable is a feature that lets users edit content directly in the page. Onblur and onfocus are events that respond to focus changes, not background processing.

So, for keeping the UI responsive while doing heavy work, web workers are the right choice.