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A web worker, as defined by the World Wide Web Consortium (W3C) and the Web Hypertext Application Technology Working Group (WHATWG), is a JavaScript script executed from an HTML page that runs in the background, independently of other user-interface scripts that may also have been executed from the same HTML page. Web workers are able to utilize multi-core CPUs more effectively. The W3C and WHATWG envision web workers as long-running scripts that are not interrupted by user-interface scripts (scripts that respond to clicks or other user interactions). Keeping such workers from being interrupted by user activities should allow Web pages to remain responsive at the same time as they are running long tasks in the background. The simplest use of workers is for performing a computationally expensive task without interrupting the user interface. The W3C and the WHATWG are currently in the process of developing a definition for an API for web workers.〔 〕 == Overview == As envisioned by WHATWG, web workers are relatively heavy-weight. They are expected to be long-lived, have a high start-up performance cost, and a high per-instance memory cost. Web workers are not intended or expected to be used in large numbers as they could hog system resources. Web Workers allow for concurrent execution of the browser threads and one or more JavaScript threads running in the background. The browser which follows a single thread of execution will have to wait on JavaScript programs to finish executing before proceeding and this may take significant time which the programmer may like to hide from the user. It allows for the browser to continue with normal operation while running in the background.The web worker specification〔 is a separate specification from the HTML5 specification〔http://dev.w3.org/html5/spec/overview.html〕〔 and can be used with HTML5. There are two types of web workers:〔 dedicated and shared workers. When web workers run in the background, they do not have direct access to the DOM but communicate with the document by message passing. This allows for multi-threaded execution of JavaScript programs. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Web worker」の詳細全文を読む スポンサード リンク
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