A JavaScript engine is a computer program that executes JavaScript (JS) code. The first JavaScript engines were mere interpreters, but all relevant modern engines use just-in-time compilation for improved performance.

JavaScript engines are typically developed by web browser vendors, and every major browser has one. In a browser, the JavaScript engine runs in concert with the rendering engine via the Document Object Model.

The use of JavaScript engines is not limited to browsers. For example, the Chrome V8 engine is a core component of the popular Node.js and Deno runtime systems.

Since ECMAScript (ES) is the standardized specification of JavaScript, ECMAScript engine is another name for these engines.

History of JavaScript Engine

The first JavaScript engine was created by Brendan Eich in 1995 for the Netscape Navigator web browser. It was a rudimentary interpreter for the nascent language Eich invented. (This evolved into the SpiderMonkey engine, still used by the Firefox browser.)

The first modern JavaScript engine was V8, created by Google for its Chrome browser. V8 debuted as part of Chrome in 2008, and its performance was much better than any prior engine.[2][3] The key innovation was just-in-time compilation, which can significantly improve execution times.

Other browser vendors needed to overhaul their interpreters to compete.[4] Apple developed the Nitro engine for its Safari browser, which had 30% better performance than its predecessor.[5] Mozilla leveraged portions of Nitro to improve its own SpiderMonkey engine.[6]

Since 2017, these browsers have added support for WebAssembly. This enables the use of pre-compiled executables for performance-critical portions of page scripts. The JavaScript engines execute WebAssembly code in the same sandbox as regular JavaScript code.

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