JavaScript is typically considered an interpreted language, but the situation is more nuanced due to the way modern JavaScript engines work.
JavaScript combines both compilation and interpretation. In its earlier versions, JavaScript relied solely on interpretation, executing code line by line and delivering immediate results. Over time, performance issues emerged due to the relatively slow nature of interpretation.
To address these performance concerns, especially starting from newer versions of JavaScript, such as those associated with the V8 engine, a Just-In-Time (JIT) compiler was introduced. This JIT compiler optimizes execution and accelerates result presentation. It achieves this by generating a bytecode, which is a set of highly efficient instructions, making code execution more efficient.
The V8 engine employs a two-step process. Initially, it employs an interpreter to decode and execute the code. As code executes repeatedly, the V8 engine identifies patterns, including frequently executed functions and commonly used variables. These patterns are then compiled to machine code, significantly enhancing performance.
Here’s a brief explanation of both terms:
- Interpreted Language:
- In an interpreted language, the source code is executed directly by an interpreter, line by line, without a separate compilation step.
- JavaScript is primarily an interpreted language in the context of web browsers. The browser’s JavaScript engine reads and executes the source code directly.
- Compiled Language:
- In a compiled language, the source code is translated into machine code or an intermediate bytecode before execution, typically in a separate compilation step.
- In recent years, some JavaScript engines have introduced just-in-time (JIT) compilation. This means that JavaScript code is not compiled ahead of time but is instead translated into machine code at runtime for improved performance.
So, JavaScript combines aspects of both interpretation and compilation:
- When a web browser loads a web page containing JavaScript, the JavaScript code is initially parsed and executed as source code by an interpreter. This allows for a rapid startup and execution of the code.
- As the JavaScript code is executed, modern JavaScript engines analyze it and may identify parts of the code that are used frequently. These parts are then compiled into optimized machine code using a JIT compiler. This compiled code is cached and can be executed much faster than the original source code.
This combination of interpretation and compilation is a key factor in the performance improvements seen in modern JavaScript engines. It allows JavaScript to execute with near-native speed for critical code paths while maintaining the flexibility and ease of use associated with interpreted languages.