Why JavaScript is Not Just an Interpreted Language: Understanding JIT Compilation

When it comes to translating code into machine code, there are two main approaches: interpreters and compilers. JavaScript is often referred to as an interpreted language, but that’s not entirely accurate, and I’ll explain why.

Interpreters vs. Compilers
An interpreter translates code line by line, which makes it great for quickly running code. It’s a fast way to start execution, but there’s a downside — it doesn’t optimize repetitive tasks. For example, if your code has a loop that repeats the same result, the interpreter will process it each time without any optimization, slowing things down.

On the other hand, a compiler takes the entire code in one go and translates it into machine code. This process takes longer at first, but after the initial compilation, it runs much faster. That’s because compilers can optimize repetitive tasks, like loops, making the code more efficient.

Enter JIT (Just-In-Time) Compiler
In the late 2000s, engineers saw the strengths of both interpreters and compilers and decided to create something better — a JIT (Just-In-Time) compiler. This hybrid approach combines the best of both worlds.

Here’s how it works:

When you give a JavaScript file to the browser, the code goes through a parser, which converts it into an Abstract Syntax Tree (AST).
At this point, there’s a decision to make:
If the code doesn’t need optimization, it goes directly to the interpreter, which translates the code to machine-readable instructions (binary).
If the code has repetitive or complex tasks, a profiler detects the need for optimization and forwards it to the compiler, which optimizes the code and translates it.

here is the best diagram for a better understanding

So, when someone asks, “Is JavaScript an interpreted language?” you can say “technically yes”, but it depends on the implementation. JavaScript uses both an interpreter and a compiler, thanks to JIT compilation.

Here’s an example to demonstrate the efficiency difference between an interpreter and a compiler.

function calculateSum() {
  let sum = 0;
  for (let i = 0; i < 1000000; i++) {
    sum += i;
  }
  return sum;
}

console.log(calculateSum());

In this example, an interpreter would go through each iteration of the loop one by one. A JIT compiler, on the other hand, would notice that the loop can be optimized, speeding up execution after the first run.

Conclusion
Understanding how JavaScript works under the hood helps you appreciate its flexibility and efficiency. The combination of interpreters and compilers (thanks to JIT) allows JavaScript to run quickly and efficiently in modern browsers.

Next time someone calls JavaScript an interpreted language, you can explain that while it starts out that way, it can also behave like a compiled language when optimization is needed.