Understanding Just-in-Time (JIT) Compiler: How it Works for JavaScript?
Tags: javascript, tutorial, beginners, programming
Published: 2024-06-30 05:23:08

---
title: Understanding Just-in-Time (JIT) Compiler: How it Works for JavaScript?
published: true
date: 2023-08-30 04:30:12 UTC
tags: #javascript, #tutorial, #beginners, #programming
series: 'Javascript' 
canonical_url: https://iamdipankarpaul.hashnode.dev/understanding-just-in-time-jit-compiler-how-it-works-for-javascript
---

### Introduction

In the world of programming, speed and efficiency are crucial. Just-in-Time (JIT) compilation is a technique used by programming languages like JavaScript to optimize the execution of code. In this blog post, we will dive into the concept of JIT compilation and explore how it works specifically for JavaScript.

### What is JIT Compiler?

Just in Time (JIT) Compiler is a type of compiler that dynamically compiles code at runtime instead of compiling it before execution. It combines the advantages of both ahead-of-time (AOT) and interpreted execution techniques. JIT compilers are commonly used in programming languages such as JavaScript to improve code performance.

### How JIT Compiler Works

1. **Parsing and Lexical Analysis:** The JIT compiler starts by parsing the JavaScript code, which involves breaking it down into smaller meaningful chunks or tokens. This process is known as **lexical analysis**.

2. **Abstract Syntax Tree (AST) Generation:** After lexical analysis, the JIT compiler generates an Abstract Syntax Tree (AST) that represents the structure and meaning of the code. The AST is a tree-like structure where each node represents an operator, value, or control structure in the code.

3. **Profiling:** The JIT compiler then analyzes the code's execution patterns by collecting runtime data. It tracks the frequency of the function calls, types of data being used. This information helps the JIT compliler identifies hotspots in the code and make informed decisions about which parts of the code should be priortized for compilation.

4. **Intermediate Representation (IR) / Bytecode Generation:** Once profiling is done, the JIT compiler converts it into an Bytecode . The bytecode is a lower-level representation of the code that is closer to machine instructions.

5. **Optimization:** In the optimization phase, the JIT compiler analyzes the bytecode to identify hotspots and apply various optimizations to improve the code's performance. These optimizations can include things like loop unrolling, inlining of function calls, constant folding, , eliminating redundant operations and dead code elimination.

6. **Machine Code Generation:** After optimization, the JIT compiler generates machine code specific to the target architecture from the optimized bytecode. Machine code is a sequence of low-level instructions that can be directly executed by the computer's processor.

7. **Execution:** Finally, the JIT compiler executes the generated machine code, which results in improved performance compared to interpreting the code directly. The JIT compiler can continuously monitor the program's execution and apply further optimizations if needed.

#### Example

Let's consider the following JavaScript code:

```javascript
function add(x, y) {
 return x + y;
}
let result = add(5, 10);
console.log(result);
```

When this code is executed with JIT compilation, the following steps take place:

1. The JIT compiler parses the code and generates an AST.
2. The AST is converted into an Intermediate Representation (IR) or Bytecode.
3. The JIT compiler applies optimizations to the bytecode, such as inlining the `add` function call.
4. The JIT compiler generates machine code from the optimized bytecode.
5. The generated machine code is executed, resulting in the output `15` being printed to the console.

### Benefits of JIT Compilation for JavaScript

JIT compilation offers several advantages for JavaScript:

1. **Increased performance:** By dynamically optimizing the code based on runtime data, JIT compilation significantly improves the execution speed of JavaScript programs.
2. **Adaptive optimization:** JIT compilers can adapt to changes in the code's execution patterns, continuously optimizing the performance as the program runs.
3. **Reduced memory footprint:** JIT compilers can optimize memory usage by eliminating unnecessary allocations and reducing the overall memory footprint of the program.
4. **Faster startup time:** JIT compilation strikes a balance between the slower initial compilation and faster subsequent executions, leading to faster startup times for JavaScript applications.

### Key Takeaways

- JIT Compiler stands for Just-In-Time Compiler. It dynamically compiles code at runtime rather than before execution. Combines advantages of both ahead-of-time (AOT) and interpreted execution.

- The JIT compilation process involves parsing, abstract syntax tree (AST) generation, profiling, bytecode generation, optimization, machine code generation, and execution.

- The JIT compilation process gives you many benefits like improved performance, memory efficiency, quicker application startup.

- JIT compilation is commonly used in programming languages like JavaScript to improve code performance.