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How the V8 engine works?

V8 is a JavaScript engine built at the google development center, in Germany. It is open source and written in C++. It is used for both client side (Google Chrome) and server side (node.js) JavaScript applications.

V8 was first designed to increase the performance of the JavaScript execution inside web browsers. In order to obtain speed, V8 translates JavaScript code into more efficient machine code instead of using an interpreter. It compiles JavaScript code into machine code at execution by implementing a JIT (Just-In-Time) compiler like a lot of modern JavaScript engines such as SpiderMonkey or Rhino (Mozilla) are doing. The main difference with V8 is that it doesn’t produce bytecode or any intermediate code.

The aim of this article is to show and to understand how V8 works, in order to produce optimized code for both client side or server side applications. If you are already asking yourself “Should I care about JavaScript performance?” then I will answer with a quote from Daniel Clifford (tech lead and manager of the V8 team): “It’s not just about making your current application run faster, it’s about enabling things that you have never been able to do in the past”.


Hidden class

JavaScript is a prototype-based language: there are no classes and objects are created by using a cloning process. JavaScript is also dynamically typed: types and type informations are not explicit and properties can be added to and deleted from objects on the fly. Accessing types and properties effectively makes a first big challenge for V8. Instead of using a dictionary-like data structure for storing object properties and doing a dynamic lookup to resolve the property location (like most JavaScript engines do), V8 creates hidden classes, at runtime, in order to have an internal representation of the type system and to improve the property access time.

Let’s have for instance a Point function and the creation of two Point objects:

hidden class

If the layouts are the same, which is the case here, p and q belong to the same hidden class created by V8. This highlights another advantage of using hidden classes: it allows V8 to group objects for which properties are the same. Here p and q use the same optimized code.

Now, let’s assume that we want to add a z property to our q object, right after its declaration (which is perfectly fine with a dynamically typed language).

How will V8 deal with this scenario? In fact, V8 creates a new hidden class everytime the constructor function declares a property and keeps track of the change in the hidden class. Why? Because if two objects are created (p and q) and if a member is added to the second object (q) after the creation, V8 needs to keep the last hidden class created (for the first object p) and to create a new one (for the second object q) with the new member.

transition information

Everytime a new hidden class is created, the previous one is updated with a class transition indicating what hidden class has to be used instead of it.

Code optimization

Because V8 creates a new hidden class for each property, hidden class creation should be kept to a minimum. To do this, try to avoid adding properties after the object creation, and always initialize object members in the same order (to avoid the creation of different tree of hidden classes).

[Update] Another trick: Monomorphic operations are operations which only work on objects with the same hidden class. V8 creates a hidden class when we call a function. If we call it again with different parameter types, V8 needs to create another hidden class: Prefer monomorphic code to polymorphic code

More example on how V8 optimized JavaScript code

Tagged values

To have an efficient representation of numbers and JavaScript objects, V8 represents both with a 32 bit value. It uses a bit to know if it is an object (flag = 1) or an integer (flag = 0) called here SMall Integer or SMI because of its 31 bits. Then, if a numeric value is bigger than 31 bits, V8 will box the number, turning it into a double and creating a new object to put the number inside.

Code optimization: Try to use 31 bit signed numbers whenever possible to avoid the expensive boxing operation into a JavaScript object.


V8 uses two different methods to handle arrays:

Code optimization: Be sure that V8 uses “Fast Elements” to handle arrays, in other words, avoid sparse arrays where keys are not next incremental numbers. Also, try to avoid pre-allocating large arrays. It is better to grow as you go. Finally, don’t delete elements in arrays: it makes the key set sparse.

How V8 compiles JavaScript code?

V8 has two compilers!

Code optimization: V8 also supports de-optimization: the optimizing compiler makes optimistic assumptions from the Inline Cache about the different types, de-optimization comes if these assumptions are invalid. For example, if a hidden class generated was not the one expected, V8 throws away the optimized code and comes back to the Full Compiler to get types again from the Inline Cache. This process is slow and should be avoided by trying to not change functions after they are optimized.


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