JavaScript Execution Context

Consider the following script,

How is the above script executed?
The execution steps are as follows:

First, assign 100 to variable x.
Assign a value of 50 to variable y.
Declare a function addNums, it adds the numbers passed as arguments.
Call the addNums() function by passing in x and y as parameters, and store the return value in the result variable.
Log the result to the console.

Internally the JavaScript engine executes by creating execution contexts.

What is JavaScript Execution Context?

When a JavaScript engine executes a script, it creates contexts. Each execution context has two phases :

Creation Phase and

Execution Phase

The Creation Phase

When a script executes for the first time, the JavaScript engine creates a global execution context. During this creation phase, it performs the following tasks:

Create a global object (window for browser and global in NodeJs env)
Create a "this" object which points to the global object
Setup a memory heap for storing variables and function references
Store the function declarations in the memory heap and variables within the global execution context are initialized with undefined.

After the creation phase, the global execution phase moves to the next phase, the Execution phase.

The Execution Phase

During the execution phase, the JS engine executes the code line by line, assigns the values to the variables, and executes the function calls.

For each function call, the JS engine creates a new Function Execution context. The function Execution context is similar to the Global Execution Context. But, instead of creating the global object, the JS engine creates the arguments object that is a reference to all the parameters of the function.

The call stack keeps the track of all the steps in the execution context.

Comments

JavaScript Runtime Environment

What is a Runtime Environment? A runtime environment is where your program will be executed. It determines what global objects your program can access and it can also impact how it runs. How does the JavaScript Environment Work? The JavaScript runtime environment provides access to built-in libraries and objects that are available to program so that it can interact with the outside world and make the code work. Different browsers have different JavaScript Runtime Environments. In the context of a browser, JS runtime is comprised of the following elements: The JavaScript Engine Web APIs The Call Back Queue The Event Loop The JavaScript Engine All the browsers have a JavaScript Engine. But all are not necessarily the same engine. Also, all the engines follow the same standard although their implementation is different. For example, Google Chrome has the V8 engine and Mozilla Firefox has Spyder Monkey as a JavaScript engine. The JavaScript Engine parses the code for us. It has two major

JavaScript Scopes

What is a variable scope? The visibility and accessibility of a variable is determined by its scope. JavaScript has three scopes: Global scope Local scope and Block scope Global Scope A variable that can be accessible everywhere in the script. Local Scope If you declare a variable inside a function are local to the function. They are called local variables. Output: I am from say I am from global Scope Chain If interpreter cannot find a variable in its local context, then it looks for it in parent contexts until it's found. This is called scope chain. Output: Hello I am message In the above script, the JavaScript performs the following: It looks up for the variable message in the current context (Functional Execution Context) of the say(). But it cannot find it. So it goes out of the function and looks in the outer execution context which is the global execution context. It found the message variable there. The way that JavaScript resolves a variable is by looking at in its c

JavaScript Hoisting

Hoisting JavaScript Hoisting refers to the process where the interpreter allocates memory for variables and function declarations prior to the execution of the code. Declarations that are made using the var keyword are initialized with the default value of undefined Declarations made using the let and the const keywords are not initialized as a part of the hoisting Hoisting takes place during the creation phase of the Execution Context. consider the following script: output: undefined Steps involved in the execution of the above script: A new Global Execution Context is created. num variable is initialized with undefined . In the execution phase, the code is executed line-by-line. In the first line it logs undefined to the console. Line two is skipped as it already defined num in the creation phase. In line three a value of 6 is assigned to num. let and const keywords let and const keywords are hoisted but not initialized Consider the following script: output: ReferenceError:

Teach Me to Code

Search This Blog