Javascript: Execution Context and Hoisting

Javascript: Execution Context and Hoisting

This article is for those who don’t know how javascript is executed.

This article is for those who don’t know how javascript is executed.

Before we start can you answer below?

Is Javascript an interpreted or a compiled language?

Yes, Javascript (JS) is an interpreted language, still has its own form of a compiler, run in what’s known as the Javascript engine.

Every web browser has its own form of a JavaScript engine Eg. Chrome has v8 and Mozilla has spider monkey etc, though they all have the same purpose. JavaScript engines simply convert JavaScript source code into a language the compiler can understand, then executes it.

Note: Comment down if you want me to cover in details what javascript engine do under the hood.

Let’s start then,

Execution Context

An environment in which the javascript code runs is what form an execution context.

The execution context decides what particular piece of code has access to variables, functions, objects, etc.

If you have read the scope article then you should know what’s the global scope and local scope(function scope).

So similarly execution context have different types —

1. Global Execution Context

Whenever the code runs for the first time or when the code is not inside any function then it gets into the global execution context. There is only one global execution context throughout the execution of code.

In the case of browser global execution context does 2 things

  1. Create a window object.

  2. The window object is referenced to this keyword.

2. Function Execution Context

Whenever the code execution found a function it creates a new function execution contexts. There can be any number of function execution contexts.

Above, Global execution context contains name variable and a function reference to func1. Whereas three function execution context containing variables and function reference will be created. The Details is explained further in the article.

Execution Stack / Call Stack

Javascript can only run one thing at one time in the browser that means it is the single thread so it queues the other action, events, function in what is called as the execution stack.

Whenever a script load in the browser, the first element in the stack is the global execution context. However, when a function executes, an execution context is created and virtually placed on top of the global execution context. Once a function has finished executing, it gets popped off of the execution stack and returning control to the context below it.

Let’s take an example and visualize the above.

An Execution Context Stack for the above code.

Step1: When the above code loads in the browser, the Javascript engine creates a global execution context and pushes it to the current execution stack.

Step2: Let’s assume that at last, we do a func1() call then the Javascript engines creates a new execution context for that function and pushes it to the top of the global execution context

Step3: Inside the func1() we have func2() call therefore the Javascript engines creates a new execution context for that function and pushes it to the top of the func1() execution context.

Step4: When the func2() function finishes, its execution context is popped off from the current stack, and the control reaches the execution context below it, that is the func1() function execution context.

Step5: When the func1() finishes, its execution stack is removed from the stack and control reaches the global execution context. Once all the code is executed, the JavaScript engine removes the global execution context from the current stack.

Execution Context Phases

There are mainly two phases of the execution context.

  1. Creation

  2. Execution

Let’s take a look one by one

Creation phase

There are several things that happen here before the function execution happen.

  1. At first, a connection to the outer environment is created for each function or variables which is what form a scope chain. This tells the execution context what should it contain and where should it look for resolving the reference for function and values for variables.
  • For the global environment, the outer environment is null. However, all environments within the global, have the global environment as its outer environment.

  • For eg: If function a, contained in function b, that means a has an outer environment b.

2. After scanning the scope chain an environment record is created where the creation and reference for global context(would be a window in a web browser), variable, function, and function arguments are done in memory.

3. At last value of ‘this’ keyword is determined (In the case of the global execution context, ‘this’ refers to the window) inside each execution context created in the 1st step.

Therefore we can represent the creation phase as

creationPhase = {
'outerEnvironmentConnection': {
        /* scope chain resolution*/ 
    },    
'variableObjectMapping': {
        /* function arguments, parameters, inner variable and function declarations are created or referenced in memory */ 
    },
    'valueOfThis': {},

}

Execution Phase

This is the phase where the code starts to run in the execution context formed in the creation phase and variable values are assigned line by line.

As the execution start, the engine looks for reference to execute the function in its creation phase object. If it doesn’t find it in its own, it will continue to move up the scope chain until it reaches the global environment.

If no references are found in the global environment it will return an error. However, if a reference is found and the function is executed correctly, the execution context of this particular function will be popped off the stack and the engine will move onto the next function, where their execution context will be added to the stack and executed, and so on.

Let's look at the above two phases via example to get a better idea around it.

So the global execution context will look something like this during the creation phase:

globalExecutionObj = {
    outerEnvironmentConnection: null,
    variableObjectMapping: {
        name: uninitialized,
        title: undefined,
        date: uninitialized,
        func1: func,
    },
    this: window //Global Object
}

Note: Above, the let (name)and const (date) defined variables do not have any value associated with them during the creation phase, but var (title)defined variables are set to undefined .

This is the reason why you can access var defined variables before they are declared (though undefined) but get a reference error when accessing letand const variables before they are declared.

This is, what we call hoisting i.e all variable declarations using var are hoisted/lifted to the top of their functional/local scope (if declared inside a function) or to the top of their global scope (if declared outside of a function) regardless of where the actual declaration has been made.

During the execution phase, the variable assignments are done. So the global execution context will look something like this during the execution phase.

globalExectutionObj = {
    outerEnvironmentConnection: null,
    variableObjectMapping: {
        name: "overflowjs.com",
        title: "Execution context",
        date: "5 july 2019",
        func1: pointer to function func1,
    },
    this: window //Global Object
}

Note: During the execution phase, if the JavaScript engine couldn’t find the value of let variable at the actual place it was declared in the source code, then it will assign it the value of undefined.

Now, when func1 is reached a new function execution context will be formed, who’s creation object look like below

func1ExecutionObj = {
    outerEnvironmentConnection: Global,
    variableObjectMapping: {
       arguments: {
            0: 10,
            length: 1
        },
        num: 10,

        author: undefined,
        val: uninitialized,
        func2: undefined
        fixed: uninitialized
        addFive: pointer to function addFive()
    },
    this: Global Object or undefined
}

During the execution phase,

func1ExecutionObj = {
    outerEnvironmentConnection: Global,
    variableObjectMapping: {
       arguments: {
            0: 10,
            length: 1
        },
        num: 10,

        author: "Deepak",
        val: 3,
        func2: pointer to function func2() 
        fixed: "Divine"
        addFive: pointer to function addFive()
    },
    this: Global Object or undefined
}

After the function completes its execution, the global environment is updated. Then the global code completes and the program finishes.

Hope you like this article. Please share with other if you can, this motivate us to write more.

Thanks!

JavaScript Tutorial: if-else Statement in JavaScript

JavaScript Tutorial: if-else Statement in JavaScript

This JavaScript tutorial is a step by step guide on JavaScript If Else Statements. Learn how to use If Else in javascript and also JavaScript If Else Statements. if-else Statement in JavaScript. JavaScript's conditional statements: if; if-else; nested-if; if-else-if. These statements allow you to control the flow of your program's execution based upon conditions known only during run time.

Decision Making in programming is similar to decision making in real life. In programming also we face some situations where we want a certain block of code to be executed when some condition is fulfilled.
A programming language uses control statements to control the flow of execution of the program based on certain conditions. These are used to cause the flow of execution to advance and branch based on changes to the state of a program.

JavaScript’s conditional statements:

  • if
  • if-else
  • nested-if
  • if-else-if

These statements allow you to control the flow of your program’s execution based upon conditions known only during run time.

  • if: if statement is the most simple decision making statement. It is used to decide whether a certain statement or block of statements will be executed or not i.e if a certain condition is true then a block of statement is executed otherwise not.
    Syntax:
if(condition) 
{
   // Statements to execute if
   // condition is true
}

Here, condition after evaluation will be either true or false. if statement accepts boolean values – if the value is true then it will execute the block of statements under it.
If we do not provide the curly braces ‘{‘ and ‘}’ after if( condition ) then by default if statement will consider the immediate one statement to be inside its block. For example,

if(condition)
   statement1;
   statement2;

// Here if the condition is true, if block 
// will consider only statement1 to be inside 
// its block.

Flow chart:

Example:

<script type = "text/javaScript"> 

// JavaScript program to illustrate If statement 

var i = 10; 

if (i > 15) 
document.write("10 is less than 15"); 

// This statement will be executed 
// as if considers one statement by default 
document.write("I am Not in if"); 

< /script> 

Output:

I am Not in if
  • if-else: The if statement alone tells us that if a condition is true it will execute a block of statements and if the condition is false it won’t. But what if we want to do something else if the condition is false. Here comes the else statement. We can use the else statement with if statement to execute a block of code when the condition is false.
    Syntax:
if (condition)
{
    // Executes this block if
    // condition is true
}
else
{
    // Executes this block if
    // condition is false
}


Example:

<script type = "text/javaScript"> 

// JavaScript program to illustrate If-else statement 

var i = 10; 

if (i < 15) 
document.write("10 is less than 15"); 
else
document.write("I am Not in if"); 

< /script> 

Output:

i is smaller than 15
  • nested-if A nested if is an if statement that is the target of another if or else. Nested if statements means an if statement inside an if statement. Yes, JavaScript allows us to nest if statements within if statements. i.e, we can place an if statement inside another if statement.
    Syntax:
if (condition1) 
{
   // Executes when condition1 is true
   if (condition2) 
   {
      // Executes when condition2 is true
   }
}

Example:

<script type = "text/javaScript"> 

// JavaScript program to illustrate nested-if statement 

var i = 10; 

if (i == 10) { 

// First if statement 
if (i < 15) 
	document.write("i is smaller than 15"); 

// Nested - if statement 
// Will only be executed if statement above 
// it is true 
if (i < 12) 
	document.write("i is smaller than 12 too"); 
else
	document.write("i is greater than 15"); 
} 
< /script> 

Output:

i is smaller than 15
i is smaller than 12 too
  • if-else-if ladder Here, a user can decide among multiple options.The if statements are executed from the top down. As soon as one of the conditions controlling the if is true, the statement associated with that if is executed, and the rest of the ladder is bypassed. If none of the conditions is true, then the final else statement will be executed.
if (condition)
    statement;
else if (condition)
    statement;
.
.
else
    statement;


Example:

<script type = "text/javaScript"> 
// JavaScript program to illustrate nested-if statement 

var i = 20; 

if (i == 10) 
document.wrte("i is 10"); 
else if (i == 15) 
document.wrte("i is 15"); 
else if (i == 20) 
document.wrte("i is 20"); 
else
document.wrte("i is not present"); 
< /script> 

Output:

i is 20

Building a Powerful Virtual Machine in JavaScript

Building a Powerful Virtual Machine in JavaScript

This JavaScript tutorial explains how to build a powerful Virtual Machine in JavaScript. A flexible, extensible, register-based virtual machine. Support for signed, unsigned and floating point operations. A call stack. Interrupt capabilities. Ability to do memory mapping for IO. An assembly language with macro and module support. A higher level, C like language. We'll use and expand the library from the parser combinators from scratch series. And finally, to be able to take the whole thing into the browser and exend it to create a sort of fantasy console - an emulator for a machine that never existed

16-Bit Virtual Machine in JavaScript 001

In this episode we begin implementing a 16-bit virtual machine from scratch in JavaScript. The concepts of computation are introduced, along with the basics of assembly language and machine code.

Memory Access and Branching (16-Bit VM in JavaScript 002)

In this video we establish the core instruction set of the VM, give the VM the capabilities to read and write to main memory, and also to make decisions about how the program should proceed with branching instructions.

What is the Stack and why do we need it? (16-Bit VM in JavaScript 003)

In this episode we understand what a stack is, how it can be implemented on the lowest level, and how it can then be harnessed to allow the Virtual Machine to run subroutines without losing state.

Implementing Stack Mechanics (16-Bit VM in JavaScript 004)

In this episode, we create an implementation for the stack mechanisms that were described in the last episode.

What is Memory Mapped I/O? (16-Bit VM in JavaScript 005)

This this episode we implement memory mapped I/O, where the address space is utilised as common bus for components to communicate.

Understanding JavaScript Decorators

Understanding JavaScript Decorators

Decorators are actually nothing more than functions that return another function, and that are called with the appropriate details of the item being decorated. Using decorators in your projects today requires some transpiler configuration. What is a Decorator? JavaScript Decorators and Property Descriptors. How to Write a Decorator. Handling API Errors. Decorating Classes. A Babel Workaround

Decorators aren’t a core feature of JavaScript yet; they’re working their way through ECMA TC39’s standardization process. That doesn’t mean we can’t get familiar with them, though. It looks like they’ll be supported natively by Node and browsers at some point in the near future, and in the meantime we’ve got Babel.

What is a Decorator?

Decorator is shorthand for “decorator function” (or method). It’s a function that modifies the behavior of the function or method passed to it by returning a new function. I said “function” a lot there. That’s an occupational hazard when you’re discussing higher-order functions.

You can implement decorators in any language that supports functions as first-class citizens, e.g. JavaScript, where you can bind a function to a variable or pass it as an argument to another function. A couple of those languages have special syntactic sugar for defining and using decorators; one of them is Python:


def cashify(fn):
    def wrap():
        print("$$$$")
        fn()
        print("$$$$")
    return wrap

@cashify
def sayHello():
    print("hello!")

sayHello()

# $$$$
# hello!
# $$$$

Let’s take a look at what’s going on there. Our cashify function is a decorator: it receives a function as an argument, and its return value is also a function. We use Python’s “pie” syntax to apply the decorator to our sayHello function, which is essentially the same thing as if we’d done this below the definition of sayHello:


def sayHello():
    print("hello!")

sayHello = cashify(sayHello)

The end result is that we print dollar signs before and after whatever we’re printing from the function we decorate.

Why am I introducing ECMAScript decorators using an example in Python? I’m glad you asked!

  • Python is a great way to explain the basics because its concept of decorators is a bit more straightforward than the way they work in JS.
  • JS and TypeScript both use Python’s “pie syntax” to apply decorators to methods and properties of classes, so it’s visually and syntactically similar.

Okay, what’s different about JS decorators?

JS Decorators and Property Descriptors

While Python decorators are passed whatever function they’re decorating as an argument, JS decorators receive quite a bit more information due to the way objects work in that language.

Objects in JS have properties, and those properties have values:


const oatmeal = {
  viscosity: 20,
  flavor: 'Brown Sugar Cinnamon',
};

But in addition to its value, each property has a bunch of other behind-the-scenes information that defines different aspects of how it works, called a property descriptor:


console.log(Object.getOwnPropertyDescriptor(oatmeal, 'viscosity'));

/*
{
  configurable: true,
  enumerable: true,
  value: 20,
  writable: true
}
*/

JS is tracking quite a few things related to that property:

  • configurable determines whether or not the type of the property can be changed, and whether it can be deleted from the object.
  • enumerable controls whether the property shows up when you enumerate the object’s properties (like when you call Object.keys(oatmeal) or use it in a for loop).
  • writable controls whether or not you can change the property’s value via the assignment operator =.
  • value is the static value of the property that you see when you access it. It’s usually the only part of the property descriptor that you see or are concerned with. It can be any JS value, including a function, which would make the property a method of the object it belongs to.

Property descriptors can also have two other properties that cause JS to treat them as “accessor descriptors” (more commonly known as getters and setters):

  • get is a function that returns the property’s value instead of using the static value property.
  • set is a special function that gets passed whatever you put on the right side of the equals sign as an argument when you assign a value to the property.

Decorating Without the Frills

JS has actually had an API for working with property descriptors since ES5, in the form of the Object.getOwnPropertyDescriptor and Object.defineProperty functions. For example, If I like the thickness of my oatmeal just the way it is, I can make it read-only using that API like so:


Object.defineProperty(oatmeal, 'viscosity', {
  writable: false,
  value: 20,
});

// When I try to set oatmeal.viscosity to a different value, it'll just silently fail.
oatmeal.viscosity = 30;
console.log(oatmeal.viscosity);
// => 20

I can even write a generic decorate function that lets me mess with the descriptor for any property of any object:


function decorate(obj, property, callback) {
  var descriptor = Object.getOwnPropertyDescriptor(obj, property);
  Object.defineProperty(obj, property, callback(descriptor));
}

decorate(oatmeal, 'viscosity', function(desc) {
  desc.configurable = false;
  desc.writable = false;
  desc.value = 20;
  return desc;
});

Adding the Shiplap and Crown Molding

The first major difference with the Decorators proposal is that it only concerns itself with ECMAScript classes, not regular objects. We’re going to need to over-engineer our breakfast in order to really demonstrate what we can accomplish, so let’s make some classes to represent our bowl of porridge:


class Porridge {
  constructor(viscosity = 10) {
    this.viscosity = viscosity;
  }

  stir() {
    if (this.viscosity > 15) {
      console.log('This is pretty thick stuff.');
    } else {
      console.log('Spoon goes round and round.');
    }
  }
}

class Oatmeal extends Porridge {
  viscosity = 20;

  constructor(flavor) {
    super();
    this.flavor = flavor;
  }
}

We’re representing our bowl of oatmeal using a class that inherits from the more generic Porridge class. Oatmeal sets the default viscosity higher than Porridge‘s default, and it adds a new flavor property. We’re also using another ECMAScript proposal, class fields, to override the viscosity value.

We can re-create our original bowl of oatmeal like so:


const oatmeal = new Oatmeal('Brown Sugar Cinnamon');

/*
Oatmeal {
  flavor: 'Brown Sugar Cinnamon',
  viscosity: 20
}
*/

Great, we’ve got our ES6 oatmeal, and we’re ready to write a decorator!

How to Write a Decorator

JS decorator functions are passed three arguments:

  1. target is the class that our object is an instance of.
  2. key is the property name, as a string, that we’re applying the decorator to.
  3. descriptor is that property’s descriptor object.

What we do inside of the decorator function depends on the purpose of our decorator. In order to decorate a method or property of an object, we need to return a new property descriptor. Here’s how we can write a decorator that makes a property read-only:


function readOnly(target, key, descriptor) {
  return {
    ...descriptor,
    writable: false,
  };
}

We’d use it by modifying our Oatmeal class like this:


class Oatmeal extends Porridge {
  @readOnly viscosity = 20;
  // (you can also put @readOnly on the line above the property)

  constructor(flavor) {
    super();
    this.flavor = flavor;
  }
}

Now our oatmeal’s glue-like consistency is immune to tampering. Thank goodness.

What if we want to do something that’s actually useful? I ran into a situation while working on a recent project where a decorator saved me a lot of typing and maintenance overhead:

Handling API Errors

In the MobX/React app I mentioned in the beginning, I have a couple of different classes that act as data stores. They each represent collections of different things that the user interacts with, and they each talk to different API endpoints for data from the server. In order to handle API errors, I made each of the stores follow a protocol when communicating over the network:

  1. Set the UI store’s networkStatus property to “loading.”
  2. Send a request to the API
  3. Handle the result
    • If successful, update local state with the response
    • If something goes wrong, set the UI store’s apiError property to the error we received
  4. Set the UI store’s networkStatus property to “idle.”

I found myself repeating this pattern a few times before I noticed the smell:


class WidgetStore {
  async getWidget(id) {
    this.setNetworkStatus('loading');

    try {
      const { widget } = await api.getWidget(id);
      // Do something with the response to update local state:
      this.addWidget(widget);
    } catch (err) {
      this.setApiError(err);
    } finally {
      this.setNetworkStatus('idle');
    }
  }
}

That’s a lot of error handling boilerplate. I decided that since I was already using MobX’s @action decorators on all the methods that updated observable properties (not shown here for the sake of simplicity), I might as well just tack on an additional decorator that allowed me to recycle my error handling code. I came up with this:


function apiRequest(target, key, descriptor) {
  const apiAction = async function(...args) {
    // More about this line shortly:
    const original = descriptor.value || descriptor.initializer.call(this);

    this.setNetworkStatus('loading');

    try {
      const result = await original(...args);
      return result;
    } catch (e) {
      this.setApiError(e);
    } finally {
      this.setNetworkStatus('idle');
    }
  };

  return {
    ...descriptor,
    value: apiAction,
    initializer: undefined,
  };
}

I could then replace the boilerplate that I was writing in each API action method with something like this:


class WidgetStore {
  @apiRequest
  async getWidget(id) {
    const { widget } = await api.getWidget(id);
    this.addWidget(widget);
    return widget;
  }
}

My error handling code is still there, but now I only need to write it once and ensure that each class that uses it has a setNetworkStatus and setApiError method.

A Babel Workaround

So what’s up with that line where I’m choosing between descriptor.value and calling descriptor.initializer? That’s a Babel thing. My hunch is that it won’t work that way when JS supports decorators natively, but it’s necessary right now because of how Babel handles arrow functions defined as class properties.

When you define a class property and assign an arrow function as its value, Babel does a little trick to bind that function to the correct instance of the class and give you the right this value. It does this by setting descriptor.initializer to a function that returns the function you wrote, with the correct this value in its scope.

An example should make things less muddy:


class Example {
  @myDecorator
  someMethod() {
    // In this case, our method would be referred to by descriptor.value
  }

  @myDecorator
  boundMethod = () => {
    // Here, descriptor.initializer would be a function that, when called, would return our `boundMethod` function, properly scoped so that `this` refers to the current instance of Example.
  };
}

Decorating Classes

In addition to properties and methods, you can also decorate an entire class. In order to do that, you really only need the first argument passed to your decorator function, target. For example, I can write a decorator that automatically registers the class it’s wrapping as a custom HTML element. I’m using a closure here to enable the decorator to receive whatever name we want to give the element as an argument:


function customElement(name) {
  return function(target) {
    customElements.define(name, target);
  };
}

We’d use it like this:


@customElement('intro-message');
class IntroMessage extends HTMLElement {
  constructor() {
    super();

    const shadow = this.attachShadow({ mode: 'open' });
    this.wrapper = this.createElement('div', 'intro-message');
    this.header = this.createElement('h1', 'intro-message__title');
    this.content = this.createElement('div', 'intro-message__text');
    this.header.textContent = this.getAttribute('header');
    this.content.innerHTML = this.innerHTML;

    shadow.appendChild(this.wrapper);
    this.wrapper.appendChild(this.header);
    this.wrapper.appendChild(this.content);
  }

  createElement(tag, className) {
    const elem = document.createElement(tag);
    elem.classList.add(className);
    return elem;
  }
}

Load that into our HTML, and we can use it like this:


<intro-message header="Welcome to Decorators">
  <p>Something something content...</p>
</intro-message>

Which gives us this in the browser:

Wrapping Up

Using decorators in your projects today requires some transpiler configuration. The most straightforward guide that I’ve seen is located in the MobX docs. It has info for TypeScript and two major versions of Babel.

Keep in mind that decorators are an evolving proposal at this point, so if you use them in production code now, you’ll probably either need to make some updates or keep using Babel’s decorators plugin in legacy mode once they become an official part of the ECMAScript specification. While it’s not even well-supported by Babel yet, the latest version of the decorators proposal already contains big changes that are not backward compatible with the previous version.

Decorators, like many bleeding edge JS features, are a useful tool to have in your kit. They can greatly simplify the sharing of behavior across different, unrelated classes. However, there’s always a cost associated with early adoption. Use decorators, but do so with a clear idea of the implications for your codebase.