Lexical Scoping in JavaScript

Lexical Scoping in JavaScript

<strong>Lexical scoping is a topic that frightens many programmers. One of the best explanations of lexical scoping can be found in Kyle Simpson’s book You Don’t Know JS: Scope and Closures. However, even his explanation is lacking because he doesn’t use a real example.</strong>

Lexical scoping is a topic that frightens many programmers. One of the best explanations of lexical scoping can be found in Kyle Simpson’s book You Don’t Know JS: Scope and Closures. However, even his explanation is lacking because he doesn’t use a real example.

One of the best real examples of how lexical scoping works, and why it is important, can be found in the famous textbook, “The Structure and Interpretation of Computer Programs” (SICP) by Harold Abelson and Gerald Jay Sussman. Here is a link to a PDF version of the book: SICP. SICP uses Scheme, a dialect of Lisp, and is considered one of the best introductory computer science texts ever written. In this article, I’d like to revisit their example of lexical scoping using JavaScript as the programming language.

The example Abelson and Sussman used is computing square roots using Newton’s method. Newton’s method works by determining successive approximations for the square root of a number until the approximation comes within a tolerance limit for being acceptable. Let’s work through an example, as Abelson and Sussman do in SICP.

The example they use is finding the square root of 2. You start by making a guess at the square root of 2, say 1. You improve this guess by dividing the original number by the guess and then averaging that quotient and the current guess to come up with the next guess. You stop when you reach an acceptable level of approximation. Abelson and Sussman use the value 0.001. Here is a run-through of the first few steps in the calculation:

Square root to find: 2
First guess: 1
Quotient: 2 / 1 = 2
Average: (2+1) / 2 = 1.5
Next guess: 1.5
Quotient: 1.5 / 2 = 1.3333
Average: (1.3333 + 1.5) / 2 = 1.4167
Next guess: 1.4167
Quotient: 1.4167 / 2 = 1.4118
Average: (1.4167 + 1.4118) / 2 = 1.4142

And so on until the guess comes within our approximation limit, which for this algorithm is 0.001.

A JavaScript Function for Newton’s Method

After this demonstration of the method the authors describe a general procedure for solving this problem in Scheme. Rather than show you the Scheme code, I’ll write it out in JavaScript:

function sqrt_iter(guess, x) {
  if (isGoodEnough(guess, x)) {
    return guess;
  }  
  else {
    return sqrt_iter(improve(guess, x), x);
  }
}

Next, we need to flesh out several other functions, including isGoodEnough() and improve(), along with some other helper functions. We’ll start with improve(). Here is the definition:

function improve(guess, x) {
  return average(guess, (x / guess));
}

This function uses a helper function average(). Here is that definition:

function average(x, y) {
  return (x+y) / 2;
}

Now we’re ready to define the isGoodEnough() function. This function serves to determine when our guess is close enough to our approximation tolerance (0.001). Here is the definition of isGoodEnough():

function isGoodEnough(guess, x) {
  return (Math.abs(square(guess) - x)) < 0.001;
}

This function uses a square() function, which is easy to define:

function square(x) {
  return x * x;
}

Now all we need is a function to get things started:

function sqrt(x) {
  return sqrt_iter(1.0, x);
}

This function uses 1.0 as a starting guess, which is usually just fine.

Now we’re ready to test our functions to see if they work. We load them into a JS shell and then compute a few square roots:

> .load sqrt_iter.js
> sqrt(3)
1.7321428571428572
> sqrt(9)
3.00009155413138
> sqrt(94 + 87)
13.453624188555612
> sqrt(144)
12.000000012408687

The functions seem to be working well. However, there is a better idea lurking here. These functions are all written independently, even though they are meant to work in conjunction with each other. We probably aren’t going to use the isGoodEnough() function with any other set of functions, or on its own. Also, the only function that matters to the user is the sqrt() function, since that’s the one that gets called to find a square root.

Block Scoping Hides Helper Functions

The solution here is to use block scoping to define all the necessary helper functions within the block of the sqrt() function. We are going to remove square() and average() from the definition, as those functions might be useful in other function definitions and aren’t as limited to use in an algorithm that implements Newton’s Method. Here is the definition of the sqrt() function now with the other helper functions defined within the scope of sqrt():

function sqrt(x) {
  function improve(guess, x) {
    return average(guess, (x / guess));
  }
  function isGoodEnough(guess, x) {
    return (Math.abs(square(guess) - x)) > 0.001;
  }
  function sqrt_iter(guess, x) {
    if (isGoodEnough(guess, x)) {
      return guess;
    }
    else {
      return sqrt_iter(improve(guess, x), x);
    }
  }
  return sqrt_iter(1.0, x);
}

We can now load this program into our shell and compute some square roots:

> .load sqrt_iter.js
> sqrt(9)
3.00009155413138
> sqrt(2)
1.4142156862745097
> sqrt(3.14159)
1.772581833543688
> sqrt(144)
12.000000012408687

Notice that you cannot call any of the helper functions from outside the sqrt() function:

> sqrt(9)
3.00009155413138
> sqrt(2)
1.4142156862745097
> improve(1,2)
ReferenceError: improve is not defined
> isGoodEnough(1.414, 2)
ReferenceError: isGoodEnough is not defined

Since the definitions of these functions (improve() and isGoodEnough()) have been moved inside the scope of sqrt(), they cannot be accessed at a higher level. Of course, you can move any of the helper function definitions outside of the sqrt() function to have access to them globally as we did with average() and square().

We have greatly improved our implementation of Newton’s Method but there’s still one more thing we can do to improve our sqrt() function by simplifying it even more by taking advantage of lexical scope.

Improving Clarity with Lexical Scope

The concept behind lexical scope is that when a variable is bound to an environment, other procedures (functions) that are defined in that environment have access to that variable’s value. This means that in the sqrt() function, the parameter x is bound to that function, meaning that any other function defined within the body of sqrt() can access x.

Knowing this, we can simplify the definition of sqrt() even more by removing all references to x in function definitions since x is now a free variable and accessible by all of them. Here is our new definition of sqrt():

function sqrt(x) {
  function isGoodEnough(guess) {
    return (Math.abs(square(guess) - x)) > 0.001;
  }
  function improve(guess) {
    return average(guess, (x / guess));
  }
  function sqrt_iter(guess) {
    if (isGoodEnough(guess)) {
      return guess;
    }
    else {
      return sqrt_iter(improve(guess));
    }
  }
  return sqrt_iter(1.0);
}

The only references to parameter x are in computations where x’s value is needed. Let’s load this new definition into the shell and test it:

> .load sqrt_iter.js
> sqrt(9)
3.00009155413138
> sqrt(2)
1.4142156862745097
> sqrt(123+37)
12.649110680047308
> sqrt(144)
12.000000012408687

Lexical scoping and block structure are important features of JavaScript and allow us to construct programs that are easier to understand and manage. This is especially important when we begin to construct larger, more complex programs.

Originally published by Michael McMillan at https://medium.com/@mmmcmillan1957/lexical-scoping-in-javascript-e876cd221b74*

Follow great articles on Twitter

Learn More

☞ The Complete JavaScript Course 2019: Build Real Projects!

☞ Become a JavaScript developer - Learn (React, Node,Angular)

☞ JavaScript: Understanding the Weird Parts

☞ Vue JS 2 - The Complete Guide (incl. Vue Router & Vuex)

☞ The Full JavaScript & ES6 Tutorial - (including ES7 & React)

☞ JavaScript - Step By Step Guide For Beginners

☞ The Web Developer Bootcamp

☞ MERN Stack Front To Back: Full Stack React, Redux & Node.js

☞ Visual Studio Code Settings and Extensions for Faster JavaScript Development

☞ Vue.js Authentication System with Node.js Backend

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

How to Retrieve full Profile of LinkedIn User using Javascript

How to Retrieve full Profile of LinkedIn User using Javascript

I am trying to retrieve the full profile (especially job history and educational qualifications) of a linkedin user via the Javascript (Fetch LinkedIn Data Using JavaScript)

Here we are fetching LinkedIn data like Username, Email and other fields using JavaScript SDK.

Here we have 2 workarounds.

  1. Configuration of linkedIn developer api
  2. Javascript Code to fetch records

Configuration of linkedIn developer api

In order to fetch records, first we need to create developer api in linkedin which will act as token/identity while fetching data from other linkedin accounts.

So to create api, navigate to https://linkedin.com/developer/apps and click on 'Create Application'.

After navigating, fill in details like name, description and other required fields and then submit.

As we submit, it will create Client ID and Client Secret shown below, which we will be using in our code while communicating to fetch records from other LinkedIn account.

Note: We need to provide localhost Url here under Oauth 2.0. I am using my localhost, but you can probably use other production URLs under Oauth 2.0 where your app is configured. It will make your api  consider the Url as trusted which fetching records.

Javascript Code to fetch records

For getting user details like first name, last name,User image can be written as,

<script type="text/javascript" src="https://platform.linkedin.com/in.js">  
    api_key: XXXXXXX //Client ID  
    onLoad: OnLinkedInFrameworkLoad //Method that will be called on page load  
    authorize: true  
</script>  
<script type="text/javascript">  
    function OnLinkedInFrameworkLoad() {  
        IN.Event.on(IN, "auth", OnLinkedInAuth);  
    }  
  
    function OnLinkedInAuth() {  
        IN.API.Profile("me").result(ShowProfileData);  
    }  
  
    function ShowProfileData(profiles) {  
        var member = profiles.values[0];  
        var id = member.id;  
        var firstName = member.firstName;  
        var lastName = member.lastName;  
        var photo = member.pictureUrl;  
        var headline = member.headline;  
        //use information captured above  
        var stringToBind = "<p>First Name: " + firstName + " <p/><p> Last Name: " + lastName + "<p/><p>User ID: " + id + " and Head Line Provided: " + headline + "<p/>"  
        document.getElementById('profiles').innerHTML = stringToBind;  
    }  
</script>    

Kindly note we need to include 'https://platform.linkedin.com/in.js' as src under script type as it will act on this Javascript SDK provided by Linkedin.

In the same way we can also fetch records of any organization with the companyid as keyword.

<head>  
    <script type="text/javascript" src="https://platform.linkedin.com/in.js">  
        api_key: XXXXXXX ////Client ID  
        onLoad: onLinkedInLoad  
        authorize: true  
    </script>  
</head>  
  
<body>  
    <div id="displayUpdates"></div>  
    <script type="text/javascript">  
        function onLinkedInLoad() {  
            IN.Event.on(IN, "auth", onLinkedInAuth);  
            console.log("On auth");  
        }  
  
        function onLinkedInAuth() {  
            var cpnyID = XXXXX; //the Company ID for which we want updates  
            IN.API.Raw("/companies/" + cpnyID + "/updates?event-type=status-update&start=0&count=10&format=json").result(displayCompanyUpdates);  
            console.log("After auth");  
        }  
  
        function displayCompanyUpdates(result) {  
            var div = document.getElementById("displayUpdates");  
            var el = "<ul>";  
            var resValues = result.values;  
            for (var i in resValues) {  
                var share = resValues[i].updateContent.companyStatusUpdate.share;  
                var isContent = share.content;  
                var isTitled = isContent,  
                    isLinked = isContent,  
                    isDescription = isContent,  
                    isThumbnail = isContent,  
                    isComment = isContent;  
                if (isTitled) {  
                    var title = isContent.title;  
                } else {  
                    var title = "News headline";  
                }  
                var comment = share.comment;  
                if (isLinked) {  
                    var link = isContent.shortenedUrl;  
                } else {  
                    var link = "#";  
                }  
                if (isDescription) {  
                    var description = isContent.description;  
                } else {  
                    var description = "No description";  
                }  
                /* 
                if (isThumbnailz) { 
                var thumbnailUrl = isContent.thumbnailUrl; 
                } else { 
                var thumbnailUrl = "http://placehold.it/60x60"; 
                } 
                */  
                if (share) {  
                    var content = "<a target='_blank' href=" + link + ">" + comment + "</a><br>";  
                    //el += "<li><img src='" + thumbnailUrl + "' alt=''>" + content + "</li>";  
                    el += "<li><div>" + content + "</div></li>";  
                }  
                console.log(share);  
            }  
            el += "</ul>";  
            document.getElementById("displayUpdates").innerHTML = el;  
        }  
    </script>  
</body>  

We can get multiple metadata while fetching records for any any organization. We can get company updates as shown below.

Conclusion

We can also fetch any company specific data like company job updates/post, total likes, comments, and number of views along with a lot of metadata we can fetch which I have shown below.

Thank you for reading !

7 Best Javascript Iframe Libraries

7 Best Javascript Iframe Libraries

Iframes let you build user experiences into embeddable ‘cross-domain components’, which let users interact with other sites without being redirected. I have compiled 7 best Javascript iframe libraries.

Iframes let you build user experiences into embeddable ‘cross-domain components’, which let users interact with other sites without being redirected. I have compiled 7 best Javascript iframe libraries.

1. Zoid

A cross-domain component toolkit, supporting:

  • Render an iframe or popup on a different domain, and pass down props, including objects and functions
  • Call callbacks natively from the child window without worrying about post-messaging or cross-domain restrictions
  • Create and expose components to share functionality from your site to others!
  • Render your component directly as a React, Vue or Angular component!
    It's 'data-down, actions up' style components, but 100% cross-domain using iframes and popups!

Download


2. Postmate

Postmate is a promise-based API built on postMessage. It allows a parent page to speak with a child iFrame across origins with minimal effort.

Download


3. Iframe Resizer

Keep same and cross domain iFrames sized to their content with support for window/content resizing, in page links, nesting and multiple iFrames

Demo

Download


4. Iframely

Embed proxy. Supports over 1800 domains via custom parsers, oEmbed, Twitter Cards and Open Graph

Demo

Download


5. React Frame component

This component allows you to encapsulate your entire React application or per component in an iFrame.

Demo

Download


6. Seamless.js

A seamless iframe makes it so that visitors are unable to distinguish between content within the iframe and content beside the iframe. Seamless.js is a JavaScript library (with no dependencies) that makes working with iframes easy by doing all the seamless stuff for you automatically.

Demo

Download


7. Porthole

A proxy to safely communicate to cross-domain iframes in javascript

Demo

Download


Thank for read!