JavaScript Interview Questions

JavaScript is a scripting language that allows you to implement complex features on web pages. JavaScript can do things like:

• Validate form input
• Check if a user is logged in
• Display a popup window
• Change the content of an HTML element
• Send data to a server
• Receive data from a server
• Update a database
• Perform calculations
• Graphically draw on a canvas
• And much more!

JavaScript is everywhere! From the browser to the server, From mobile to desktop, From IoT to machine learning, It is everywhere!

JavaScript and Java are completly 2 different programming language with different syntax and different purpose.

In early days, JavaScript was named as LiveScript and later it was renamed to JavaScript because of the popularity of Java.

Because of similarity in name, people often confuse JavaScript with Java. But they are completly different.

The following table shows the differences between JavaScript and Java:

There are two types of equality operators in JavaScript:

• == (equality operator)
• === (strict equality operator)

The == operator compares two values for equality. If the values being compared are not of the same type, JavaScript will convert one type to another before making the comparison.

Whereas the === operator compares two values for equality. If the values being compared are not of the same type, then the values are considered unequal.

For any strict equality comparison, always use the === operator.

console.log(1 == '1'); // true
console.log(1 === '1'); // false

let and var are two keywords used to declare variables in JavaScript.

However, there are some differences between them:

• Scope: The main difference between let and var is in the scope. The scope of a variable declared with let is limited to the block in which it is declared, whereas the scope of a variable declared with var is the entire enclosing function.
• Re-declaration: Variables declared with let cannot be re-declared in the same scope. But variables declared with var can be re-declared in the same scope.
• Hoisting: let are not hoisted meaning they can't be used before they are declared. But var are hoisted so they can be used before they are declared.

null and undefined are two special values in JavaScript. They seem similar but they are different.

A Scriptting language is generally interpreted rater than compiled. These programs are not converted into machine language.

These are generally used for automation purposes, repetative tasks or adding some functionality to a software.

Example of scripting languages are: JavaScript, VBScript, Perl, Python, PHP, etc.

On the other hand a Programming language is a language that is used to write complex programs and applications.

These programs are compiled into machine language and then executed. For example C, C++, Java, etc.

JavaScript can handle or process only one task at a time that's why it is called single threaded.

To handle multiple tasks JavaScript uses asynchronous programming. But this doesn't mean multiple tasks are executed at any moment. Actually tasks are queued in a certain way that it seems to handle multiple tasks.

JavaScript has only one call stack. The call stack is a data structure that records where we are in the program.

ECMAScript is a scripting language specification standardized by Ecma International. It was created to standardize JavaScript to help foster multiple independent implementations.

ECMAScript is the official name of the language. JavaScript is the common name of the language.

It has different versions like es5, es6, etc.

Type coercion is a process by which a value is converted from one type to another.

There are 2 types of type coercion in JavaScript:

1. Implicit coercion: It is an automatic conversion of data type in certain situations. For example, comparing 2 values of different types using == operator.

   // Implicit coercion
   console.log(1 == '1'); // true

2. Explicit coercion: This is type of conversion where we explicitly use some function to convert type. Example Number(), String(), Boolean(), etc.

   // Explicit coercion
   console.log(Number('1')); // 1

Arrow function is a new way to write a function in JavaScript. It was introduced in ES6.

Arrow functions has few advantages over normal functions:

• Syntax of arrow function is shorter
• this in arrow function is same as this in parent scope.
• If there is only one statement in arrow function then it is automatically returned.

Strict more is a feature in JavaScript that allows us to write secure and error free code.

It can be implemented by adding "use strict" at the top of the file or function.

This forces javascript to throw errors for some common mistakes like:

• Using undeclared variables
• Assigning value to a read-only property
• Deleting undeletable properties
• Using with statement
• Using eval function

Both forEach and for-of loops are used for iteration purposes.

• forEach: It is an array method that iterates over and array and execute a callback function for each element of the array. It doesn't return anything.

  const arr = [1, 2, 3, 4, 5];
  arr.forEach((element) => {
    console.log(element);
  });

• for-of: It can iterate not only over arrays but also over strings, object, sets, etc. Values of each element is assigned to a variable and then that variable is used in the loop.

  const str = 'Hello World';
  for (const element of str) {
    console.log(element);
  }

Hoisting is mechanism in JavaScript that moves some variable and function declarations to the top of the scope they are declared in.

It is done by the JavaScript engine before the code is executed.

It is done for 2 types of declarations:

Due to this phenomenon, you can use a variable or function before it is declared (but later in the code it will be declared).

Note: Not all declarations are hoisted. Only var and normal function declarations are hoisted.

// Hoisting
a = 10;
console.log(a); // 10
var a;

hello("Jack");
function hello(name) {
  console.log("Hello " + name);
}