Top 50+ JavaScript Interview Questions and Answers

Answer:

JavaScript is a lightweight, interpreted programming language with object-oriented capabilities that allows you to build interactivity into otherwise static HTML pages. The general-purpose core of the language has been embedded in Netscape, Internet Explorer, and other web browsers.

Answer:

Following are the major differences between Java & JavaScript:

• Java is an OOP programming language. JavaScript is an OOP scripting language.
• It creates applications that run in a virtual machine or browser. The code is run on a browser only.
• Java code needs to be compiled. JavaScript code are all in the form of text.

Answer:

The data types supported by JavaScript are:

• Undefined
• Null
• Boolean
• String
• Symbol
• Number
• Object

Answer:

Following are the features of JavaScript:

• It is a lightweight, interpreted programming language.
• It is designed for creating network-centric applications.
• It is complementary to and integrated with Java.
• It is an open and cross-platform scripting language.

Answer:

The scope of a variable is the region of your program in which it is defined. JavaScript variable will have only two scopes.

• Global Variables − A global variable has global scope which means it is visible everywhere in your JavaScript code.
• Local Variables − A local variable will be visible only within a function where it is defined. Function parameters are always local to that function.

Answer:

The JavaScript this keyword refers to the object it belongs to. This has different values depending on where it is used. In a method, this refers to the owner object and in a function, this refers to the global object.

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A callback is a plain JavaScript function passed to some method as an argument or option. It is a function that is to be executed after another function has finished executing, hence the name ‘call back‘. In JavaScript, functions are objects. Because of this, functions can take functions as arguments, and can be returned by other functions.

Answer:

The type of operator is used to get the data type of its operand. The operand can be either a literal or a data structure such as a variable, a function, or an object. It is a unary operator that is placed before its single operand, which can be of any type. Its value is a string indicating the data type of the operand.

Answer:

The simplest way to create a cookie is to assign a string value to the document.cookie object, which looks like this-

Syntax :

document.cookie = “key1 = value1; key2 = value2; expires = date”;

Answer:

The three possible ways of defining a variable in JavaScript are:

• Var – The JavaScript variables statement is used to declare a variable and, optionally, we can initialize the value of that variable. Example: var a =10; Variable declarations are processed before the execution of the code.
• Const – The idea of const functions is not allow them to modify the object on which they are called. When a function is declared as const, it can be called on any type of object.
• Let – It is a signal that the variable may be reassigned, such as a counter in a loop, or a value swap in an algorithm. It also signals that the variable will be used only in the block it’s defined in.

Answer:

Undefined means a variable has been declared but has not yet been assigned a value. On the other hand, null is an assignment value. It can be assigned to a variable as a representation of no value. Also, undefined and null are two distinct types: undefined is a type itself (undefined) while null is an object.

Answer:

Event bubbling is a way of event propagation in the HTML DOM API, when an event occurs in an element inside another element, and both elements have registered a handle for that event. With bubbling, the event is first captured and handled by the innermost element and then propagated to outer elements. The execution starts from that event and goes to its parent element. Then the execution passes to its parent element and so on till the body element.

Answer:

NaN is a short form of Not a Number. Since NaN always compares unequal to any number, including NaN, it is usually used to indicate an error condition for a function that should return a valid number. When a string or something else is being converted into a number and that cannot be done, then we get to see NaN.

Answer:

A prompt box is a box which allows the user to enter input by providing a text box. The prompt() method displays a dialog box that prompts the visitor for input. A prompt box is often used if you want the user to input a value before entering a page. When a prompt box pops up, the user will have to click either “OK” or “Cancel” to proceed after entering an input value.

Answer:

There are three types of errors:

• Load time errors: Errors that come up when loading a web page, like improper syntax errors, are known as Load time errors and generate the errors dynamically.
• Runtime errors: Errors that come due to misuse of the command inside the HTML language.
• Logical errors: These are the errors that occur due to the bad logic performed on a function with a different operation.

Answer:

The different functional components in JavaScript are-

• First-class functions: Functions in JavaScript are utilized as first-class objects. This usually means that these functions can be passed as arguments to other functions, returned as values from other functions, assigned to variables, or can also be stored in data structures.
• Nested functions: The functions, which are defined inside other functions, are called Nested functions. They are called ‘every time the main function is invoked.

Answer:

JavaScript escape characters enable you to write special characters without breaking your application. Escape characters (Backslash) is used when working with special characters like single quotes, double quotes, apostrophes and ampersands. Place backslash before the characters to make it display.

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Hoisting is the default behavior of JavaScript where all the variable and function declarations are moved on top. In simple words, we can say that Hoisting is a process in which, irrespective of where the variables and functions are declared, they are moved on top of the scope. The scope can be both local and global.

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Temporal Dead Zone is a behavior that occurs with variables declared using let and const keywords before they are initialized.

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BOM is the Browser Object Model where users can interact with browsers that is a window, an initial object of the browser. The window object consists of a document, history, screen, navigator, location, and other attributes. Nevertheless, the window’s function can be called directly as well as by referencing the window.