JavaScript : MCQs & PYQs with Explanation

Option C

In JavaScript, the + operator works as both addition and string concatenation. If any operand is a string, the operation becomes concatenation. In the given code, the expression is evaluated from left to right.
First, '4'+3 results in "43" because the number 3 is converted into a string. Then "43"+12.89 results in "4312.89", as the number is again converted into a string and concatenated.
Hence, the final output is 4312.89.

Option A

The HTML <script> tag is used to embed or reference JavaScript code in an HTML page. JavaScript is commonly used to make webpages interactive and dynamic.
<script>
    alert("Hello, Suresh Kulahry");
</script>
It can also be linked to an external JavaScript file using the src attribute as <script src="myscript.js"></script>

Option A

In JavaScript, the Document object represents the actual HTML page and its content that is loaded in the browser.
It provides access to HTML elements such as headings, paragraphs, images, forms, buttons, and other elements.

Option A

In JavaScript, the main difference between var and let lies in their scope. A variable declared with var has either global scope (if declared outside a function) or function scope (if declared inside a function). It does not follow block scope, meaning it can still be accessed outside the block { } in which it is defined.
On the other hand, a variable declared with let is block scoped, which means it is only accessible within the block { } where it is declared.
 

// Using var
if (true) {
    var x = 10;
}
console.log(x); // No Error, print 10 (var is not block scoped).
// Using let
if (true) {
    let y = 20;
}
console.log(y); // Error, y is not defined (let is block scoped).

Option D

JavaScript is commonly used to make an HTML webpage dynamic and interactive. When JavaScript code is written directly inside an HTML document, it is placed between the <script> and </script> tags.
The <script> tag is used to include JavaScript in an HTML document.
<script>
    // JavaScript code
</script>

• JavaScript can also be placed in a separate external .js file as <script src="myscript.js"></script>

Option B

In JavaScript, a callback function is a function that is passed as an argument (parameter) to another function. The receiving function can then execute the callback function at the appropriate time.
Callback functions are widely used in JavaScript for event handling, timers, asynchronous operations, array methods, etc.
JavaScript supports functions as first-class objects, which means a function can be stored in a variable, passed as an argument to another function, and returned from another function.
E.g.:
function greet(name, callback) {
    console.log("Hello " + name);
    callback();
}
function message() {
    console.log("Welcome!");
}
greet("Suresh", message);
Here:

• greet() is the main function.
• message() is passed as an argument to greet().
• message() is executed inside greet().
• Therefore, message() is a callback function.

Option B

In JavaScript, the Window object is the top-level or master container of the browser window. It represents the browser's window or tab and provides access to objects such as the Document, Location, History, and Screen.
Window
  │
  ├── Document
  ├── Location
  ├── History
  └── Screen
E.g.:

• window.document
• window.location
• window.screen

The document object represents the actual HTML document (webpage content) displayed inside the browser window.

Option C

A cookie does not have a standard attribute called Lifetime for specifying how long it remains in the browser. The lifetime of a cookie is normally controlled using the Expires attribute or the Max-Age attribute. Therefore, statement (i) is False.
The Secure attribute is a Boolean attribute of a cookie. When the Secure attribute is present, the browser sends the cookie only over a secure HTTPS connection. Therefore, statement (ii) is True.

Option C

In JavaScript, the alert() method of the Window object is used to display a dialog box containing a message.
E.g.:

• alert("Hello, Suresh Kulahry");
• window.alert("Hello, Suresh Kulahry");

Important Window Dialog Methods:

Option C

JavaScript is primarily an interpreted language, and its code is executed by the JavaScript Engine inside a Web Browser such as Chrome, Firefox, etc. During execution, the browser reads the code and ignores comments because they are meant only for developers, not for execution.