Basic and most common JavaScript interview questions and answers.
Question 1: What is closure in JavaScript?
Answer: A closure is a function that has access to variables that are declared and defined in the parent function scope even after the outer function has returned.
function init() {
var name = 'Mozilla'; // name is a local variable created by init
function displayName() { // displayName() is the inner function, a closure
alert(name); // use variable declared in the parent function
}
displayName();
}
init();
Question 2: What is the use of window objects in JavaScript?
Answer: The window object is created automatically by the browser that represents the window of a browser. It is a browser object, not a JavaScript Object.
Methods of the window object:
| Method | Description |
|---|---|
| alert() | displays the alert box containing a message with the ok button. |
| confirm() | displays the confirm dialog box containing messages with the ok and cancel buttons. |
| prompt() | displays a dialog box to get input from the user. |
| open() | opens the new window. |
| close() | closes the current window. |
| setTimeout() | performs an action after a specified time like calling function, evaluating expressions, etc. |
Question 3: What is the use of the History object in JavaScript?
Answer: A History object of the browser is used to roll back the previous user history and actions. Such as Back and forward from a current web page or another page.
| Methods | Description |
|---|---|
back() | specifies a method that loads the previous URL from the history list. |
forward() | specifies a method that loads the next URL from the history list. |
go() | specifies a method that loads a specific URL from the history list. |
pushState() | used to push the given data onto the session history stack with the specified title |
replaceState() | used to update the most recent entry on the history stack to the specified data, title, and, if provided, URL |
Question 4: What is difference between == and ===?
Answer:
- = is used for assigning values to a variable in JavaScript.
- == is used for comparison between two variables irrespective of the data type of variable.
- === is used for comparison between two variables but this will check strict type, which means it will check datatype and compare two values.