JavaScript Interview Questions - Set 5

JavaScript Interview Questions

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This page consists of JavaScript interview questions and answers.

Q1: What are global variables in JavaScript?

Global variables are the variables that are available throughout the code.

If we create a variable without the var keyword then it becomes a global variable.

Q2: What is the this keyword in JavaScript?

The this keyword is used to refer to the object from where it is called.

Q3: What is the use of "use strict"; in JavaScript?

The "use strict"; in JavaScript is used to execute the code in strict mode.

So, for example in strict mode we can't use undeclared variables.

"use strict"; is a literal expression and not a statement and is ignored by older JavaScript versions.

Q4: What is setTimeout in JavaScript?

The setTimeout function executes a function after a given delay.

In the following example we will get "Hello World" message in the browser console after 5000ms (milliseconds) delay.

setTimeout(function() {
  console.log('Hello World');
}, 5000)

Q5: What is setInterval in JavaScript?

The setInterval function is used to execute a function repeatedly after a given delay. The execution stops only when we stop the timer using the clearInterval function.

In the following example we will get "Hello World" after every 3 seconds (3000ms) for 5 times.

var count = 0;
var t = setInterval(function() {
  console.log('Hello World');
  count++;
  if (count === 5) {
    clearInterval(t);
    console.log('End');
  }
}, 3000);

Q6: What is the difference between View state and Session state?

View state is specific to a page in a session. Whereas, Session state is specific to a user or browser session and is accessible from all pages.

Q7: What are the different loop structure available in JavaScript?

Following are the loop structure available in JavaScript.

  • while
  • do-while
  • for

Q8: Write JavaScript code to print individual characters of a given string 'Hello World' in the browser console.

var str = 'Hello World';
for (var i = 0, len = str.length; i < len; i++) {
  console.log(str[i]);
}

Q9: What is the output for the following JavaScript code?

console.log(typeof typeof null);

The output is "string".

Explanation:

The typeof null will give us "object". This is in double quotes so, it is a string.

Next, the typeof "object" will give us "string".

Hence, the final answer is "string".

Q10: What is the use of Void(0) in JavaScript?

Void(0) is used to prevent a page from refreshing when used with hyperlinks.

In the following example we have a hyperlink which when clicked will not refresh the page or jump to the top of the page.

<a href="javascript:void(0);" ondblclick="alert('Hello World')">Double click me</a>
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