Important Interview Questions: Javascript Version - Part 3

Explain how to read and write a file using JavaScript?

const fs = require('fs')

fs.writeFile('index.js', 'Content of the file goes here', (error) => {
    if(error) {
        console.log(error)
    }
    console.log('file created!')
})

fs.readFile('input.js', (err, data) => {
    if(err) {
        console.log(err)
    }
    console.log(`The data in the file is: ${data.toString()}`)
})

1. First we require fs module (Node File System module) which reads and writes in a file.
2. The function writeFile() is used to create a file. This function accepts 2 arguments and a function. The first one takes the name of the file, the second argument takes the data of the file, and lastly, the function takes the error (if any).
3. The function readFile() is used to read a file. This function accepts the name of the file to be searched and then there is a function with two arguments. First for the error, and second for the data inside the file.

What are all the looping structures in JavaScript?

The different looping structures in JavaScript are:

• for loop
• while loop
• foreach loop
• do …while loop
• for-in loop

What is called Variable typing in Javascript?

JavaScript is a dynamically typed, that means values of a variable can be reassigned various time.

var a // undefined
a = 'Hello World' // string
a = 30 // number
a = true // boolean
a = {} // object

How can you convert the string of any base to integer in JavaScript?

In JavaScript parseInt() function is used to convert the string to an integer. This function returns an integer of the base which is specified in the second argument of the parseInt() function. The parseInt() function returns Nan (not a number) when the string doesn’t contain number.

Syntax:

parseInt(value, radix)

Explain the difference between == and ===?

• == converts the variable values to the same type before performing the comparison. This is called type coercion.
• === does not do any type conversion (coercion) and returns true only if both values and types are identical for the two variables being compared.

What would be the result of 3+5+"7"?

Since 3 and 5 are integers, they will add up to 8. Since 7 is a string, it will be concatenated to an integer. Then the result 60 will be printed.

Explain how to detect the operating system on the client machine?

All the window.navigator properties can be listed by

console.log(navigator)

What do mean by NULL in Javascript?

The value null represents the intentional absence of any object value. It is one of JavaScript’s primitive values and is treated as falsy for boolean operations.

What is the function of the delete operator?

The delete keyword is used to delete a property as well as the value of an object.

var student = {"name": "Ishan", "roll": 19}

delete student.roll // deletes the roll property

What is an undefined value in JavaScript?

Undefined value means the

• Variable used in the code doesn’t exist
• Variable is not assigned to any value
• Property doesn’t exist