JavaScript Day 4 — Mastering JavaScript Basics: A Comprehensive Guide for Interview Preparation

100 Days of JavaScript Interview Prep: A Beginner’s Journey

Welcome to Day 4 –100 Days of JavaScript Interview Prep: A Beginner’s Journey

Get ready to ace your next JavaScript interview with this comprehensive guide! Join us as we document a 100-day adventure filled with learning, practice, and growth.

DAY 4 (March 26, 2024) — Data Types, Variables, Loops Practical Questions

Note if someone wants to try online while reading :)

Can explore below online javascript Compiler — Happy Coding!

Online JavaScript Compiler

We will use online JavaScript Compiler today to run all examples of questions

Question 1

What will be the result of typeof null in JavaScript?

Answer:

The result will be "object".

This is a historical quirk in JavaScript and is considered a mistake.

typeof null returning "object" is a well-known issue in JavaScript that cannot be fixed due to backward compatibility concerns.

Question 2

How can you check if a variable is an array in JavaScript without using the typeof operator?

Answer:

let myarr = [1, 2, 3];
console.log(Array.isArray(myarr)); 

You can use the Array.isArray() method to check if a variable is an array. For example:

Question 3

Explain the difference between == and === operators in JavaScript.

Answer:

The == operator compares the values of two operands after converting them to a common type, while === (strict equality) compares both value and type without type conversion

== (Equality Operator):

5 == "5"; 
// true, because the string "5" is coerced to the number 5
0 == false; 
// true, because the boolean false is coerced to the number 0
null == undefined; 
// true, because they are both considered equal in value (but not in type)

=== (Strict Equality Operator):

The === operator checks for equality of values as well as equality of types, without performing type coercion.

5 === "5"; 
// false, because the types are different (number vs. string)
0 === false; 
// false, because the types are different (number vs. boolean)
null === undefined; 
// false, because although they are both considered equal in value, their types are different

Question 4

What will be the output of console.log(+'10') in JavaScript?

Answer:

The output will be 10.

The unary plus operator (+) converts its operand to a number, so '10' is converted to the number 10.

Question 5

How can you convert a string to a number in JavaScript?

Answer:

You can use parseInt() or parseFloat() functions for converting strings to integers or floating-point numbers, respectively.

Using the parseInt() function:

• The parseInt() function parses a string and returns an integer.

let str = "123";
let num = parseInt(str);
console.log(num); // Output: 123
console.log(typeof(num)) // number 

Using the parseFloat() function:

• The parseFloat() function parses a string and returns a floating-point number.

let str = "3.14";
let num = parseFloat(str);
console.log(num); // Output: 3.14

Question 6

How can you convert a string to a number in JavaScript with Number or + Operator or parseFloat() or parseInt() with unary plus for numeric strings?

Answer:

Using the Number() function:

• The Number() function converts the given value to a number.

let str = "42";
let num = Number(str);
console.log(num); // Output: 42

Using the + operator:

• The unary plus operator (+) can also convert a string to a number.

let str = "99";
let num = +str;
console.log(num); // Output: 99
console.log(typeof(num))

Using parseFloat() or parseInt() with unary plus for numeric strings:

• If you have a string containing only numeric characters, you can use the unary plus operator directly.

let str = "4.56";
let num = +str;
console.log(num); // Output: 4.56
console.log(typeof(num))

let str = "456";
let num = +str;
console.log(num); // Output: 456
console.log(typeof(num))

Question 7

What is the result of 0 == false in JavaScript? Explain.

Answer:

The result is true.

In JavaScript, 0 is considered falsy, so it's loosely equal to false

Question 8

How can you check if a variable is defined in JavaScript?

Answer:

In JavaScript, you can check if a variable is defined using several methods. Here are some commonly used approaches:

1 Using the typeof operator:

• The typeof operator returns a string indicating the type of the unevaluated operand.
• You can use typeof to check if a variable is defined by comparing it with the string 'undefined'.

let x;
if (typeof x !== 'undefined') {
    console.log('x is defined');
} else {
    console.log('x is not defined');
}

2 Using strict equality (===) with undefined:

• You can directly check if the variable is strictly equal to undefined.

let y;
if (y !== undefined) {
    console.log('y is defined');
} else {
    console.log('y is not defined');
}

3. Using the in operator:

• The in operator checks if a property is in an object.
• You can use it to check if a variable is defined in the global scope by checking if it’s a property of the window object (in browsers).

if ('z' in window) {
    console.log('z is defined');
} else {
    console.log('z is not defined');
}

4. Using a try-catch block:

• You can try to access the variable within a try block and catch the ReferenceError if the variable is not defined.

try {
    let a;
    let b = a; // This line will throw a ReferenceError if 'a' is not defined
    console.log('a is defined');
} catch (error) {
    console.log('a is not defined');
}

Question 9

Write a JavaScript function to check if a number is even or odd without using the modulus operator %.

Answer:

function isEven(num) {
    return (num / 2) === Math.floor(num / 2);
}

Question 9

Explain the concept of truthy and falsy values in JavaScript?

Answer:

In JavaScript, values are evaluated in Boolean contexts, such as conditions in if statements or while loops.

Truthy and falsy values are concepts that describe how JavaScript treats non-boolean values when they are coerced to Boolean in such contexts.

Truthy Values:

• Values that are considered truthy evaluate to true when converted to a Boolean.

Examples of truthy values include:

• Non-empty strings ("hello", "true", "0").
• Non-zero numbers (positive or negative).
• Non-empty objects (including arrays and functions).
• The boolean value true.
• When used in a condition, truthy values cause the condition to be true.

Falsy Values:

• Values that are considered falsy evaluate to false when converted to a Boolean.

Examples of falsy values include:

• The empty string "".
• The number 0 (zero).
• The boolean value false.
• null.
• undefined.
• NaN (Not a Number).
• When used in a condition, falsy values cause the condition to be false.

let x = "hello";
if (x) {
    console.log("x is truthy");
} else {
    console.log("x is falsy");
}

let y = 0;
if (y) {
    console.log("y is truthy");
} else {
    console.log("y is falsy");
}

In this example, "hello" is a truthy value, so the first condition evaluates to true and "x is truthy" is logged to the console.

However, 0 is a falsy value, so the second condition evaluates to false and "y is falsy" is logged to the console.

Question 10

What will be the output of console.log(1 + '1' - 1) in JavaScript?

Answer:

The output will be 10.

The concatenation of '1' and 1 results in the string '11',which is then converted back to a number during arithmetic operations, resulting in 10

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