JavaScript Coding Questions and Answers | Learn JavaScript Coding Interview Questions Answers to become Full Stack JavaScript Developer | Front-end Developer

## JavaScript Coding Questions and Answers

Q:- Reverse a given string using JavaScript?
var str = "Full Stack Tutorials"; var output = str .split("") .reverse() .join(""); document.write(output);
Output:

slairotuT kcatS lluF

Q:- Find the sum of all elements/numbers of a given array?

There are many ways to solve this problem, we will see following ways.

1. using array reduce() method
var arr = [1, 2, 5, 10, 20]; var sum = arr.reduce((a, i) => { return a + i; }); document.write(sum);
2. using loops (e.g. - for loop)
var arr = [1, 2, 5, 10, 20]; var sum = 0; for (let i in arr) { sum += arr[i]; } document.write(sum);
Output:

38

Q:- Tell the output of following code?
const a = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]; for (let i = 0; i < 10; i++) { setTimeout(() => console.log(a[i]), 1000); } for (var i = 0; i < 10; i++) { setTimeout(() => console.log(a[i]), 1000); }
Output:
1 2 3 4 5 6 7 8 9 10 undefined undefined undefined undefined undefined undefined undefined undefined undefined undefined
Q:- What will be the output of following js code snippet?
const number = undefined + 11; if (number === NaN) { document.write("NaN"); } else if (number === 11) { document.write("11"); } else { document.write("other"); }
Output:

other

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Q:- JavaScript startsWith and endsWith?
let name = "Full Stack Tutorials, Latest Interview Questions and Answers!"; //startsWith console.log(name.startsWith("Full")); // true console.log(name.startsWith("full")); // false console.log(name.startsWith("Tutorials")); // false console.log(name.startsWith("Tutorials", 11)); // true //endsWith console.log(name.endsWith("Answers!")); // true console.log(name.endsWith("answers!")); // false

Note:- Both startsWith & endsWith are case-sensitive.

Q:- Find the output?
var a=3; var b=a++; var c=++a; console.log(a,b,c)
Output:

5 3 5

Q:- What will be the output of both the fucntions?
function func1() { return { name: "Full Stack Tutorials", }; } console.log(func1());
Output:
//Output: {name: "Full Stack Tutorials"}
function func2() { return; { name: "Full Stack Tutorials"; } } console.log(func2());
Output:
//Output: undefined
Reason:-

The reason for this has to do with the fact that semicolons are technically optional in JavaScript (although omitting them is generally really bad form).

As a result, when the line containing the return statement (with nothing else on the line) is encountered in func2(), a semicolon is automatically inserted immediately after the return statement.

No error is thrown since the remainder of the code is perfectly valid, even though it doesnâ€™t ever get invoked or do anything (it is simply an unused code block that defines a property bar which is equal to the string "Full Stack Tutorials").

This behavior also argues for following the convention of placing an opening curly brace at the end of a line in JavaScript, rather than on the beginning of a new line. As shown here, this becomes more than just a stylistic preference in JavaScript.

Q:- What will be output of following?
console.log(typeof undefined); console.log(typeof null); console.log(typeof NULL); console.log(typeof typeof 1);
Output:
//Output: undefined object undefined string
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Q:- How to convert an Object {} into an Array [] in JavaScript?
let obj = { id: "1", name: "Test User", age: "25", profession: "Developer" }; //Method 1: Convert the keys to Array using - Object.keys() console.log(Object.keys(obj)); // ["id", "name", "age", "profession"] // Method 2 Converts the Values to Array using - Object.values() console.log(Object.values(obj)); // ["1", "Test User", "25", "Developer"] // Method 3 Converts both keys and values using - Object.entries() console.log(Object.entries(obj)); //[["id", "1"],["name", "Test User"],["age", "25"],["profession", "Developer"]]
Q:- How to convert an Array [] to Object {} in JavaScript?
let arr = ["1", "Test User", "25", "Developer"]; let arr1 = [ ["id", "1"], ["name", "Test User"], ["age", "25"], ["profession", "Developer"], ]; //Method 1: Using Object.assign() method console.log(Object.assign({}, arr)); //{0: "1", 1: "Test User", 2: "25", 3: "Developer"} // Method 2 Using Spread operator console.log({ ...arr }); //{0: "1", 1: "Test User", 2: "25", 3: "Developer"} // Method 3: Using Object.fromEntries() method console.log(Object.fromEntries(arr1)); //{id: "1", name: "Test User", age: "25", profession: "Developer"}
Q:- Tell the output of following code?
const a = { x: 1, y: 2 }; const b = a; b.x = 3; console.log(a); console.log(b);
Output:
{ x: 3, y: 2 } { x: 3, y: 2 }

Reason: both the object are pointing to same reference.

Questions for you!
Q:- What is the output?
let a = 10; var a = 20; console.log(a);