ES6 fundamentals

Question 1

Define the concept of “scope”.

Answer 1

Scope is a way to identify where and whether we can use a variable.

Question 2

What are the 3 types of scope?

Answer 2

1. Block 2. Functional 3. Global

Question 3

Are the variables declared with let and const available outside of their declaration block?

Answer 3

No, let and const variables have block scope - they are not visible outside of the block they were declared in.

Question 4

Are the variables declared with var, let, const accessible outside of the function they were declared in?

Answer 4

No, they are visible only inside the function.

Question 5

Can you reassign variables declared with let, const and var?

Answer 5

Only those decalred with let and var.

Question 6

If you are in non-strict mode and you access an undefined variable declared withvar, you get undefined. What happens if you are in strict-mode?

Answer 6

You get ReferenceError

Question 7

What is a template literal?

Answer 7

String literals that allow embedding expressions.

Question 8

What is an arrow function?

Answer 8

An arrow function is an anonymous function with a special syntax: () => {}

Question 9

How does this keyword work in arrow function?

Answer 9

In arrow functions, this is lexically bound, meaning it uses this from the surrounding code that contains the arrow function.

Question 10

When should you avoid using arrow functions and use regular functions instead?

Answer 10

You should avoid using arrow functions for object methods and event listener callbacks because this behaves differently in arrow functions compared to regular functions.

Question 11

In what situations is it recommended to use arrow functions?

Answer 11

Arrow functions are recommended when you need a concise coding style, such as for map and reduce operations.

Question 12

What is object destructuring in ECMAScript 2015?

Answer 12

Object destructuring is the process of creating variables from object properties.

Question 13

How can you use object destructuring to retrieve values from a nested object?

Answer 13

You can use object destructuring with nested objects like this: const { dept: { address: { street } } } = employee;

Question 14

What is an alias in object destructuring?

Answer 14

An alias in object destructuring allows you to rename the variable when destructuring. For example, const { name: firstname } = user;

Question 15

How can you handle dynamic property names with object destructuring?

Answer 15

You can handle dynamic property names with object destructuring using square brackets, like this: const { [key]: returnValue } = employee;

Question 16

In what situations would you destructure objects in function arguments and return values?

Answer 16

Destructuring objects in function arguments and return values is useful when you want to work with specific properties of the object passed as an argument or returned from a function.

Question 17

How do spread and rest syntax differ in ECMAScript 2015?

Answer 17

Spread takes an iterable (like an array) and expands it into individual elements, while rest syntax helps collect elements together in a destructuring context.

Question 18

How can you clone an object using spread syntax?

Answer 18

You can clone an object using spread syntax like this: const newUser = { …user }

Question 19

What is the difference between spread and rest syntax in assignment statements?

Answer 19

In assignment statements, rest syntax is used on the left side of the assignment to collect values, while spread syntax is used on the right side to spread values.

Question 20

How can you merge two objects using spread syntax?

Answer 20

You can merge two objects using spread syntax like this: const final = { …user1, …user2 }

Question 21

What is a JavaScript module?

Answer 21

A JavaScript module is a JavaScript file that encapsulates and exports features for use in other modules.

Question 22

What keyword is used to export features from a module?

Answer 22

The export keyword.

Question 23

How do you import features from another module?

Answer 23

Using import keyword

Question 24

How can you import all features of a module as a namespace?

Answer 24

You can import all features of a module as a namespace using the import * as NamespaceName from ‘module’; syntax.

Question 25

What is the purpose of the default keyword when exporting from a module?

Answer 25

The default keyword is used to export a single feature as the default export from a module.

Question 26

What data structure does the JavaScript engine use to track the current function in execution?

Answer 26

The "function execution stack"

Question 27

What is the purpose of the callback queue or task queue in JavaScript?

Answer 27

The callback queue or task queue in JavaScript is used to store and manage callback functions for asynchronous execution.

Question 28

What are promises in JavaScript?

Answer 28

Promises are special objects in JavaScript that represent the eventual completion of an action.

Question 29

How many states can a Promise be in, and what are they?

Answer 29

A Promise can be in one of three states: Pending (initial state), Fulfilled (successful completion), or Rejected (encountered an error).

Question 30

What is the event loop's priority when dealing with the job queue and callback queue?

Answer 30

The event loop gives priority to the job queue items over the callback queue items when the stack is free, ensuring that jobs are executed before callbacks.

Question 31

What ia job queue in Javascript?

Answer 31

Every time a promise occurs in the code, the executor function of the promise gets into the job queue.

Question 32

Who is responsible for managing the execution of asynchronous code and handling callback functions in a non-blocking manner in Javascript?

Answer 32

The event loop

Question 33

What is the output of the main function? function f1() { console.log('f1'); } function f2() { console.log('f2'); } function f3() { console.log('f3'); } function main() { console.log('main'); setTimeout(f1, 50); setTimeout(f3, 30); new Promise((resolve, reject) => resolve('I am a Promise, right after f1 and f3! Really?') ).then(resolve => console.log(resolve)); new Promise((resolve, reject) => resolve('I am a Promise after Promise!') ).then(resolve => console.log(resolve)); f2(); } main();

Answer 33

main f2 I am a Promise, right after f1 and f3! Really? I am a Promise after Promise! f3 f1

Question 34

How does the event loop execute tasks from the job queue and callback queue?

Answer 34

The event loop executes tasks from the job queue before moving to the callback queue when the stack is free, ensuring that jobs are given priority in execution.