ES6

Question 1

What is a code block? What are some examples of a code block?

Answer 1

In computer programming, a block or code block or block of code is a lexical structure of source code which is grouped together. Blocks consist of one or more declarations and statements.

In JavaScript, blocks are denoted by curly braces {} , for example, the if else, for, do while, while, try catch and so on:

Question 2

What does block scope mean?

Answer 2

A block scope is the area within if, switch conditions or for and while loops. Generally speaking, whenever you see {curly brackets}, it is a block. In ES6, const and let keywords allow developers to declare variables in the block scope, which means those variables exist only within the corresponding block.

https://dev.to/sandy8111112004/javascript-introduction-to-scope-function-scope-block-scope-d11

Question 3

What is the scope of a variable declared with const or let?

Answer 3

block scope

Question 4

What is the difference between let and const?

Answer 4

const is a signal that the identifier won’t be reassigned.

let is a signal that the variable may be reassigned, such as a counter in a loop, or a value swap in an algorithm. It also signals that the variable will be used only in the block it’s defined in, which is not always the entire containing function.

https://medium.com/javascript-scene/javascript-es6-var-let-or-const-ba58b8dcde75#:~:text=%60const%60%20is%20a%20signal%20that,always%20the%20entire%20containing%20function.

Question 5

Why is it possible to .push() a new value into a const variable that points to an Array?

Answer 5

This happens because your constant is actually storing a reference to the array. When you join something into your array you are not modifying your constant value, but the array it points to. The same would happen if you assigned an object to a constant and tried to modify any property of it.

If you want to freeze an array or object so it can’t be modified, you can use the Object.freeze method, which is already part of ECMAScript 5.

https://stackoverflow.com/questions/23436437/why-can-i-change-a-constant-object-in-javascript

Question 6

How should you decide on which type of declaration to use?

Answer 6

Use let when you know that the value of a variable will change.
Use const for every other variable.
Do not use var.

https://codeburst.io/javascript-var-let-or-const-which-one-should-you-use-2fd521b050fa

Question 7

What is the syntax for writing a template literal?

Answer 7

using backtiks

Question 8

What is “string interpolation”?

Answer 8

String interpolation is replacing placeholders with values in a string literal. The string interpolation in JavaScript is performed by template literals (strings wrapped in backticks ` ) and ${expression} as a placeholder.

Question 9

What is destructuring, conceptually?

Answer 9

Destructuring broadly means extracting data from arrays or objects. Using destructuring, we can break a complex array or object into smaller parts. Destructuring also offers us the ability to extract multiple data at once from an object or array.

Question 10

What is the syntax for Object destructuring?

Answer 10

let { property1: variable1, property2: variable2 } = object;

Question 11

What is the syntax for Array destructuring?

Answer 11

let [x, y, z] = getScores();

console. log(x); // 70
console. log(y); // 80
console. log(z); // 90

Question 12

How can you tell the difference between destructuring and creating Object/Array literals?

Answer 12

objects: curly braces
array: brackets

Question 13

What is the syntax for defining an arrow function?

Answer 13

An arrow function expression is an anonymous function expression written with the “fat arrow” syntax ( => ). Rewrite the sum function with arrow function syntax: const sum = (a, b) => { return a + b }

Question 14

When an arrow function’s body is left without curly braces, what changes in its functionality?

Answer 14

The parenthesis are returning a single value, the curly braces are executing multiple lines of code.

with the curly braces, its a statement so you need a ‘return’ keyword.

const add = (x, y) => x+y; //expression.. 
const add = (x, y) => { return x+y; } //explicitly saying to return

Question 15

How is the value of this determined within an arrow function?

Answer 15

Arrow functions have a lexical this and its value within the arrow function is determined by the surrounding scope. You probably assume that this gets a different value within the person object literal.

Question 16

What are the three states a Promise can be in?

Answer 16

A promise may be in one of 3 possible states: fulfilled, rejected, or pending.

Promise users can attach callbacks to handle the fulfilled value or the reason for rejection.

Question 17

How do you handle the fulfillment of a Promise?

Answer 17

The then() method returns a Promise. It takes up to two arguments: callback functions for the success and failure cases of the Promise.

Question 18

How do you handle the rejection of a Promise?

Answer 18

The catch() method returns a Promise and deals with rejected cases only. It behaves the same as calling Promise.prototype.then(undefined, onRejected) (in fact, calling obj.catch(onRejected) internally calls obj.then(undefined, onRejected)). This means that you have to provide an onRejected function even if you want to fall back to an undefined result value - for example obj.catch(() => {}).

Question 19

What is “syntactic sugar”?

Answer 19

syntactic sugar is syntax within a programming language that is designed to make things easier to read or to express.

Question 20

What is the typeof an ES6 class?

Answer 20

object ?
Class − A class in terms of OOP is a blueprint for creating objects. A class encapsulates data for the object.

Question 21

Describe ES6 class syntax.

Answer 21

class Polygon { 
   constructor(height, width) { 
      this.height = height; 
      this.width = width; 
   } 
}

Question 22

What is “refactoring”?

Answer 22

code refactoring is the process of restructuring existing computer code—changing the factoring—without changing its external behavior.

Question 23

How are ES Modules different from CommonJS modules?

Answer 23

ES modules are the standard for JavaScript, while CommonJS is the default in Node. js.

https://blog.logrocket.com/commonjs-vs-es-modules-node-js/#:~:text=ES%20modules%20are%20the%20standard,encapsulating%20JavaScript%20code%20for%20reuse.

Question 24

What kind of modules can Webpack support?

Answer 24

Webpack supports the following module types natively: ECMAScript modules. CommonJS modules. AMD modules.