ES6

Question 1

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

Answer 1

JavaScript statements can be grouped together in code blocks, inside curly brackets {…}.
The purpose of code blocks is to define statements to be executed together.
One place you will find statements grouped together in blocks, is in JavaScript functions:

Question 2

2. What does block scope mean?

Answer 2

You can’t access variables defined within the block OUTSIDE the block.

Block Level Scope: This scope restricts the variable that is declared inside a specific block, from access by the outside of the block. The let & const keyword facilitates the variables to be block scoped. In order to access the variables of that specific block, we need to create an object for it. Variables declared with the var keyword, do not have block scope.

Question 3

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

Answer 3

In order to access the variables of a specific block, we need to create an object for it.

Question 4

4. What is the difference between let and const?

Answer 4

let is mutable
let CAN be updated but cannot be re-declared into the scope.
let CAN be declared without initialization.
let can NOT be accessed without initialization as it returns an error.

const is read only
const – can NOT be updated or redeclare into the scope. (It is immutable)
const can NOT be declared without initialization.
const can NOT be accessed without initialization, SINCE it cannot be declared without initialization.
YOU CANNOT REASSIGN A CONST VARIABLE

Question 5

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

Answer 5

When you’re adding to an array or object you’re not re-assigning or re-declaring the constant. It is already declared and assigned, you’re just adding to the “list” that the constant points to.

Question 6

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

Answer 6

If the variable is going to be re-assigned at any time, use let
If the variable is constant, use CONST.

Prefer CONST – use const until you can’t.

Question 7

1. What is the syntax for writing a template literal?

Answer 7

Define a variable
Use the back tick ‘ ` ‘ to input your data
End with a semicolon

${} (dollar sign and curly braces.
READ: JavaScript expression.

Question 8

2. What is “string interpolation”?

Answer 8

Creating strings by using substitution of placeholders ( ${} )
JavaScript string interpolation is the process of embedding an expression into part of a string. A template literal is used to embed expressions. You can add values such as variables and mathematical calculations into a string using interpolation.

Question 9

1. What is destructuring, conceptually?

Answer 9

The destructuring assignment syntax is a JavaScript expression that makes it possible to unpack values from arrays, or properties from objects, into distinct variables.
(Explode out properties into variables and use them right away – Tim)

Question 10

2. What is the syntax for Object destructuring?

Answer 10

let { property1: variable1, property2: variable2 } = object;
keyword (var, let, or const)
for objects use curly braces.
And within the curly braces, the property (key name) separated by a colon if you want another name (alias)
Followed by a closing curly brace
Assignment operator (=)
Object you are destructuring properties from.

Question 11

3. What is the syntax for Array destructuring?

Answer 11

const [ first, second, third… elements in the array] = theArray

THIS IS WRONG  const foo = [‘one’, ‘two’, ‘three’];

const [red, yellow, green] = foo;

console. log(red); // “one”
console. log(yellow); // “two”
console. log(green); // “three”

Question 12

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

Answer 12

Dead giveaway – which side of the brackets show up of the equal sign.
Left side is destructuring
Right side is object literal.

Question 13

1. What is the syntax for defining an arrow function?

Answer 13

(param1, param2) => {
return;
};

Another way…
param => expression;

Another way…
() => expression;

Question 14

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

Answer 14

It is evaluated as an expression rather than a statement.

Ex: const add = (x, y) => x + y
If no curly braces, …

Question 15

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

Answer 15

It takes the “this” from the enclosing function.
In a normal function, “this” is called during call time.
In arrow functions, “this” is called when the function is DEFINED and it gets its value from the enclosing function.

Question 16

1. What are the three states a Promise can be in?

Answer 16

a. pending: initial state, neither fulfilled nor rejected.
b. fulfilled: meaning that the operation was completed successfully.
c. rejected: meaning that the operation failed.

Question 17

2. How do you handle the fulfillment of a Promise?

Answer 17

a. Use the “then” and the callback function.

Question 18

3. How do you handle the rejection of a Promise?

Answer 18

a. Use the “catch” method
b. You pass the rejection callback handler.
c. Essentially, a promise is a returned object to which you attach callbacks, instead of passing callbacks into a function.