JavaScript ES6

Question 1

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

Answer 1

Code inside curly braces, like a function or if block

Question 2

What does block scope mean?

Answer 2

Scoped inside of a code block. Not accessible outside the block.

Question 3

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

Answer 3

block scoped

Question 4

What is the difference between let and const?

Answer 4

let is allows re-assignment. const does not allow re-assignment.

Question 5

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

Answer 5

The address is immutable, but the object the address refers, too, is not. Therefore the object can be modified unless deep freezed.

Question 6

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

Answer 6

If you intend to re-assign the value. Const protects the variable from being re-assigned.

Question 7

What is the syntax for writing a template literal?

Answer 7

${variable_name inside back ticks

Question 8

What is “string interpolation”?

Answer 8

The ability to substitute part of the string for the values of variables or expressions.

Question 9

What is destructuring, conceptually?

Answer 9

Assigning properties of an object or elements of an array to individual variables.

Question 10

What is the syntax for Object destructuring?

Answer 10

{ key1, key2, key3 } = obj

Question 11

What is the syntax for Array destructuring?

Answer 11

[ var1, var2] = array

Question 12

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

Answer 12

destructoring is on the left side of the = sign, creating is on the right side

Question 13

What are the three states a Promise can be in?

Answer 13

pending, fulfilled, rejected

Question 14

How do you handle the fulfillment of a Promise?

Answer 14

then clause

Question 15

How do you handle the rejection of a Promise?

Answer 15

catch clause

Question 16

What is “syntactic sugar”?

Answer 16

Syntax designed to make code easier to read or express

Question 17

What is the typeof an ES6 class?

Answer 17

function

Question 18

Describe ES6 class syntax.

Answer 18

class keyword followed by name and braces. prototype methods in the class body.

Question 19

What is “refactoring”?

Answer 19

process of restructuring code without changing its functionality to improve readability and maintainability

Question 20

How are ES Modules different from CommonJS modules?

Answer 20

ES Modules can be statically analyzed (for static checking, optimization), better support for cyclic dependencies

Question 21

What kind of modules can Webpack support?

Answer 21

ECMAScript modules, CommonJS modules, AMD modules, Assets, WebAssembly modules

Question 22

What must the return value of myFunction be if the following expression is possible?
myFunction()();

Answer 22

return is a function

Question 23

What does this code do?
const wrap = value => () => value;

Answer 23

return value function

Question 24

In JavaScript, when is a function’s scope determined; when it is called or when it is defined?

Answer 24

when it’s defined

Question 25

What allows JavaScript functions to “remember” values from their surroundings?

Answer 25

closure