JS Fundamentals 1

Question 1

ECMAScript

Answer 1

JS standardization for browsers.
The “ES” in ES5, ES6, etc.
ES6 - 2015
Transpilers and Polyfills are used to help fill in browser limitations.

Question 2

JS Runtime

Answer 2

Engine that interprets JS code.
Can be part of a browser, server, or other runtime env

Question 3

Single-Threaded Runtime

Answer 3

This is the Defining Feature of JS.
Runtime consists of a Stack and a Queue.
An Event Loop takes work from the Queue to the Stack.

Question 4

Shadow DOM

Answer 4

Creates a boundary around part of the UI that prevents a parent from changing a child.
It does this by forcing events started across the boundary to rescope their targets.

Question 5

Object Literal Notation

Answer 5

Declarative.
const bike = { gears: 10 }
Best for one-off objects.

Question 6

New Objects with Constructors

Answer 6

Best for creating 2 or more objects with the same properties.

Question 7

3 shapes of properties (on an object)

Answer 7

Primitives
Objects
Arrays

Question 8

6 Primitive types

Answer 8

String
Number
Boolean
Null
Undefined
Symbol (value guaranteed to be unique)

Question 9

Prototype Inheritance

Answer 9

A prototype is an object that sits in memory and defines properties or functions.
Objects that share the same prototype inherit those properties.

Question 10

Class-based Inheritance

Answer 10

JS does not have true class-based inheritance.
Constructor functions are syntactical sugar for Prototypal Inheritance.

Question 11

DOM events / browser events relation to JS

Answer 11

DOM and browser events are not part of JS.
They are APIs implemented for JS in the browser.

Question 12

Runtime events path

Answer 12

Event is emitted in Runtime.
Message is created in the Engine.
Message placed in the Event Queue.
Once the Stack is free, the Event Handler is invoked, creating a Frame on the Call Stack.

Question 13

Function Declaration

Answer 13

A statement that uses the ‘function’ keyword to create a function.
These can be called BEFORE they are declared.

Question 14

Function Expression

Answer 14

A function created with a variable.
These must be called AFTER they are declared.
const doubleUp = (num) => {}

Question 15

Factory Function

Answer 15

A pattern where a function returns a function.
Useful for one-off reusable functions, especially to capture variable references in a closure.

Question 16

Hoisting

Answer 16

The use of ‘var’ does not recognize non-function blocks of code (such as ‘if’ statement or ‘for loop’), therefore its context is assigned to the nearest outer function (hoisting).

Question 17

Scope of let, const, var

Answer 17

let and const are block-scoped.
var is global or function-scoped (allows for hoisting).

Question 18

THIS

Answer 18

THIS is a pointer to the state of the current execution of code.
When a fn is invoked, there is always an object container around it (THIS).
Using the ‘new’ keyword for a constructor shifts THIS into the newly created object.

Question 19

Context

Answer 19

The object container of a function where it is invoked (not where it is declared)

Question 20

Closure

Answer 20

When functions are declared they can reference variables in the scope they are contained in, as well as those declared inside of or passed into the function.