JavaScript

Question 1

What is ECMA?

Answer 1

ECMA = an organisation that defines standards for technologies

Question 2

What is ECMAScript?

Answer 2

A specification developed by ECMA

JavaScript is a programming language that conforms to the ECMAScript specification

Question 3

What are classes?

Answer 3

Classes are syntactic sugar over prototypes and prototypical inheritance.

Because JavaScript is dynamically typed, classes in JS differ to those in say C#.

Question 4

What are Value (Primitive) Types?

Answer 4

Number 
String 
Boolean
Undefined
Null
Symbol

Primitives are copied by their VALUE.

Question 5

What are Reference Types?

Answer 5

All reference types are essentially OBJECTS!

e.g. Functions & Arrays

Objects are copied by their REFERENCE

Question 6

PRIMITIVE TYPES:

let x = 10;
let y = x;

x = 20;

What is the value of y?

Answer 6

Primitives are copied by their VALUE.

x = 20;
y = 10;

These variables are independent of each other.

With primitive types, the value 10 is stored inside x:
let x = 10

When we copy that var, the value that is stored inside it is copied into the new var
let x = 10;
let y = x; // y = 10;

x = 20;

This is not the case when we use reference types (objects)

Question 7

REFERENCE TYPES:

let x = { value: 10 };
let y = x;

x.value = 20;

What is the value of y?

Answer 7

Objects are copied by their REFERENCE

let x = { value: 10 };
let y = x;

x.value = 20;
y = 20;

When an object is used, it does not get stored inside ‘x’ itself.

It gets stored somewhere else in memory & the address of that location in memory is what gets stored in the variable ‘x’.

e.g. 
let x = { value: 10 };

object gets stored in memory
1233 = {value:10}

SO:
x = 1234 (1234 holds a reference to that place in memory which holds the object)

therefore when assigning ‘y’ the value of ‘x’, we actually assign it the reference in memory

Question 8

What is the difference between scope & closure in JavaScript?

Answer 8

In scope, the variables are temporary.
E.G.
- a function within a function
- child function has two variables x & y
- variables are re-created & re-initalised each time the child function is called
- after the function has completed, the variables in the child function die in the scope

In closure, the variables within the parent function stay initialised

Question 9

What is classical inheritance?

we have prototypical inheritance in JS, NOT classical but it’s good to know the difference

Answer 9

Instead of redefining common methods, they can be defined and then inherited by other classes.

It allows us to re-use code easily & eliminate redundant code.

Reduces opportunity for bugs (no spaghetti code!).

Makes it easier to change/maintain code as you just need to look in one place to make adjustments.

E.G.

base/super/parent class
= Shape
has a function = startLocation()

derived/sub/child class
= Circle, Square, Triangle

inheritance relationship = IS-A

E.G. Circle IS-A Shape, and inherits it’s function

Question 10

What is a prototype?

Answer 10

Every object in JS, except ___ has a prototype (parent).

This object inherits everything defined in the prototype.

Think of it as a starting base set up for all objects.

Question 11

What are Falsy values?

Answer 11

undefined
null
0
false
''
NaN

Question 12

What are Truthy values?

Answer 12

Anything that returns true from a boolean statement/is not falsy

Question 13

What is coercion?

Answer 13

Implicit & Explicit

EXPLICIT:
var a = "42" //string
var b = Number(a);  // a becomes a number

IMPLICIT:
var a = "42" //string
var b = a * 1 ;  // 42 becomes the number because we perform a math operation on it.

Question 14

What is scope?

Answer 14

Availability of variables and functions within local, global or lexical scope?

Question 15

What are the two different types of equality?

Answer 15

Strict comparison === checks for value & type equality (no coercion)

Abstract comparison == checks for value equality only (with coercion allowed)

E.G:
var a = "42";
var b = 42;

a == b; // true
a === b; // false

Question 16

What is an array?

Answer 16

An array is an object that holds values (of any type) in numerically indexed positions.

Question 17

Explain null & undefined:

Answer 17

UNDEFINED = something hasn’t been initialised with a value.

NULL = something is currently unavailable.

Question 18

What is strict mode?

Answer 18

The use strict literal is entered at the top of a JavaScript program or at the top of a function and it helps you write safer JavaScript code by throwing an error if a global variable is created by mistake. For example, the following program will throw an error:

function doSomething(val) {
  "use strict"; 
  x = val + 10;
}`

It will throw an error because x was not defined and it is being set to some value in the global scope, which isn’t allowed with use strict The small change below fixes the error being thrown:

Question 19

What is a stack?

Answer 19

???

Question 20

What is a callback function?

Answer 20

A function that is passed to another function as an argument.

Question 21

How can you iterate over OBJECTS?

Answer 21

For…in loop

Question 22

How can you iterate over ARRAYS?

Answer 22

for loop, while loop

Higher order functions:

• map
• forEach
• filter
• indexOf

Question 23

What is a JSON?

Answer 23

JSON (JavaScript Object Notation) is a lightweight format that is used for exchange of data.

It is based on a subset of JavaScript language (the way objects are built in JavaScript).