YDKJS scopes and closures

Question 1

why is it important to figure out is js compiled or interpreted?

Answer 1

because it will determine Scope

Question 2

what are the compilation steps?

Answer 2

1 - tokenization/lexing: var a = 2 -> var, a ,= , 2
2 - creation of AST abstract syntax tree
3 - code generation

Question 3

what are the 2 phases for js program?

Answer 3

1 - parsing/compiling

2 - execution

Question 4

how do we prove that js is compiled?

Answer 4

1 - syntax errors
2 - early errors
3 - hoisting

Question 5

how to modify scope at runtime?

Answer 5

1 - using eval

2 - with -> turns object into a block scope with entries as variables

Question 6

what is the scope of js called?

Answer 6

lexical scope

Question 7

what is the scope of let and const

Answer 7

it’s nearest {}

Question 8

what is the scope of var?

Answer 8

it’s nearest enclosing function

Question 9

what happens when assigning a variable that was not declared before?

Answer 9

1 - non-strict mode will be created globally

2 - strict mode will throw a reference error

Question 10

when does a lookup stop for a variable?

Answer 10

once it finds the first declaration for it upwards

Question 11

what happens if we cannot find the declaration of a variable in any lexical scope during compilation?

Answer 11

the variable is left undefined until runtime because the variable might be declared in another file

Question 12

what is shadowing?

Answer 12

it’s when a block scope variable steps in for the outer scope variable which makes that outer scope impossible to modify

Question 13

how to access a global variable even if shadowed?

Answer 13

by using window.

but it only works for var and function nothing else

Question 14

can let shadow var?

Answer 14

yes

Question 15

can var shadow let?

Answer 15

no unless it is outside the boundary of a function because var hops until in functions the function

Question 16

what happens to function expression with a name?

var askQuestion = function ofTheTeacher() ?

Answer 16

the function identifier is accessed only in the function scope and it’s value cannot be changed

Question 17

what does function expression with a named identifier called?

Answer 17

named function expression

var askQuestion = function ofTheTeacher()

Question 18

what does function expression with no identifier called?

Answer 18

anonymous function expression

var askQuestion = function()

Question 19

is there a difference in lexical scope between arrow functions and normal ones?

Answer 19

no

Question 20

what is hoisting?

Answer 20

the ability to access a variable in the beginning of the scope before it’s created

Question 21

what is the difference between function declaration hoisting and function expression hoisting?

Answer 21

1 - function declaration identifier n is hoisted and initialized to its function value (again,
called function hoisting).

2 - A var variable is also hoisted, and
then auto-initialized to undefined

Question 22

can you redeclare a var?

Answer 22

yes if only declared before as a var

Question 23

can you redeclare a let?

Answer 23

no even if the other declare is a var

Question 24

why can you redeclare a var and not a let?

Answer 24

just stylistic no technical reason

Question 25

what is TDZ?

Answer 25

temporal dead zone period of time from the entering of a scope to where the auto-initialization of the variable occurs.

Question 26

are let and const hoisted?

Answer 26

yes but not initialized but still cannot be accessed even with initialization call because of TDZ

Question 27

is it necessary that every curly braces is a scope?

Answer 27

no it becomes a scope once a let or const is inside it

Question 28

what is a behavior of closure?

Answer 28

closure is a behavior of function only and doesn't deal with class or objects

Question 29

does a closure have to close over a function?

Answer 29

no it only has to close over an outer scope

Question 30

are closures value oriented or variable oriented?

Answer 30

variable oriented they close over variables not value

Question 31

how does a closure get treated in a loop?

Answer 31

1 - if defined by var there is only one variable for the whole loop so it will be invoked by the last value 2 - if defined by let there will be a new variable for each iteration

Question 32

what is a closure?

Answer 32

Closure is observed when a function uses variable(s) from outer scope(s) even while running in a scope where those variable(s) wouldn’t be accessible.