JS109

Question 1

What is the broader term for

Variables declared with let and var
Constants declared with const
Properties of the global object
Function names
Function parameters
Class names

Answer 1

Identifier

Question 2

Are object keys ever identifiers?

Answer 2

No, not all of them.
But all of them are on the global object

Question 3

What happens when you do a variable declaration?

Answer 3

Asks the JavaScript engine to reserve space for a variable with a particular name.

Question 4

What is the name for =

Answer 4

assignment operator

Question 5

assignment operator

Answer 5

=

Question 6

let function = function() {
??what is this space??
}

Answer 6

the function body

Question 7

Difference between function call and invocation?

Answer 7

Same
But “invocation” and “invoking” are easier to distinguish noun and verb.

You invoke a function to call it or write a function invocation that will be called when the program runs

Question 8

Are parameters just variables

Answer 8

Yes basically

Question 9

A function that always return boolean are called…

Answer 9

a predicate

Question 10

What is a predicate?

Answer 10

predicates are functions that always return boolean

Question 11

All JavaScript function calls evaluate to a value t or f

Answer 11

T
If it’s not defined it returns undefined

Question 12

When a method mutates the object it was called on it’s called

Answer 12

mutates the caller

Question 13

What is it called when you pass a function as the argument into a function?
like this:
add(subtract(80, 10), times(subtract(20, 6), add(30, 5)));

Answer 13

function composition

Question 14

What is this operator called %

Answer 14

remainder operator
modulo operation

Question 15

You cannot invoke a function expression before it is declared t or f

Answer 15

T

Question 16

What does it mean to be first class functions? (2 things)

Answer 16

They are treated like values and can be assigned to variables. They can also be parsed into other functions as arguments

Question 17

Describe how the callstack works:
1 function first() {
2 console.log(“first function”);
3 }
4
5 function second() {
6 first();
7 console.log(“second function”);
8 }
9
10 second();

Answer 17

https://launchschool.com/books/javascript/read/functions#callstack
Push to the stack
Pop from the stack
Stack frame

The call stack helps JavaScript keep track of what function is executing as well as where execution should resume when the function returns.

main, initial stack frame called the main function

main

Program execution reaches a function invocation, and updates the main stack frame with the current program location. So it knows where to come back to when it’s done executing this function.

Then a new stack frame is pushed onto the stack for this new function:

second
main: line 10

Main is inaccessible and dormant until second completes.
The program reaches another function invocation. The second frame is updated with the line so JS knows where to resume execution later.

first
second: line 6
main: line 10

Etc. When it comes back, it starts executing again immediately after the invocation.

second: line 6
main: line 10

second: line 7
main: line 10

main:line 10

then an empty stack when the program finishes.

Question 18

What is one level of the stack called?

Answer 18

Stack frame

Question 19

What is the conditional part of a loop called?

Answer 19

A clause

Question 20

Another name for strict equality operator and non-strict equality operator

Answer 20

identity operator
loose equality operator

Question 21

What kind of operators are these:
||
&&
!

Answer 21

logical operators

Question 22

What’s this called
undefined ?? null
How does it differ from ||

Answer 22

nullish coalescing operator

|| will also return left operator if falsy

0 ?? 2
will return 0
0 || 2
will return 2

Question 23

When should the ternary operator be used?

Answer 23

When choosing between return 2 values
Not when choosing between 2 actions

Question 24

What kind of statement are break, continue, return?

Answer 24

jump statements

Question 25

What are these operators called?
++
–

Answer 25

increment
decrement

Question 26

Methods like forEach have a special term, what is it?

Answer 26

array looping abstractions

Question 27

What is the part of a recursive function that ends the recursion?

Answer 27

baseline condition

Question 28

What is an adjective for primitive immutable values?

Answer 28

atomic
base unit
Even something like 0 + 0 evaluates to a new value of 0.

Question 29

Kinds of errors
Syntax
Reference
Type

Answer 29

Syntax- like a grammar mistake. The specific syntax is wrong and JS doesn’t know what to do

Reference- variable used when not declared or can’t be found

Type error- Short for data type error. When a value is used outside the scope of it’s data type.
ex:
let num = 15;
console.log(num.split(“”));

Question 30

What happens when an object has 2 keys with the same string?

Answer 30

The second one will override the first

Question 31

What is a template literal? (allows 2 things)

Answer 31

literals delimited with backtick (`) characters, allowing for multi-line strings (usually new lines in a string literal automatic end the line), string interpolation with embedded expressions,

Question 32

What does array.reverse return, does it mutate?

Answer 32

Mutates
Returns reference to array

Question 33

What does array.pop return, does it mutate?

Answer 33

Mutates, removes that element from array
returns that element

Question 34

What does array.push return, does it mutate?

Answer 34

Mutates, add the argument to the end of the array
returns the new length of the array

Question 35

Describe the ternary operator

Answer 35

It’s basically an alternative to an if else statement.
It takes 3 operands, the only operator to do so.
A condition,
followed by a question mark then an expression to execute if the condition is truthy follow by a colon, then a expression to execute if the condition is falsy.
Can chain it for a chained conditionals.

Question 36

demonstrate nested ternany operatorr

Answer 36

https://medium.com/javascript-scene/nested-ternaries-are-great-361bddd0f340

Question 37

can you exclude any part of the ternany operator

Answer 37

no
false ? console.log(‘a’)
will result in a syntax error

Question 38

What does require do?
Can you write to the JSON file?
Can you edit the JSON while using it?

Answer 38

It allows access to modules which allows splitting the code into different files.
These different files have different scopes (unlike using script in web)

No. require caches the result. You can make changes and those changes will be available later in code, but it will not effect the original JSON file.

Question 39

Difference between block, local, and function scope? and define
What scopes do var, let and const have?
What are the 5 scoping rules

Answer 39

Global: available everywhere
Local scope: not everywhere. Only between curly braces. There are 2 kinds of local scope. Block scope and function scope.
They each have the same rules, but one describes local scoping rules for functions, and blocks the rest
This is because var is function scoped.

var is globally scoped outside of functions. Function scoped in a function.
let and const are block scoped. So they are globally scoped when outside of all

Scoping rules
1
outer scope variables accessible within inner scope
2
inner scope variables not accessible within outer scope
3
peer scopes do not conflict
4
nested blocks/functions have their own variable scope
5
inner scope variables can shadow outer scope

Question 40

What is a side effect? What is a pure function?

What is output compared to return?

Answer 40

A side effect is when a function does something that you can’t see (read, write, error, mutate, reassign)
A pure function has the same output for the same input. No side effects. It doesn’t depend on something else.

It reassigns any non-local variable. Reassigning a variable in the outer scope would be a side effect.

It mutates the value of any object referenced by a non-local variable. Mutating an array or object argument, for instance, would be a side effect.

It reads from or writes to a file, network connection, browser, or the system hardware. Side effects like this include writing to the console log and reading input from the terminal.

It raises an exception without handling it.

(possibly incorrect) A return is a what a function will evaluate to.
Output is what the program gives back to us. (a keystroke in a text box, outputs the key that we stroke).
“Any information processed by and sent out from a computer or other electronic device is considered output. An example of output is anything viewed on your computer monitor screen, such as the words you type on your keyboard. Without some type of output that a human could see, feel, or hear, a human could not interact with the computer.”

Question 41

Expression vs statement

Answer 41

Expression is anything that evaluates to a value that can be captured and used in subsequence code.

A statement is any syntactic unit of code that expresses an action for the computer to perform. A statement is not an expression. It’s return cannot be used in subsequent code (will throw a syntax error)

This includes:
variable, function, and class declarations
loops and if statements
return and break statements
assignments: a = 3;
standalone expressions: console.log(“Hello”);

Question 42

Pass by value vs pass by reference and give example

Answer 42

value
function() {return 6}
function(6)

Reference
function() {return []}
function([])

Question 43

What does Math.pow return?

Answer 43

the first argument to the power of the second

Question 44

exceptions: throwing and catching (incomplete)

Answer 44

https://www.google.com/search?q=exceptions%3A+throwing+and+catching+js&oq=exceptions%3A+throwing+and+catching+js&aqs=chrome..69i57j69i58.1037j0j7&sourceid=chrome&ie=UTF-8

Question 45

Best way to prevent errors

Answer 45

Look through, try edge cases, create guard clauses

Question 46

If you don’t want to plan for every permuation of a situations, how can you catch errors
What can be omitted?

Answer 46

try {
// Do something that might fail here and throw an exception.
} catch (error) {
// This code only runs if something in the try clause throws an exception.
// “error” contains the exception object.
} finally {
// This code always runs even if the above code throws an exception.
}

Finally can be ommitted always
Catch can be ommitted if there’s no error
You can’t ommit both

Question 47

What argument will go into the parameter in a catch function

Answer 47

The error object

Question 48

What is function composition

Answer 48

When you pass the function call as an argument in another function

Question 49

What is a template literal? (allows 2 things, also a third)

Answer 49

literals delimited with backtick (`) characters,
allowing for multi-line strings,
string interpolation with embedded expressions,

and special constructs called tagged templates.