JS109 Flashcards
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
Identifier
Are object keys ever identifiers?
No, not all of them.
But all of them are on the global object
What happens when you do a variable declaration?
Asks the JavaScript engine to reserve space for a variable with a particular name.
What is the name for =
assignment operator
assignment operator
=
let function = function() {
??what is this space??
}
the function body
Difference between function call and invocation?
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
Are parameters just variables
Yes basically
A function that always return boolean are called…
a predicate
What is a predicate?
predicates are functions that always return boolean
All JavaScript function calls evaluate to a value t or f
T
If it’s not defined it returns undefined
When a method mutates the object it was called on it’s called
mutates the caller
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)));
function composition
What is this operator called %
remainder operator
modulo operation
You cannot invoke a function expression before it is declared t or f
T
What does it mean to be first class functions? (2 things)
They are treated like values and can be assigned to variables. They can also be parsed into other functions as arguments
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();
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.
What is one level of the stack called?
Stack frame
What is the conditional part of a loop called?
A clause
Another name for strict equality operator and non-strict equality operator
identity operator
loose equality operator
What kind of operators are these:
||
&&
!
logical operators
What’s this called
undefined ?? null
How does it differ from ||
nullish coalescing operator
|| will also return left operator if falsy
0 ?? 2
will return 0
0 || 2
will return 2
When should the ternary operator be used?
When choosing between return 2 values
Not when choosing between 2 actions
What kind of statement are break, continue, return?
jump statements
What are these operators called?
++
–
increment
decrement
Methods like forEach have a special term, what is it?
array looping abstractions
What is the part of a recursive function that ends the recursion?
baseline condition
What is an adjective for primitive immutable values?
atomic
base unit
Even something like 0 + 0 evaluates to a new value of 0.
Kinds of errors
Syntax
Reference
Type
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(“”));
What happens when an object has 2 keys with the same string?
The second one will override the first
What is a template literal? (allows 2 things)
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,
What does array.reverse return, does it mutate?
Mutates
Returns reference to array
What does array.pop return, does it mutate?
Mutates, removes that element from array
returns that element
What does array.push return, does it mutate?
Mutates, add the argument to the end of the array
returns the new length of the array
Describe the ternary operator
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.
demonstrate nested ternany operatorr
https://medium.com/javascript-scene/nested-ternaries-are-great-361bddd0f340
can you exclude any part of the ternany operator
no
false ? console.log(‘a’)
will result in a syntax error
What does require do?
Can you write to the JSON file?
Can you edit the JSON while using it?
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.
Difference between block, local, and function scope? and define
What scopes do var, let and const have?
What are the 5 scoping rules
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
What is a side effect? What is a pure function?
What is output compared to return?
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.”
Expression vs statement
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”);
Pass by value vs pass by reference and give example
value
function() {return 6}
function(6)
Reference
function() {return []}
function([])
What does Math.pow return?
the first argument to the power of the second
exceptions: throwing and catching (incomplete)
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
Best way to prevent errors
Look through, try edge cases, create guard clauses
If you don’t want to plan for every permuation of a situations, how can you catch errors
What can be omitted?
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
What argument will go into the parameter in a catch function
The error object
What is function composition
When you pass the function call as an argument in another function
What is a template literal? (allows 2 things, also a third)
literals delimited with backtick (`) characters,
allowing for multi-line strings,
string interpolation with embedded expressions,
and special constructs called tagged templates.
