JS Array Methods

Question 1

What does
~~~
pop()
~~~
do?

Answer 1

Removes the last element of an array, decrementing the array length, and returning the value that was removed. (This alters the original array)
no arguments

https://www.codecademy.com/resources/docs/javascript/arrays/pop

Question 2

What does
push()
do?

Answer 2

Adds one or more elements to the end of an array and returns the new length. (Original array is altered)

arguement(s) is new elements (of any datatype) ex:
arr.push(item1, ‘item2’, …);

Question 3

What does
~~~
shift()
~~~
do?

Answer 3

Removes and returns the first element of an array. All subsequent elements shift down one place.

no arguements needed. Alters original array.

Question 4

What does
~~~
unshift()
~~~
do?

Answer 4

Adds one or more elements to the beginning of array and returns new length.

Alters original array. Arguements consist of new elements to insert. ex:
arr.unshift( item1, item 2, …);

Question 5

What does
~~~
.slice()
~~~
do?

Answer 5

returns a shallow copy of all or parts of an array without modifying the original.

Creates new ‘shallow’ array. Arguements consist of (arguement index) up to but not including (2nd arguement index) ex:
arr.slice (0-5) //will copy index 0 to 4.

second arguement is not needed.
if no arguements, entire array is copied.

arguements can be negative (indicating offset from end of sequence) but 2nd arguement behavior still applies.

Question 6

What does
.splice()
do?

Answer 6

swiss army method - it can insert, remove, and replace elements.

syntax: arr.splice(starting index, deleteCount, newInsertedElement(s),…)

Question 7

What does
.slice()
do?

Answer 7

Returns a new array, copying elements from startIndex, and endIndex(not including end) arguements.

syntax: arr.slice(startIndex, endIndex)

Question 8

What does
.concat()
do?

Answer 8

creates a new array, concatinating(putting together) the array it’s used on, with other array, array values, values, or objects.

syntax: arr.concat([3,4], 5, 6)

Question 9

In order to use .concat() on an array-like Object, how can we have said object treated as an array by .concat()?

Answer 9

If the object has the property of :
\[Symbol.isConcatSpreadable]: true
it’s elements will be added with the behavior of normal arrays.

Question 10

What is
.forEach()?

Answer 10

.forEach() method is used to iterate/run a function for every element of the array

syntax: arr.ForEach(function(element, index, array){}

Question 11

What does
.indexOf()
do?

Answer 11

looks for item starting from indexStart, and returns the index where it was found. Otherwise returns -1.

syntax: arry.IndexOf(item , indexStart)

indexStart can be omitted.

Question 12

What does
.include()
do?

Answer 12

Looks for item starting from indexStart, returns true if found.
syntax: arr.include(item, indexStart)
indexStart can be omitted.

Question 13

What does
find()
do?

Answer 13

Looks for an object with a specific condition, if found it returns true and the item is returned. If item is not found, ‘undefined’ is returned.

syntax: arr.find(function(item, index, array){});

example: users.find(item => item.id == 1);

Question 14

What
does findIndex()/findLastIndex()
do?

Answer 14

Similar to find(), it finds the first of an element, but returns the index, rather than the item itself.

syntax: arr.findIndex(item, index, array)

Question 15

What does filter(func) do?

Answer 15

Similar to find(), but instead of the first, and 1 item it encounters, it returns an array of all matching elements.

syntax: arr.filter(function(item,index,array){});

Question 16

What does .map(func) do?

Answer 16

It calls the functon for each element of the array, and returns the array of results.

syntax: arr.map(function(item, index, array){});

Question 17

what does
.sort(func)
do?

Answer 17

sorts the elements in place, changing its element order. The original array is modified.

parameter/method of sorting can be your own function to establish rules of sorting order.

by default, items are sorted as strings.

note: using .localeCompare method to correctly sort letters.

Question 18

What does .reverse() do?

Answer 18

reverse the order of elements in an array.

no parameters used.

Question 19

What does str.split(delim) do?

Answer 19

Used on a string variable, and used delimiting character to split the string into an array.

ex: str.split(‘, ‘) will split words seperated by a comma into elements in an array.

Question 20

What does join() do?

Answer 20

creates a string using elements joined by provided parameter.
ex: arr.join(;) results in “item1;item2;item3”

Question 21

What do
reduce()/reduceRight()
do?

Answer 21

Its a interesting tool used to calculate a single value based on an array.
syntax: arr.reduce(function(accumulator, item, index, array){},[initial]);
ex:
arr.reduce((sum, current) => sum + current, 0);
can be used to add up the values of an array, using 0 as the initial starting value.

Question 22

How can we determine if an item/obj is an array?

Answer 22

using .isArray will return true or false.

Question 23

How can we think of
.some(fn) / .every(fn)
as?

Answer 23

Similar to || and && operators.
.some will return true of any case is true, while .every will only return true if every item case is true.

Question 24

What is arr.fill?

Answer 24

method used for filling the array with repeated values from indexStart to indexEnd
syntax: arr.fill(value, start, end)

Question 25

what’s the difference between .find() and filter()?

Answer 25

.find()’s return value returns undefined if no match.