Array Methods

Question 1

Check whether a thing is an array

SIMPLE LANGUAGE:
Is the argument an Array?

Answer 1

Array.isArray(input)

Question 2

Iterate over each array element. The return from a callback function is passed to the next iteration.

SIMPLE LANGUAGE:
Analyze all array elements, allow a variable to pass from one iteration to the next

Answer 2

myArray.reduce(callback function(acc, arg) => {
acc += arg}, accStart
)

ex.
> let arr = [2, 3, 5, 7]
> arr.reduce((accumulator, element) => accumulator + element, 0)

let apple = [1, 2, 3, 4, 5, 6]
console.log(apple.reduce((acc, value) =>
[(acc[0] + value), (acc[1] - value)], [0,0] ));

logs: [ 21, -21 ]

Question 3

Analyze all of an array’s elements, returns the elements in a new array based on whether a function for each element is true.

SIMPLE LANGUAGE:
Create a new array from old array filtered based on some condition.

Answer 3

myArray.filter(element => if true return element as part of new array)

Question 4

Return boolean if a string contains a specified substring
or
if an array element matches the entirety of the argument

SIMPLE LANGUAGE:
Does the array contain this entry?
Does the string contain this substring?

Answer 4

myStringOrArray.includes(‘value’)

Question 5

SIMPLE LANGUAGE:

Cut (mutate) an array and return the cut section

Answer 5

myArray.splice(start index, number of indexes)

Question 6

SIMPLE LANGUAGE:

Add argument to the end of an array and return the new array length

Answer 6

myArray.push(new element);

Question 7

SIMPLE LANGUAGE:

Add argument to the beginning of an array and return the new array length

Answer 7

myArray.unshift(new element);

Question 8

Return a combined array or string from the argument.
Returns string if method is used on a string.
Returns array if method is used on an array.

SIMPLE LANGUAGE:
Add string arguments to the end of a string
Add arrays, or other arguments to the end of an array.

Answer 8

myStringorArray.concat( , )

does not mutate.

Question 9

Remove the last element from an array and return the element

Answer 9

myArray.pop();

Question 10

Remove the first element from an array and return the element

Answer 10

myArray.shift();

Question 11

Return a copied section from an array (or string)

What happens if the first argument equals the second?

What happens if there is no second argument?

Answer 11

myArray.slice(first index, last index exclusive)

If first index = last index, an empty array is returned

It returns an array/string from that index to the last

Question 12

Reorganize an array’s elements in place (mutating). It also returns the organized array

No arg defaults to string comparison

Answer 12

myArray.sort( )

Question 13

Reverse an array’s elements in place (mutating).

It also returns the reversed array

Answer 13

Array.prototype.reverse( )

Question 14

Return the Array with nested arrays unnested.

can choose level of unesting

Answer 14

myArray.flat( number of levels to unnest)

Question 15

Return combined array elements into a string with argument in between

Answer 15

myArray.join(‘ ‘)

Question 16

What’s the difference between Array.from and .map() ?

Answer 16

Array.from() works with array-like objects like strings
Array.from() is meant to create arrays from array like objects and can take a callback function

.map() is for arrays
.map() is meant to map a new array off an existing one using a callback function. It has more functionality
.map() is 15 times faster

Question 17

Return true if at least 1 array element returns true from a callback function

SIMPLE LANGUAGE:
Do some of the array element follow this pattern?

Answer 17

myArray.some( )

Question 18

Return true if all array elements returns true from a callback function

SIMPLE LANGUAGE:
Does every array element follow this pattern?

Answer 18

myArray.every()

Question 19

What’s the difference between .some() and .includes()?

Answer 19

.includes() is just a simple comparison of the argument and array values.
.some() lets you use a callback function to test whatever logic you want.

Question 20

Returns the first element that returns true from a callback function.

SIMPLE LANGUAGE:
What’s the first element in the array that follows this pattern?

Answer 20

myArray.find( callback fuction)

Question 21

Replace an array’s elements between certain indexes with a SINGLE entry
Returns the changed array

SIMPLE LANGUAGE
Replace each consecutive elements with a 1 argument.

Answer 21

myArray.fill(replacement, start ind, end exclusive index)
mutates
[1, 2, 3, 4, 5, 6, 7, 8].fill(“apple”, 4, 6)
Expected output:
[ 1, 2, 3, 4, ‘apple’, ‘apple’, 7, 8 ]

Question 22

Return the Array elements based on a callback function and with first-level nested arrays unnested.

Does it map first or flat first?

SIMPLE LANGUAGE
Map an array based on callback and flat the result

Answer 22

myArray.flatMap( callback function)

It maps then flats.

Question 23

What is returned from:
[1,2].concat([3,4]);

[1,2].concat([[3,4]]);

‘string’.concat(342432)

Answer 23

output=> [1,2,3,4]

output=> [1,2,[3,4]]

output=> “string342432”

Question 24

What is returned from:

[1, 2, 3].pop().shift()

Answer 24

an error
Because it is evaluated from left to right
[1, 2, 3].pop() evaluates to 3
and 3.shift() give an error

Question 25

What are the similarities and difference between .substring() and .slice()? What happens when only 1 argument? If start index > end index, what happens? What do indexes start and finish? what do they work on? What happens when second argument larger than length? how do they repond to negative numbers or NaN?

Answer 25

Similarities - Both go from start index (inc) to end index (exc) - If end index is excluded they go to the end - If end index > length, end index = length; Differences - slice works with strings and arrays, substring only works on strings. - If start index > end index, .slice() will return empty string/ array. .substring() will swap the indexes. - both act different with negative arguments and NaN https: //stackoverflow.com/questions/2243824/what-is-the-difference-between-string-slice-and-string-substring

Question 26

What is logged to console from this: let apple = [1, 2, 3, 4, 5, 6] console.log(apple.reduce((acc, value) => [(acc[0] + value), (acc[1] - value)], [0,0] )); Why?

Answer 26

logs: [ 21, -21 ] The accumulator is set to: [0,0] The callback function returns 2 elements in an array (acc[0] + value) and (acc[1] - value) The first accumulator element plus the value The second accumulator element minus the value

Question 27

``` What is the difference between each of these (what do they do and return?): Array.prototype.includes() Array.prototype.some( ) Array.prototype.every() Array.prototype.filter() Array.prototype.find() Array.prototype.indexOf() ```

Answer 27

.includes() - returns boolean based on whether the argument is within the array .some( ) - iterates over each element and returns boolean if at least 1 iteration returns true based on a callback function .every() - iterates over each element and returns boolean if all iterations return true based on a callback function .filter() - iterates over each element and returns an array based on whether the elements evaluate to true based on a callback function .find() - returns the value of the first element that returns true from a callback function .indexOf() returns the first index of an element that matches the argument

Question 28

What's the difference between .map() and .forEach() ? | 2 differences

Answer 28

.forEach does a function for each element in an array. .map() returns an array based on a callback function return. 1.  .forEach() returns undefined (and can only be used at the end of a chain usefully) .map() returns the new mapped array (and is therefore chainable from) 2. .map() does not mutate and returns an array .forEach() just allows a callback function iterating over array elements

Question 29

What does Array.prototype.includes() do? and/or return?

Answer 29

Determines whether an array includes a certain value among its entries, returning true or false as appropriate. includes(searchElement) includes(searchElement, fromIndex) (There is also a String.prototype.includes())

Question 30

What does Array.isArray() do? and/or return?

Answer 30

Returns boolean based on whether the passed value is an Array. Array.isArray(value)

Question 31

What does Array.prototype.reduce() do? and/or return?

Answer 31

Executes a user-supplied “reducer” callback function on each element of the array, in order, passing in the return value from the calculation on the preceding element. The final result of running the reducer across all elements of the array is a single value. ``` // Arrow function reduce((previousValue, currentValue) => { /* ... */ } ) reduce((previousValue, currentValue, currentIndex) => { /* ... */ } ) reduce((previousValue, currentValue, currentIndex, array) => { /* ... */ } ) reduce((previousValue, currentValue, currentIndex, array) => { /* ... */ }, initialValue) ``` // Callback function reduce(callbackFn) reduce(callbackFn, initialValue) ``` // Inline callback function reduce(function(previousValue, currentValue) { /* ... */ }) reduce(function(previousValue, currentValue, currentIndex) { /* ... */ }) reduce(function(previousValue, currentValue, currentIndex, array) { /* ... */ }) reduce(function(previousValue, currentValue, currentIndex, array) { /* ... */ }, initialValue) ```

Question 32

What does Array.prototype.filter() do? and/or return?

Answer 32

Creates a new array with all elements that pass the test implemented by the provided function. ``` // Arrow function filter((element) => { /* ... */ } ) filter((element, index) => { /* ... */ } ) filter((element, index, array) => { /* ... */ } ) ``` // Callback function filter(callbackFn) filter(callbackFn, thisArg) ``` // Inline callback function filter(function(element) { /* ... */ }) filter(function(element, index) { /* ... */ }) filter(function(element, index, array){ /* ... */ }) filter(function(element, index, array) { /* ... */ }, thisArg) ```

Question 33

What does Array.prototype.splice() do? and/or return?

Answer 33

changes the contents of an array by removing or replacing existing elements and/or adding new elements in place Returns the cut section splice(start) splice(start, deleteCount) splice(start, deleteCount, item1) splice(start, deleteCount, item1, item2, itemN)

Question 34

What does Array.prototype.push() do? and/or return?

Answer 34

adds one or more elements to the end of an array and returns the new length of the array. push(element0) push(element0, element1) push(element0, element1, /* ... ,*/ elementN)

Question 35

What does Array.prototype.unshift() do? and/or return?

Answer 35

adds one or more elements to the beginning of an array and returns the new length of the array. unshift(element0) unshift(element0, element1) unshift(element0, element1, /* ... ,*/ elementN)

Question 36

What does Array.prototype.concat() do? and/or return?

Answer 36

The concat() method combines two or more arrays AND/OR adds the arguments to the target array. This method does not change the existing arrays, but instead returns a new array. concat() concat(value0) concat(value0, value1) concat(value0, value1, ... , valueN) (There is also a String.prototype.concat())

Question 37

What does Array.prototype.pop() do? and/or return?

Answer 37

removes the last element from an array and returns that element. This method changes the length of the array. pop()

Question 38

What does Array.prototype.shift() do? and/or return?

Answer 38

removes the first element from an array and returns that removed element. This method changes the length of the array. shift()

Question 39

What does Array.prototype.slice() do? and/or return?

Answer 39

returns a shallow copy of a portion of an array into a new array object selected from start to end (end not included) where start and end represent the index of items in that array. The original array will not be modified. slice() slice(start) slice(start, end)

Question 40

What does Array.prototype.sort() do? and/or return? How does it's algorithm work?

Answer 40

sorts the elements of an array in place and returns the sorted array. The default sort order is ascending, built upon converting the elements into strings, then comparing their sequences of UTF-16 code units values. Mutates ``` // Functionless sort() ``` ``` // Arrow function sort((firstEl, secondEl) => { /* ... */ } ) ``` ``` // Compare function sort(compareFn) ``` ``` // Inline compare function sort(function compareFn(firstEl, secondEl) { /* ... */ }) ``` If [1, -2, 15, 2, 0, 8].sort(function(a, b) { }); It will put a number in a It will put a number in b If the return of the function is positive, it will put b after a.

Question 41

What does Array.prototype.reverse() do? and/or return?

Answer 41

reverses an array in place. The first array element becomes the last, and the last array element becomes the first. Mutates Returns the reversed array reverse()

Question 42

What does Array.prototype.flat() do? and/or return?

Answer 42

creates a new array with all sub-array elements concatenated into it recursively up to the specified depth. flat() flat(depth)

Question 43

What does Array.prototype.join() do? and/or return?

Answer 43

returns a new string by concatenating all of the elements in an array (or an array-like object), separated by commas or a specified separator string. If the array has only one item, then that item will be returned without using the separator. join() join(separator)

Question 44

What does Array.from() do? and/or return?

Answer 44

static method creates a new, shallow-copied Array instance from an array-like or iterable object. ``` // Arrow function Array.from(arrayLike, (element) => { /* ... */ } ) Array.from(arrayLike, (element, index) => { /* ... */ } ) ``` ``` // Mapping function Array.from(arrayLike, mapFn) Array.from(arrayLike, mapFn, thisArg) ``` ``` // Inline mapping function Array.from(arrayLike, function mapFn(element) { /* ... */ }) Array.from(arrayLike, function mapFn(element, index) { /* ... */ }) Array.from(arrayLike, function mapFn(element) { /* ... */ }, thisArg) Array.from(arrayLike, function mapFn(element, index) { /* ... */ }, thisArg) ```

Question 45

What does Array.prototype.map() do? and/or return?

Answer 45

creates a new array populated with the results of calling a provided function on every element in the calling array. ``` // Arrow function map((element) => { /* ... */ }) map((element, index) => { /* ... */ }) map((element, index, array) => { /* ... */ }) ``` ``` // Callback function map(callbackFn) map(callbackFn, thisArg) ``` ``` // Inline callback function map(function(element) { /* ... */ }) map(function(element, index) { /* ... */ }) map(function(element, index, array){ /* ... */ }) map(function(element, index, array) { /* ... */ }, thisArg) ```

Question 46

What does Array.prototype.some() do? and/or return?

Answer 46

"Are there some values in the array for which the given callback returns a truthy value? tests whether at least one element in the array passes the test implemented by the provided function. It returns true if, in the array, it finds an element for which the provided function returns true; otherwise it returns false. It doesn't modify the array. ``` // Arrow function some((element) => { /* ... */ } ) some((element, index) => { /* ... */ } ) some((element, index, array) => { /* ... */ } ) ``` // Callback function some(callbackFn) some(callbackFn, thisArg) ``` // Inline callback function some(function(element) { /* ... */ }) some(function(element, index) { /* ... */ }) some(function(element, index, array){ /* ... */ }) some(function(element, index, array) { /* ... */ }, thisArg) ```

Question 47

What does Array.prototype.every() do? and/or return?

Answer 47

The every() method tests whether all elements in the array pass the test implemented by the provided function. It returns a Boolean value. ``` // Arrow function every((element) => { /* ... */ } ) every((element, index) => { /* ... */ } ) every((element, index, array) => { /* ... */ } ) ``` // Callback function every(callbackFn) every(callbackFn, thisArg) ``` // Inline callback function every(function(element) { /* ... */ }) every(function(element, index) { /* ... */ }) every(function(element, index, array){ /* ... */ }) every(function(element, index, array) { /* ... */ }, thisArg) ```

Question 48

What does Array.prototype.find() do? and/or return?

Answer 48

The find() method returns the value of the first element in the provided array that satisfies the provided testing function. If no values satisfy the testing function, undefined is returned. ``` // Arrow function find((element) => { /* ... */ } ) find((element, index) => { /* ... */ } ) find((element, index, array) => { /* ... */ } ) ``` // Callback function find(callbackFn) find(callbackFn, thisArg) ``` // Inline callback function find(function(element) { /* ... */ }) find(function(element, index) { /* ... */ }) find(function(element, index, array){ /* ... */ }) find(function(element, index, array) { /* ... */ }, thisArg) ```

Question 49

What does Array.prototype.fill() do? and/or return?

Answer 49

The fill() method changes all elements in an array to a static value, from a start index (default 0) to an end index (default array.length). It returns the modified array. Mutates fill(value) fill(value, start) fill(value, start, end)

Question 50

What does Array.prototype.flatMap() do? and/or return?

Answer 50

The flatMap() method returns a new array formed by applying a given callback function to each element of the array, and then flattening the result by one level. It is identical to a map() followed by a flat() of depth 1, but slightly more efficient than calling those two methods separately. ``` // Arrow function flatMap((currentValue) => { /* ... */ } ) flatMap((currentValue, index) => { /* ... */ } ) flatMap((currentValue, index, array) => { /* ... */ } ) ``` // Callback function flatMap(callbackFn) flatMap(callbackFn, thisArg) ``` // Inline callback function flatMap(function(currentValue) { /* ... */ }) flatMap(function(currentValue, index) { /* ... */ }) flatMap(function(currentValue, index, array){ /* ... */ }) flatMap(function(currentValue, index, array) { /* ... */ }, thisArg) ```

Question 51

What does Array.prototype.indexOf() do? and/or return?

Answer 51

returns the first index at which a given element can be found in the array, or -1 if it is not present. indexOf(searchElement, fromIndex) (There is also an indexof for strings)

Question 52

``` Which of these take a callback function: Array.prototype.includes() Array.prototype.some( ) Array.prototype.every() Array.prototype.filter() Array.prototype.find() Array.prototype.indexOf() ```

Answer 52

.some() .every() .filter() .find()

Question 53

Return the first index in an array that matches the given argument Can also start from a given index

Answer 53

Array.prototype.indexOf()

Question 54

What's the difference between .map() and .filter() and Array.from()

Answer 54

.filter() creates a new array based on whether the elements return boolean on a callback function .map creates a new array based on the return of a callback function Array.from() is for array like object. It is slower than .map. but otherwise functions similarly

Question 55

Return the elements of an array as a string separated by commas 2 ways

Answer 55

Array.prototype.toString() | String(arr)

Question 56

What does: Array.prototype.toString() do and/or return?

Answer 56

Return the elements of an array as a string separated by commas

Question 57

t or f. Why? alert( 0 == [0] ); alert( '0' == [0] ); alert( [0] == [0] ); alert( 0 == [] ); alert( '' == [] ); alert('0' == [] );

Answer 57

false- non empty arrays compared to primitives will convert to strings, '0' true- false- these arrays are unique objects true- empty arrays will convert to 0 when compared to numbers true- empty arrays will convert to '' when compared to strings false- '0' == ''

Question 58

Insert elements into the middle of an array

Answer 58

arr.splice( 2, 0, 'insert this', 'and this') | It wil insert it AT position 2. It will push the old position 2 to position 3

Question 59

What happens when you use negative indexes with array methods? How about as an array index? what is returned from each of these? arr[-1] arr[-1] = 1; arr[-1]

Answer 59

It starts from the end of the array. undefined, looking for a string '-1' as a key likely makes the -1 a key as a regular object property returns 1 as the object value it will be listed in the array as: '-1': 1

Question 60

Returns the index of the first element in the array that satisfies the provided testing function. Otherwise, it returns -1, indicating that no element passed the test.

Answer 60

Array.prototype.findIndex()

Question 61

What does: Array.prototype.findIndex() do and/or return?

Answer 61

Returns the index of the first element in the array that satisfies the provided testing function. Otherwise, it returns -1, indicating that no element passed the test.

Question 62

How is the array altered? | [9, 21, 1].sort()

Answer 62

``` [1, 21, 9] The elements are sorted as strings. How do you make it not do this? (scroll down) . . . . . . ``` ``` . . . . . . . . ``` . . . To make it not do this: ``` function compare(a, b) { if (a > b) return 1; // if the first value is greater than the second if (a == b) return 0; // if values are equal if (a < b) return -1; // if the first value is less than the second } [9, 21, 1].sort(compare() ) ``` ``` // Functionless sort() ``` ``` // Arrow function sort((firstEl, secondEl) => { /* ... */ } ) ``` ``` // Compare function sort(compareFn) ``` ``` // Inline compare function sort(function compareFn(firstEl, secondEl) { /* ... */ }) ```

Question 63

What does Array.prototype.reduceRight() do? and/or return?

Answer 63

same as reduce but starts on the end of the array

Question 64

Do a reduce method but start from the end

Answer 64

Array.prototype.reduceRight()

Question 65

What is "thisArg"?

Answer 65

A parameter in many methods that sets the value of this. thisArg refers to context which callback should be called, basically it is what this refers to inside callback. For example: var myObject = { name: 'myObject' }; [1,2].forEach(function(item) { console.log(item); // 1, 2 console.log(this === myObject); // true }, myObject)