JS Array

Question 1

What is an Array

https://www.w3schools.com/jsref/jsref_obj_array.asp

Answer 1

The Array object is used to store multiple values in a single variable.

Example: const cars = [“Saab”, “Volvo”, “BMW”];

Question 2

Array at()

https://www.w3schools.com/jsref/jsref_array_at.asp

Answer 2

The at() method returns an indexed element from an array.

The at() method returns the same as [].

index indicates the position of the element within the array (starting from 1)

Question 3

Array concat()

https://www.w3schools.com/jsref/jsref_concat_array.asp

Answer 3

The concat() method concatenates (joins) two or more arrays.

The concat() method returns a new array, containing the joined arrays.

The concat() method does not change the existing arrays.

the ( + ) just works the same way

array1.concat(array2, array3, …, arrayX)

Question 4

Array constructor

https://www.w3schools.com/jsref/jsref_constructor_array.asp

Answer 4

The constructor property returns the function that created the Array prototype.

array.constructor

function Array() { [native code] }

Question 5

copyWithin()

https://www.w3schools.com/jsref/jsref_copywithin.asp

Answer 5

The copyWithin() method copies array elements to another position in the array.

The copyWithin() method overwrites the existing values.

The copyWithin() method does not add items to the array.

array.copyWithin(target, start, end)

Question 6

Array entries()

https://www.w3schools.com/jsref/jsref_entries.asp

Answer 6

The entries() method returns an Array Iterator object with key/value pairs:
[0, “Banana”]
[1, “Orange”]
[2, “Apple”]
[3, “Mango”]
The entries() method does not change the original array.

array.entries()

Question 7

Array every()

https://www.w3schools.com/jsref/jsref_every.asp

Answer 7

The every() method executes a function for each array element.

The every() method returns true if the function returns true for all elements.

The every() method returns false if the function returns false for one element.

The every() method does not execute the function for empty elements.

The every() method does not change the original array

array.every(function(currentValue, index, arr), thisValue)

Question 8

fill()

https://www.w3schools.com/jsref/jsref_fill.asp

Answer 8

The fill() method fills specified elements in an array with a value.

The fill() method overwrites the original array.

Start and end position can be specified. If not, all elements will be filled.

array.fill(value, start, end)

Question 9

filter()

https://www.w3schools.com/jsref/jsref_filter.asp

Answer 9

The filter() method creates a new array filled with elements that pass a test provided by a function.

The filter() method does not execute the function for empty elements.

The filter() method does not change the original array.

array.filter(function(currentValue, index, arr), thisValue)

Question 10

find()

https://www.w3schools.com/jsref/jsref_find.asp

Answer 10

The find() method returns the value of the first element that passes a test.

The find() method executes a function for each array element.

The find() method returns undefined if no elements are found.

The find() method does not execute the function for empty elements.

The find() method does not change the original array.

array.find(function(currentValue, index, arr),thisValue)

Question 11

findIndex()

https://www.w3schools.com/jsref/jsref_findindex.asp

Answer 11

The findIndex() method executes a function for each array element.

The findIndex() method returns the index (position) of the first element that passes a test.

The findIndex() method returns -1 if no match is found.

The findIndex() method does not execute the function for empty array elements.

The findIndex() method does not change the original array.

array.findIndex(function(currentValue, index, arr), thisValue)

Question 12

flat()

https://www.w3schools.com/jsref/jsref_array_flat.asp

Answer 12

The flat() method concatenates sub-array elements.

array.flat(depth)

Question 13

flatMap()

https://www.w3schools.com/jsref/jsref_array_flatmap.asp

Answer 13

The flatMap() method maps all array elements and creates a new flat array.

flatMap() creates a new array from calling a function for every array element.

flatMap() does not execute the function for empty elements.

flatMap() does not change the original array.

array.flatMap(function(currentValue, index, arr), thisValue)

Question 14

forEach()

https://www.w3schools.com/jsref/jsref_foreach.asp

Answer 14

The forEach() method calls a function for each element in an array.

The forEach() method is not executed for empty elements.

array.forEach(function(currentValue, index, arr), thisValue)

Question 15

Array.from()

https://www.w3schools.com/jsref/jsref_from.asp

Answer 15

The Array.from() method returns an array from any object with a length property.

The Array.from() method returns an array from any iterable object.

Array.from(object, mapFunction, thisValue)

Question 16

includes()

https://www.w3schools.com/jsref/jsref_includes_array.asp

Answer 16

The includes() method returns true if an array contains a specified value.

The includes() method returns false if the value is not found.

The includes() method is case sensitive.

array.includes(element, start)

Question 17

indexOf()

https://www.w3schools.com/jsref/jsref_includes_array.asp

Answer 17

The indexOf() method returns the first index (position) of a specified value.

The indexOf() method returns -1 if the value is not found.

The indexOf() method starts at a specified index and searches from left to right.

By default the search starts at the first element and ends at the last.

Negative start values counts from the last element (but still searches from left to right).

array.indexOf(item, start)

Question 18

Array.isArray()

https://www.w3schools.com/jsref/jsref_isarray.asp

Answer 18

The isArray() method returns true if an object is an array, otherwise false.

Array.isArray(obj)

Question 19

join()

https://www.w3schools.com/jsref/jsref_join.asp

Answer 19

The join() method returns an array as a string.

The join() method does not change the original array.

Any separator can be specified. The default is comma (,).

array.join(separator)

Question 20

keys()

https://www.w3schools.com/jsref/jsref_keys.asp

Answer 20

The keys() method returns an Array Iterator object with the keys of an array.

The keys() method does not change the original array.

array.keys()

Question 21

lastIndexOf()

https://www.w3schools.com/jsref/jsref_lastindexof_array.asp

Answer 21

The lastIndexOf() method returns the last index (position) of a specified value.

The lastIndexOf() method returns -1 if the value is not found.

The lastIndexOf() starts at a specified index and searches from right to left.

By defalt the search starts at the last element and ends at the first.

Negative start values counts from the last element (but still searches from right to left).

array.lastIndexOf(item, start)

Question 22

length of .length

https://www.w3schools.com/jsref/jsref_length_array.asp

Answer 22

The length property sets or returns the number of elements in an array.

array.length | array.length = number

Question 23

map()

https://www.w3schools.com/jsref/jsref_map.asp

Answer 23

map() creates a new array from calling a function for every array element.

map() does not execute the function for empty elements.

map() does not change the original array.

array.map(function(currentValue, index, arr), thisValue)

Question 24

pop()

https://www.w3schools.com/jsref/jsref_pop.asp

Answer 24

The pop() method removes (pops) the last element of an array.

The pop() method changes the original array.

The pop() method returns the removed element.

array.pop()

Question 25

prototype

https://www.w3schools.com/jsref/jsref_prototype_array.asp

Answer 25

prototype allows you to add new properties and methods to arrays.

prototype is a property available with all JavaScript objects.

Array.prototype.name = value

Question 26

push()

https://www.w3schools.com/jsref/jsref_push.asp

Answer 26

The push() method adds new items to the end of an array.

The push() method changes the length of the array.

The push() method returns the new length.

array.push(item1, item2, …, itemX)

Question 27

reduce()

https://www.w3schools.com/jsref/jsref_reduce.asp

Answer 27

The reduce() method executes a reducer function for array element.

The reduce() method returns a single value: the function’s accumulated result.

The reduce() method does not execute the function for empty array elements.

The reduce() method does not change the original array.

array.reduce(function(total, currentValue, currentIndex, arr), initialVa

Question 28

reduceRight()

https://www.w3schools.com/jsref/jsref_reduceright.asp

Answer 28

The reduceRight() method executes a reducer function for each array element.

The reduceRight() method works from right to left.

The reduceRight() method returns a single value: the function’s accumulated result.

The reduceRight() method does not execute the function for empty elements.

array.reduceRight(function(total, currentValue, currentIndex, arr), init

Question 29

reverse()

https://www.w3schools.com/jsref/jsref_reverse.asp

Answer 29

The reverse() method reverses the order of the elements in an array.

The reverse() method overwrites the original array.

array.reverse()

Question 30

shift()

https://www.w3schools.com/jsref/jsref_shift.asp

Answer 30

The shift() method removes the first item of an array.

The shift() method changes the original array.

The shift() method returns the shifted element.

array.shift()

Question 31

slice()

https://www.w3schools.com/jsref/jsref_slice_array.asp

Answer 31

The slice() method returns selected elements in an array, as a new array.

The slice() method selects from a given start, up to a (not inclusive) given end.

The slice() method does not change the original array.

array.slice(start, end)

Question 32

some()

https://www.w3schools.com/jsref/jsref_some.asp

Answer 32

The some() method checks if any array elements pass a test (provided as a callback function).

The some() method executes the callback function once for each array element.

The some() method returns true (and stops) if the function returns true for one of the array elements.

The some() method returns false if the function returns false for all of the array elements.

The some() method does not execute the function for empty array elements.

The some() method does not change the original array.

array.some(function(value, index, arr), this)

Question 33

sort()

https://www.w3schools.com/jsref/jsref_sort.asp

Answer 33

The sort() sorts the elements of an array.

The sort() overwrites the original array.

The sort() sorts the elements as strings in alphabetical and ascending order.

array.sort(compareFunction)

Question 34

splice()

https://www.w3schools.com/jsref/jsref_splice.asp

Answer 34

The splice() method adds and/or removes array elements.

The splice() method overwrites the original array.

array.splice(index, howmany, item1, ….., itemX)

Question 35

toString()

https://www.w3schools.com/jsref/jsref_tostring_array.asp

Answer 35

The toString() method returns a string with array values separated by commas.

The toString() method does not change the original array.

array.toString()

Question 36

unshift()

https://www.w3schools.com/jsref/jsref_unshift.asp

Answer 36

The unshift() method adds new elements to the beginning of an array.

The unshift() method overwrites the original array.

array.unshift(item1, item2, …, itemX)

Question 37

valueOf()

https://www.w3schools.com/jsref/jsref_valueof_array.asp

Answer 37

The valueOf() method returns the array itself.

The valueOf() method does not change the original array.

fruits.valueOf() returns the same as fruits.

array.valueOf()