Array methods

Question 1

Array.prototype.at()

Answer 1

Takes an integer value and returns the item at that index, allowing for positive and negative integers. Negative integers count back from the last item in the array.
~~~
[1, 2, 3].at(1) // will return 2
~~~

Question 2

Array.prototype.concat()

Answer 2

Merge two or more arrays. Does not change the existing arrays, but instead returns a new array.
~~~
let newArr = [1,2,3].concat([4,5,6]) // will return [1,2,3,4,5,6]
~~~

Question 3

Array.prototype.copyWithin()

Answer 3

Shallow copies part of this array to another location in the same array and returns this array without modifying its length.
Parameters - target (where to start copying to), start (where to start copying from), end (optional - where to stop copying from, if omited, copy to end of array up to length of the array)
~~~
[1,2,3,4,5].copyWithin(1, 3) // will return [1, 4, 5, 4, 5]
~~~

Question 4

Array.prototype.every()

Answer 4

Tests whether all elements in the array pass the test implemented by the provided function. It returns a Boolean value.
~~~
[1,2,3,4,5].every(item => item < 10) // returns true
~~~

Question 5

Array.prototype.fill()

Answer 5

Changes all elements within a range of indices in an array to a static value. It returns the modified array.
Parameters - value (what to fill array with), start (optional - where to start filling), end (optional - where to end filling, up to and not including end, if absent fill to end)
~~~
[1,2,3,4,5].fill(1, 5) // returns [1, 5, 5, 5, 5]
~~~

Question 6

Array.prototype.filter()

Answer 6

Creates a shallow copy of a portion of a given array, filtered down to just the elements from the given array that pass the test implemented by the provided function
~~~
[1, 2, 3, 4, 5].filter(item => item % 2 == 0) // returns [2, 4]
~~~

Question 7

Array.prototype.find()

Answer 7

Returns the first element in the provided array that satisfies the provided testing function. If no values satisfy the testing function, undefined is returned.
~~~
[5, 8, 12, 25, 114].find(item => item > 10) // returns 12
~~~

Question 8

Array.prototype.findIndex()

Answer 8

Returns the index of the first element in an array that satisfies the provided testing function. If no elements satisfy the testing function, -1 is returned.
~~~
[5, 8, 12, 56, 145].findIndex(item => item > 13) // returns 3
~~~

Question 9

Array.prototype.findLast()

Answer 9

Iterates the array in reverse order and returns the value of the first element that satisfies the provided testing function. If no elements satisfy the testing function, undefined is returned.
e.g.
~~~
[5, 8, 50, 130, 12].findLast(item => item > 30) //returns 130
~~~

Question 10

Array.prototype.findLastIndex()

Answer 10

Iterates the array in reverse order and returns the index of the first element that satisfies the provided testing function. If no elements satisfy the testing function, -1 is returned.
~~~
[5, 8, 130, 48, 12].findLastIndex(item => item > 30) // returns 3
~~~

Question 11

Array.prototype.flat()

Answer 11

Creates a new array with all sub-array elements concatenated into it recursively up to the specified depth.
Parameters - depth (optional - the depth level of the flattening, defaults to 1)
~~~
[1,2,[3,4]].flat() // returns [1,2,3,4]
~~~

Question 12

Array.prototype.flatMap()

Answer 12

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 (arr.map(…args).flat()), but slightly more efficient than calling those two methods separately.
~~~
[1, 2, 1}.flatMap((num) => (num === 2 ? [2, 2] : 1)) // returns [1, 2, 2, 1]
~~~

Question 13

Array.prototype.forEach()

Answer 13

Executes a provided function once for each array element. Return value is undefined.
~~~
[1, 2, 3].forEach(item => console.log(item)) // logs each item to the console

Question 14

Array.prototype.includes()

Answer 14

Determines whether an array includes a certain value among its entries, returning true or false as appropriate.
~~~
[5, 10, 15].includes(10) // returns true
~~~

Question 15

Array.prototype.indexOf()

Answer 15

Returns the first index at which a given element can be found in the array, or -1 if it is not present.

 [5, 10, 15, 20].indexOf(15) // returns 2

Question 16

Array.prototype.join()

Answer 16

Creates and returns a new string by concatenating all of the elements in this array, 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.
~~~
[‘c’, ‘a’, ‘t’].join(“”) // returns ‘cat’
~~~

Question 17

*

Array.prototype.keys()

Answer 17

Returns a new array iterator object that contains the keys for each index in the array.

Question 18

Array.prototype.lastIndexOf()

Answer 18

Returns the last index at which a given element can be found in the array, or -1 if it is not present. The array is searched backwards, starting at fromIndex.
Parameters - search element (item searched for), fromindex (optional - index to start searching from)
~~~
[1, 2, 3, 1].lastIndexOf(1) // returns 3
~~~

Question 19

Array.prototype.map()

Answer 19

Creates a new array populated with the results of calling a provided function on every element in the calling array.
~~~
[1, 2, 3].map(item => item * 2) // returns [2, 4, 6]
~~~

Question 20

Array.prototype.pop()

Answer 20

Removes the last element from an array and returns that element. This method changes the length of the array.
~~~
[1, 2, 3, 4].pop() // returns 4
~~~

Question 21

Array.prototype.push()

Answer 21

Adds the specified elements to the end of an array and returns the new length of the array.
~~~
[‘apple’, ‘bears’, ‘cows’].push(‘dust’) // returns 4
~~~

Question 22

Array.prototype.reduce()

Answer 22

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.
~~~
[1, 2, 3, 4].reduce((acc, curr) => acc + curr) // returns 10
~~~

Question 23

Array.prototype.reverse()

Answer 23

Reverses an array in place and returns the reference to the same array, the first array element now becoming the last, and the last array element becoming the first. In other words, elements order in the array will be turned towards the direction opposite to that previously stated.
~~~
[1, 2, 3].reverse() // returns [3, 2, 1]

Question 24

Array.prototype.shift()

Answer 24

Removes the first element from an array and returns that removed element. This method changes the length of the array.
~~~
[1, 2, 3, 4].shift() // returns 1
~~~

Question 25

Array.prototype.slice()

Answer 25

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.
Parameters - start (index to start slicing), end (optional, slice up to but not including end)
~~~
[1, 2, 3, 4, 5].slice(2) // returns [3,4,5]
~~~

Question 26

Array.prototype.some()

Answer 26

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.
~~~
[1, 2, 3, 4, 5].some(item => item % 2 === 0) // returns true
~~~

Question 27

Array.prototype.sort()

Answer 27

Sorts the elements of an array in place and returns the reference to the same array, now sorted. The default sort order is ascending, built upon converting the elements into strings, then comparing their sequences of UTF-16 code units values.
~~~
[‘cow’, ‘apple’, ‘bison’].sort() // returns [‘apple’, ‘bison’, ‘cow’]
[1, 56, 1000, 2].sort() // returns [1, 1000, 2, 56] -> this needs to be [1, 56, 1000, 2].sort((a, b) => a-b) to give [1, 2, 56, 1000]
~~~

Question 28

Array.prototype.splice()

Answer 28

Changes the contents of an array by removing or replacing existing elements and/or adding new elements in place.
Parameters - start (index to start splice), delete count (optional, how many items to remove), item1 , item2 etc, (what to insert at the splice)
~~~
[‘jan’, ‘march’, ‘april’].splice(1, 0, ‘feb’) // returns [‘jan’, ‘feb’, ‘march’, ‘april’]
[‘jan’, ‘feb’, march’, ‘may’].splice(3, 1, ‘april’) // returns [‘jan’, ‘feb’, ‘march’, ‘april’]
~~~

Question 29

Array.prototype.toReversed()

Answer 29

The copying counterpart of the reverse() method. It returns a new array with the elements in reversed order.

Question 30

Array.prototype.toSorted()

Answer 30

The copying version of the sort() method. It returns a new array with the elements sorted in ascending order.

Question 31

Array.prototype.toSpliced()

Answer 31

The copying version of the splice() method. It returns a new array with some elements removed and/or replaced at a given index.

Question 32

Array.prototype.toString()

Answer 32

Returns a string representing the specified array and its elements.

 [1, 2, 3].toString() // returns "1,2,3"

Question 33

Array.prototype.unshift()

Answer 33

Adds the specified elements to the beginning of an array and returns the new length of the array.
[1, 2, 3].unshift(5) // returns 4

Question 34

Array.prototype.values()

Answer 34

Returns a new array iterator object that iterates the value of each item in the array.

Question 35

Array.prototype.with()

Answer 35

The copying version of using the bracket notation to change the value of a given index. It returns a new array with the element at the given index replaced with the given value.
Parameters - index (where to make the change), value (what to replace item at given index with)
~~~
[1, 2, 3, 4, 5].with(2, 6) // returns [1, 2, 6, 4, 5]
~~~