Array Methods

Question 1

How would I obtain the length of an array?

Answer 1

array.length

const myArray = [1, 2, 3, 4]
console.log(myArray.length) // prints 4

Question 2

How would I add one or more elements to the end of an array?

Answer 2

array.push()

const myArray = [1, 2, 3, 4]
console.log(myArray.push(5)) // prints [1, 2, 3, 4, 5]

Returns the new length of the array.

Question 3

How would I remove and return the last element from an array?

Answer 3

array.pop()
~~~
const myArray = [1, 2, 3, 4]
const lastElement = myArray.pop()
~~~

console.log(myArray) // prints [1, 2, 3]
console.log(lastElement) // prints 4

Mutates the original array.

Question 4

How would I remove and return the first element of an array?

Answer 4

array.shift()
~~~
const myArray = [1, 2, 3, 4];
const firstElement = myArray.shift();
~~~

console.log(myArray); // prints [2, 3, 4]
console.log(firstElement); // prints 1

Mutates the original array.

Question 5

How would I add one or more elements to the beginning of an array?

Answer 5

array.unshift()
~~~
const myArray = [1, 2, 3, 4];
myArray.unshift(0);

console.log(myArray);
``` // prints [0, 1, 2, 3, 4]

Question 6

How would I return a portion of an array as a new array?

Answer 6

array.slice(a, b)
Parameter a is starting index.
Parameter b is ending ending index, which is not included.

const myArray = [1, 2, 3, 4];
const newArray = myArray.slice(1,  3)

console.log(newArray) // prints [2, 3];
console.log(myArray) // prints [1, 2, 3, 4];

The original array is not mutated.

Question 7

How would I remove and/or replace existing elements at any location in an array?

Answer 7

array.splice(a, b, c)
Parameter a is starting index (required)
Parameter b is number of elements to remove (required)
Paremeter c is one or more elements to be inserted (optional)

~~~
const myArray = [1, 2, 3, 4];
my array.splice(2, 1, “a”, “b”);

console.log(myArray)
``` // prints [1, 2, “a”, “b”, 4]

Question 8

How would I combine two or more arrays?

Answer 8

Method 1:
array.concat()
~~~
const array1 = [1, 2]
const array2 = [3, 4]
const array3 = [5, 6]
const combinedArray = array1.concat(array2, array3)
~~~

Method 2:
Spread operator

const combinedArray = [...array1, ...array2, ...array3]

Question 9

How would I check whether all elements in an array pass a certain condition?

Answer 9

array.every(function)
~~~
const numbers = [2, 4, 6, 8];
const areAllEven = numbers.every(num => num % 2 === 0);

console.log(areAllEven);
``` // true

Question 10

How would I create a new array with all elements that pass a provided test?

Answer 10

array.filter(function)

~~~
function isBigEnough(value) {
return value >= 10;
}

const filtered = [12, 5, 8, 130, 44].filter(isBigEnough);
``` // filtered is [12, 130, 44]

Question 11

How would I return the value of the first element in an array that satisfies a provided test?

Answer 11

array.find(function)

~~~
const numbers = [2, 4, 6, 8, 10];
const foundNumber = numbers.find(function (num) {
return num > 5;
})

console.log(foundNumber)
``` // 6

Returns undefined if no element satisfies condition.

Question 12

How would I return the index of the first element in an array that satisfies a provided test?

Answer 12

array.findIndex(function)

~~~
const numbers = [2, 4, 6, 8, 10];
const firstEvenIndex = numbers.findIndex(num => num % 2 === 0);

console.log(firstEvenIndex)
``` // 0

Returns -1 if no elements pass the test.

Question 13

How would I create a new array with sub-array elements flatted by a specified depth?

Answer 13

array.flat()

~~~
const arr = [1, 2, [3, 4]];
const flattened = arr.flat();

console.log(flattened);
``` // [1, 2, 3, 4]

Question 14

How would I loop through an array and perform a function on each element using the .forEach() method?

Answer 14

array.forEach(element, index, array)

element parameter is current value of element being processed
index parameter is index of current element being processed
array parameter is array that forEach() is being called on

Example : Looping through array and logging each element

const myArray = ['apple', 'banana', 'orange'];
myArray.forEach((element) => {
  console.log(element); //apple banana orange
});

Example 2: Modifying the elements of an array

~~~
const myArray = [1, 2, 3];

myArray.forEach((element, index, array) => {
array[index] = element * 2;
});

console.log(myArray);
``` //[2, 4, 6]

Question 15

How would I check whether an array contains a certain value?

Answer 15

array.includes(value)
const arr = ["apple", "banana", "orange"];

console.log(arr.includes("banana")); // true
console.log(arr.includes("BANANA")); // false
console.log(arr.includes("grape")); // false

Returns boolean value.

Question 16

How would I find the index of a certain element in an array?

Answer 16

array.indexOf(a, b)
Parameter a is the element sought
Parameter b is the starting index from which to search (optional)

const animals = [“cat”, “dog”, “fish”];
const index = animals.indexOf(“bird”);
console.log(index); // Output: -1

Returns -1 if the element is not found

Question 17

How would I concatenate all elements of an array into a single string?

Answer 17

array.join(separator)
The separator parameter is optional and will default to a comma if nothing else is provided. Otherwise, joins the elements of the array using the separator.

const fruits = [‘apple’, ‘banana’, ‘kiwi’];
const fruitsString = fruits.join(); // ‘apple,banana,kiwi’

Question 18

How would I locate the index of the last occurance of a specified value in an array?

Answer 18

array.lastIndexOf(a, b)
Parameter a is the value
Parameter b is index from which to start the search (optional)

Searches array from right to left.

Question 19

How would I reverse the order of the elements in an array?

Answer 19

array.reverse()

Mutates the original array

Question 20

How would I determine whether at least one element in an array meets a certain condition?

Answer 20

array.some(function(currentValue, index, array))

const numbers = [1, 5, 8, 12, 3];
const result = numbers.some(function(num, index) {
return num > index;
});

console.log(result); // true

Returns boolean

Question 21

How would I sort the elements of an array in place?

Answer 21

array.sort()

const numbers = [4, 2, 7, 1, 3];
numbers.sort();

console.log(numbers); // Output: [1, 2, 3, 4, 7]

Using .sort() with a compare function:

const numbers = [4, 2, 7, 1, 3];
numbers.sort((a, b) => a - b);
console.log(numbers); // Output: [1, 2, 3, 4, 7]

Question 22

How would I return a string representation of an array?

Answer 22

array.toString()
Returns elements of the array separated by commas, with no spaces beteween the elements.

const fruits = [‘apple’, ‘banana’, ‘cherry’];
const fruitsString = fruits.toString();

console.log(fruitsString); // “apple,banana,cherry”

Question 23

How would I create a new array from an interable object?

Answer 23

array.from()
console.log(Array.from(‘foo’)) // [‘f’, ‘o’, ‘o’]
console.log(Array.from([1, 2, 3], x => x + x)) // [2, 4, 6]

Question 24

How would I change all elements in an array to a static value?

Answer 24

array.fill()
let a = [1, 2, 3, 4]
let b = a.fill(0, 2, 4)
console.log(b) // [1, 2, 0, 0]

Question 25

How would I test whether a variable is an array?

Answer 25

array.isArray()

Returns boolean