Array

Question 1

Method determines whether an array includes a certain element, returning true or false as appropriate.

Answer 1

var array1 = [1, 2, 3];

console.log(array1.includes(2));
// expected output: true

var pets = [‘cat’, ‘dog’, ‘bat’];

console.log(pets.includes('cat'));
// expected output: true

console.log(pets.includes('at'));
// expected output: false

Question 2

Method returns the value of the first element in the array that satisfies the provided testing function. Otherwise undefined is returned.

Answer 2

var array1 = [5, 12, 8, 130, 44];

var found = array1.find(function(element) {
  return element > 10;
});

console.log(found);
// expected output: 12

Question 3

Method returns a string representing the elements of the array. The elements are converted to Strings using their toLocaleString methods and these Strings are separated by a locale-specific String (such as a comma “,”).

Answer 3

var array1 = [1, 'a', new Date('21 Dec 1997 14:12:00 UTC')];
var localeString = array1.toLocaleString('en', {timeZone: "UTC"});

console.log(localeString);
// expected output: "1,a,12/21/1997, 2:12:00 PM",
// This assumes "en" locale and UTC timezone - your results may vary

Question 4

Method tests whether at least one element in the array passes the test implemented by the provided function.

Answer 4

var array = [1, 2, 3, 4, 5];

var even = function(element) {
  // checks whether an element is even
  return element % 2 === 0;
};

console.log(array.some(even));
// expected output: true

Question 5

Method is used to merge two or more arrays. This method does not change the existing arrays, but instead returns a new array.

Answer 5

var array1 = ['a', 'b', 'c'];
var array2 = ['d', 'e', 'f'];

console.log(array1.concat(array2));
// expected output: Array ["a", "b", "c", "d", "e", "f"]

Question 6

Method returns a new Array Iterator object that contains the keys for each index in the array.

Answer 6

var array1 = ['a', 'b', 'c'];
var iterator = array1.keys(); 

for (let key of iterator) {
console.log(key); // expected output: 0 1 2
}

Question 7

Method changes the contents of an array by removing existing elements and/or adding new elements.

Answer 7

var months = [‘Jan’, ‘March’, ‘April’, ‘June’];
months.splice(1, 0, ‘Feb’);
// inserts at 1st index position
console.log(months);
// expected output: Array [‘Jan’, ‘Feb’, ‘March’, ‘April’, ‘June’]

months.splice(4, 1, ‘May’);
// replaces 1 element at 4th index
console.log(months);
// expected output: Array [‘Jan’, ‘Feb’, ‘March’, ‘April’, ‘May’]

Question 8

Method adds one or more elements to the beginning of an array and returns the new length of the array.

Answer 8

var array1 = [1, 2, 3];

console.log(array1.unshift(4, 5));
// expected output: 5

console.log(array1);
// expected output: Array [4, 5, 1, 2, 3]

Question 9

Method 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.

Answer 9

var animals = [‘Dodo’, ‘Tiger’, ‘Penguin’, ‘Dodo’];

console.log(animals.lastIndexOf('Dodo'));
// expected output: 3

console.log(animals.lastIndexOf('Tiger'));
// expected output: 1

Question 10

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

Answer 10

var plants = [‘broccoli’, ‘cauliflower’, ‘cabbage’, ‘kale’, ‘tomato’];

console.log(plants.pop());
// expected output: "tomato"

console.log(plants);
// expected output: Array ["broccoli", "cauliflower", "cabbage", "kale"]

plants.pop();

console.log(plants);
// expected output: Array ["broccoli", "cauliflower", "cabbage"]

Question 11

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

Answer 11

var array1 = [5, 12, 8, 130, 44];

function findFirstLargeNumber(element) {
  return element > 13;
}

console.log(array1.findIndex(findFirstLargeNumber));
// expected output: 3

Question 12

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

Answer 12

var beasts = [‘ant’, ‘bison’, ‘camel’, ‘duck’, ‘bison’];

console.log(beasts.indexOf('bison'));
// expected output: 1

// start from index 2
console.log(beasts.indexOf('bison', 2));
// expected output: 4

console.log(beasts.indexOf('giraffe'));
// expected output: -1

Question 13

Returns the number of elements in that array.

Answer 13

var clothing = [‘shoes’, ‘shirts’, ‘socks’, ‘sweaters’];

console.log(clothing.length);
// expected output: 4

Question 14

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

Answer 14

var array1 = [1, 2, 3];

var firstElement = array1.shift();

console.log(array1);
// expected output: Array [2, 3]

console.log(firstElement);
// expected output: 1

Question 15

Method sorts the elements of an array in place and returns the array. Javascript sort algorithm on V8 is now stable. The default sort order is according to string Unicode code points.

The time and space complexity of the sort cannot be guaranteed as it is implementation dependent.

Answer 15

var months = [‘March’, ‘Jan’, ‘Feb’, ‘Dec’];
months.sort();
console.log(months);
// expected output: Array [“Dec”, “Feb”, “Jan”, “March”]

var array1 = [1, 30, 4, 21];
array1.sort();
console.log(array1);
// expected output: Array [1, 21, 30, 4]

Question 16

Method fills all the elements of an array from a start index to an end index with a static value. The end index is not included.

Answer 16

var array1 = [1, 2, 3, 4];

// fill with 0 from position 2 until position 4
console.log(array1.fill(0, 2, 4));
// expected output: [1, 2, 0, 0]

// fill with 5 from position 1
console.log(array1.fill(5, 1));
// expected output: [1, 5, 5, 5]

console.log(array1.fill(6));
// expected output: [6, 6, 6, 6]

Question 17

Method applies a function against an accumulator and each value of the array (from right-to-left) to reduce it to a single value.

Answer 17

const array1 = [[0, 1], [2, 3], [4, 5]].reduceRight(
(previousValue, currentValue) => previousValue.concat(currentValue)
);

console.log(array1);
// expected output: Array [4, 5, 2, 3, 0, 1]

Question 18

Method executes a reducer function (that you provide) on each member of the array resulting in a single output value.

Answer 18

const array1 = [1, 2, 3, 4];
const reducer = (accumulator, currentValue) => accumulator + currentValue;

// 1 + 2 + 3 + 4
console.log(array1.reduce(reducer));
// expected output: 10

// 5 + 1 + 2 + 3 + 4
console.log(array1.reduce(reducer, 5));
// expected output: 15

Question 19

Method returns a new Array Iterator object that contains the values for each index in the array.

Answer 19

const array1 = ['a', 'b', 'c'];
const iterator = array1.values();

for (const value of iterator) {
console.log(value); // expected output: “a” “b” “c”
}

Question 20

Method creates and 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.

Answer 20

var elements = [‘Fire’, ‘Wind’, ‘Rain’];

console.log(elements.join());
// expected output: Fire,Wind,Rain

console.log(elements.join(''));
// expected output: FireWindRain

console.log(elements.join('-'));
// expected output: Fire-Wind-Rain

Question 21

Method creates a new, shallow-copied Array instance from an array-like or iterable object.

Answer 21

console.log(Array.from('foo'));
// expected output: Array ["f", "o", "o"]

console.log(Array.from([1, 2, 3], x => x + x));
// expected output: Array [2, 4, 6]

Question 22

Method tests whether all elements in the array pass the test implemented by the provided function.

Answer 22

function isBelowThreshold(currentValue) {
  return currentValue < 40;
}

var array1 = [1, 30, 39, 29, 10, 13];

console.log(array1.every(isBelowThreshold));
// expected output: true

Question 23

Method returns a shallow copy of a portion of an array into a new array object selected from begin to end (end not included). The original array will not be modified.

Answer 23

var animals = [‘ant’, ‘bison’, ‘camel’, ‘duck’, ‘elephant’];

console.log(animals.slice(2));
// expected output: Array ["camel", "duck", "elephant"]

console.log(animals.slice(2, 4));
// expected output: Array ["camel", "duck"]

console.log(animals.slice(1, 5));
// expected output: Array ["bison", "camel", "duck", "elephant"]

Question 24

Method returns a string representing the source code of the array.

Answer 24

var alpha = new Array(‘a’, ‘b’, ‘c’);

alpha.toSource();
//returns ['a', 'b', 'c']

Question 25

Method executes a provided function once for each array element.

Answer 25

var array1 = [‘a’, ‘b’, ‘c’];

array1.forEach(function(element) {
console.log(element);
});

// expected output: "a"
// expected output: "b"
// expected output: "c"

Question 26

Method adds one or more elements to the end of an array and returns the new length of the array.

Answer 26

var animals = [‘pigs’, ‘goats’, ‘sheep’];

console.log(animals.push('cows'));
// expected output: 4

console.log(animals);
// expected output: Array ["pigs", "goats", "sheep", "cows"]

animals.push(‘chickens’);

console.log(animals);
// expected output: Array ["pigs", "goats", "sheep", "cows", "chickens"]

Question 27

Method returns a new Array Iterator object that contains the key/value pairs for each index in the array.

Answer 27

var array1 = [‘a’, ‘b’, ‘c’];

var iterator1 = array1.entries();

console.log(iterator1.next().value);
// expected output: Array [0, "a"]

console.log(iterator1.next().value);
// expected output: Array [1, "b"]

Question 28

Method creates a new Array instance with a variable number of arguments, regardless of number or type of the arguments.

Answer 28

Array.of(7); // [7]
Array.of(1, 2, 3); // [1, 2, 3]

Array(7); // [ , , , , , , ]
Array(1, 2, 3); // [1, 2, 3]

Question 29

Method determines whether the passed value is an Array.

Answer 29

Array.isArray([1, 2, 3]); // true
Array.isArray({foo: 123}); // false
Array.isArray(‘foobar’); // false
Array.isArray(undefined); // false

Question 30

Method shallow copies part of an array to another location in the same array and returns it, without modifying its size.

Answer 30

var array1 = [1, 2, 3, 4, 5];

// place at position 0 the element between position 3 and 4
console.log(array1.copyWithin(0, 3, 4));
// expected output: Array [4, 2, 3, 4, 5]

// place at position 1 the elements after position 3
console.log(array1.copyWithin(1, 3));
// expected output: Array [4, 4, 5, 4, 5]

Question 31

Method returns a string representing the specified array and its elements.

Answer 31

var array1 = [1, 2, ‘a’, ‘1a’];

console.log(array1.toString());
// expected output: "1,2,a,1a"

Question 32

Method creates a new array with the results of calling a provided function on every element in the calling array.

Answer 32

var array1 = [1, 4, 9, 16];

// pass a function to map
const map1 = array1.map(x => x * 2);

console.log(map1);
// expected output: Array [2, 8, 18, 32]

Question 33

Method reverses an array in place. The first array element becomes the last, and the last array element becomes the first.

Answer 33

var array1 = ['one', 'two', 'three'];
var reversed = array1.reverse(); 

console.log(array1);
// expected output: Array ['three', 'two', 'one']

console.log(reversed);
// expected output: Array ['three', 'two', 'one']

Question 34

Method creates a new array with all elements that pass the test implemented by the provided function.

Answer 34

var words = [‘spray’, ‘limit’, ‘elite’, ‘exuberant’, ‘destruction’, ‘present’];

const result = words.filter(word => word.length > 6);

console.log(result);
// expected output: Array ["exuberant", "destruction", "present"]