collections.md

JavaScript interview code challenges on Collections

Q1

Show the different ways of creating an array

Arrays are the collection of values in javascript. Array is a special type of object in JavaScript
Arrays values are indexed from 0 and have special property length which stores the count of elements present in array

// literal form
const arr = [];

// consturctor form
const arr = new Array();

// pre defined number of slots
const arr = new Array(10);

// with values
const arr = [1, true, "string"];

// constructor form with values
const arr = new Array(1, true, "string");

References

https://javascript.info/array

Q2

Write a program to iterate over an array and print all the values of it

Arrays can be iterated by using its index to fetch the values
Arrays also can be iterated with for each style loops

for(let i =0; i < arr.length; i++){
    console.log(arr[i]);
}

for(let index in arr){
    console.log(arr[index]);
}

for(let value of arr){
    console.log(value);
}

arr.forEach(val => console.log(val));

Q3

Write a program to append a single or multiple values in to an array

Values to the array can be appended using push method of array

arr.push(1);

arr.push(1, 2);

arr.push(...otherArr);

Notes

To remove the elements from the end of the array pop operation can be used but one element at a time

References

Q4

Write a program to prepend a single or multiple values in to an array

Values to the array can be prepended using unshift method of array

arr.unshift(1);

arr.unshift(1, 2);

arr.unshift(...arr);

Notes

To remove the elements from the start of the array shift operation can be used but one element at a time

References

Q5

Remove an element from the array from a given index

Values of the array can be removed from any position using splice method of array

const position = 2;
const count = 1;
arr.splice(position, count);

Notes

'count' indicates the number of elements to be removed from the index 'position'

References

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/splice

Q6

Insert an element to the array at a given index..

Values of the array can also be inserted to any position using splice method of array
2nd argument is passed as 0 which inserts the elements without replacing
The values passed after 2nd argument are considered for insertion

const position = 2;
arr.splice(position, 0, 6);

Notes

Multiple values can be inserted with , sepeation for values

Q7

Show the different ways of emptying an array which has values

Array can be emptied by giving a new reference of an empty array
Setting the length of the array to 0 will automatically makes the array empty
pop operation on array can also be used to empty the array where each elements get removed

arr = [];

arr.length = 0;

while(arr.length > 0){
    arr.pop();
}

arr.splice(0, arr.length)

Notes

splice operation used here to empty the array is a trick by passing the length of the array as argument, where all the elements of the array get removed

References

https://www.javascripttutorial.net/array/4-ways-empty-javascript-array/

Q8

Check if given input is an array or not

Array.isArray is a method which checks if the given argument is an array or not
Alternatively the toString method present on Object prototype can be used to check if it is an array

Array.isArray(arr);

Object.prototype.toString.call(arr) === '[object Array]'

Notes

typeof operator cannot be used to check if a value is an array or not because array is an object and typeof arr returns us "object"

References

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/isArray

Q9

Write a function which can concatenate 2 arrays. If only one array is passed it will duplicate it

Function can take 2 arguments which concatenates arrays
2nd array parameter can be defaulted to 1st array if the value is not passed

function mergeArray(arr1, arr2 = arr1){
    return [...arr1, ...arr2];
}

function mergeArray(arr1, arr2 = arr1){
    return arr1.concat(...arr2);
}

function mergeArray(arr1, arr2 = arr1){
    arr1.push(...arr2);
    return arr1;
}

Notes

When 2nd argument is not passed, the case is same as duplicating the array

References

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/concat

Q10

Write a program to replace 3 center elements of the 1st array by center 3 elements of the 2nd array

slice method on array can be used to fetch the values of range in the array
splice method on array can be used to replace the value of range in the array

const a = [1, 2, 3, 4, 5];
const b = [4, 0, 0, 0, 8];

const startPositionFor1stArray = a.length / 2 - 1;
const startPositionFor2ndArray = b.length / 2 - 1;
a.splice(startPositionFor1stArray, 3, ...b.slice(startPositionFor2ndArray, startPositionFor2ndArray + 3));

Notes

The center most 3 values of array 'a' is replaced by 'b'

Q11

Show how an array in JavaScript can act like a stack and queue

Stack is a 'Last In First Out' data structure can be achieved using push and pop operations
Queue is a 'First In First Out' data structure can be achieved using push and shift operations

// To add the value to the stack
arr.push(value);

// To remove the value from the stack
arr.pop();

// To add the value to the queue
arr.push(value);

// To remove the value from the queue
arr.shift();

Q12

Sort the given array of integers in ascending or descending order

sort method sorts the elements of an array in place and returns the sorted array
It receives a function as an argument, which is used for comparision

arr.sort((a, b)=> a - b);       // ascending
arr.sort((a, b)=> b - a);       // descending

Notes

If function is not passed an argument, default sorting will happen

References

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/sort

Q13

Square all the positive numbers of the array and return the output array

filter is the method on Array which can be used to filter. It receives a function which can return boolean to filter the elements
map is the method on Array which can be used to map the values to new values. It receives a function which can return the modified value

const positiveArr = arr.filter((value) => value >= 0);
const squaredPositiveArr = arr.map((value) => value * value);

const squaredPositiveArr = arr.filter((value) => value >= 0).map((value) => value * value);

Notes

2nd solution uses chaining

References

Q14

Check if the user with the name "John" exists in the array of objects

const doesJohnExist = arr.some((obj) => obj.name === "John");

const jonhObject = arr.find((obj) => obj.name === "John");
const doesJohnExist = jonhObject ? true : false;

const jonhIndex = arr.findIndex((obj) => obj.name === "John");
const doesJohnExist = jonhIndex < 0 ? false : true;

Q15

Generate an array of objects with properties id and full name from an array of objects where each object will have id, firstname and lastname

To manipulate array of objects map method can be used

const employeesListWithFullName = arr.map((obj) => { return { id, fullName: obj.firstName + " " + obj.lastName } });

Q16

Create an array by removing all the holes of the array

Holes are undefined value present inside array
Holes do not get iterated in filter which will just fetch all the values except undefined

const uniqueArr = arr.filter(value => true);

Notes

Holes can be formed when an array value by index is deleted. Example: delete arr[index]

Q17

Write a program to calculate the sum of all the values of an array

Sum of the values of an array can calculated by iterating and adding all the values of the array
reduce method of array can be used efficiently to calculate the sum with or without initial value

const sum = arr.reduce((acc, value) => acc + value, 0);

const sum = arr.reduce((acc, value) => acc + value);

const sum = 0;
for(let value of arr){
    sum = sum + value;
}

Notes

reduce method takes a function as 1st argument and initial value as 2nd argument. The return value of current iteration will be 1st argument for the next iteration along with the next element of the array

References

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/reduce

Q18

Get the maximum value from a numbers array along with its index

Math.max is a method which returns maximum value from a given list of values
reduce can also be designed to return the maximum value of each comparision

const max = Math.max(...arr);
arr.indexOf(max);           // position of max number

let max = a[0], position = 0;
for(let index in arr){
    if(arr[index] > max){
        position = index
        max = value;
    }
}

position;                   // position of max number

const max = arr.reduce((a, b) => a < b ? a : b);
arr.indexOf(max);           // position of max number

Notes

Though 2nd solution is verbose compared but has good performance

References

Q19

Find the number of occurences of minimum value in the numbers list

filter method can be used to fetch all the minimum values and we can get the count of those valuses

const min = Math.min(...arr);
minArr = arr.filter((value) => value === min);
minArr.length;                                  // count of minimum value occurences

Q20

Create an array of length n with all the values of it set to 10

fill is a method on Array prototype which fills all the slots of array with the value given passed as the argument

const n = 5;
const arr = new Array(n);
arr.fill(10);

Notes

If an object is passed the object reference is copied to all the slots and not the different objects

References

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/fill

Q21

Optimize the given statements having lot of logical checks to use a compact and cleaner logic

// Example1
browser === "chrome" || browser === "firefox" || browser === "IE" || browser === "safari"

// Example2
browser !== "chrome" && browser !== "firefox" && browser !== "IE" && browser !== "safari"

Examples can be modified to store the values of comparision in an array and check for the presence of value if it is present inside array

// Example1
// browser === "chrome" || browser === "firefox" || browser === "IE" || browser === "safari"

const browserList = ["chrome", "firefox", "IE", "safari"];
browserList.includes(browser);

// Example2
// browser !== "chrome" && browser !== "firefox" && browser !== "IE" && browser !== "safari"

const browserList = ["chrome", "firefox", "IE", "safari"];
!browserList.includes(browser);

Notes

Generally this use case can be implemented for if conditions

Q22

Write a program to iterate over a 2 dimensional array and print all the values of it

Arrays can be iterated by using its index to fetch the values
Arrays also can be iterated with for each style loops, with one loop to iterate the rows and inside it for cells

for (let i = 0; i < arr.length; i++) {
    for (let j = 0; j < arr[i].length; j++) {
        console.log(arr[i][j]);
    }
}

for (let i = 0; i < arr.length; i++) {
    for (let j = 0; j < arr[i].length; j++) {
        console.log(arr[i][j]);
    }
}

for (let rowArr of arr) {
    for (let value of rowArr) {
        console.log(value);
    }
}

arr.forEach(rowArr => rowArr.forEach(val => console.log(val)));

Q23

Write a program to store values in to a set

Set lets us store unique values of any type
Set can be created empty & then added with values or can be initialized also

const set = new Set();
set.add(1);
set.add(true);
set.add("text");
set.add(1);

set;            // 1, true, "text"

const set = new Set([1, 2, 3]);

set;            // 1, 2, 3

References

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set

Q24

Write a program to store values in to a map

Map holds key-value pairs and remembers the original insertion order of the keys
Map can be created empty & then added with values or can be initialized also with key-value pairs

const map = new Map();
map.set(1, 1000);
map.set(true, false);
map.set("text", "String");

map;                                                            // [1, 1000] [true, false] ["text", "String"]

const map = new Map([[1, "One"], [2, "two"], [3, "three"]]);
map;                                                            // [1, "One"] [2, "two"] [3, "three"]

Notes

Unlike objects, Map can have any primitive or object as the key

References

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map

Q25

Write a code to iterate over a set

set is an iterable object and can be iterated using for..of loop
set can also be iterated by simple forEach loop

for(let val of set) console.log(val);

set.forEach(value => console.log(value));

Q26

Write a code to iterate over a map

map is an iterable object and can be iterated using for..of loop
map can also be iterated by simple forEach loop

for(let val of map) console.log(val[0], val[1]);

for(let key of map.keys()) console.log(key, map.get(key));

map.forEach((value, key) => console.log(key, value));

Q27

Show how map is different from object to store key value pairs with coding example

Map does not contain any keys by default unlike objects which has keys from its prototype
Map's keys can be any value (including functions, objects, or any primitive) unlike object where keys are only strings
The keys in Map are ordered in a simple, straightforward way
The number of items in a Map is easily retrieved from its size property
Map is an iterable object

map.set(1, 'Mapped to a number');           // primitive number as key
map.set('1', 'Mapped to a string');         // string value as key
map.set({}, 'Mapped to a object');          // object as key
map.set([], 'Mapped to an array');          // array as key
map.set(()=>{}, 'Mapped to a function');    // function as key

Notes

Maps perform better than objects in most of the scenarios involving addition and removal of keys

Q28

Write the code to remove the duplicates from the array

Set is a data structure which does not allow duplicate elements

const set = new Set(...arr);
const distinctArr = [...set];

Q29

Design a flat function which flattens an array to any depth

Flat function can be used to flatten the array by recursive call

function flat(arr){
    const flatArr = [];
    arr.forEach((value) => {
        if(Array.isArray(value)){
            flat(value);
        }
        else{
            flatArr.push(value);
        }
    });
    return flatArr;
}

Q30

Check if all the students of have passed or not (40 is the pass marks)

every is a method on Array prototype which returns true only if all the elements condition satisfies the condition

const isAllPass = students.every((student) => student.marks >= 40);

Q31

Get the average of all the salaries which is greater than 10000 from the department of "IT" from the array of objects)

const itEmployeesWithSalaryGT10K = employees.filter((employee) => employee.salary > 10000 && employee.dept === 'IT );
const itTotalSalaryGT10K = itEmployeesWithSalaryGT10K.reduce((acc, value) => acc + value, 0);
const itAvgSalaryGT10K = itTotalSalaryGT10K / itEmployeesWithSalaryGT10K.length;

Q32

Extract the list of all the elements from the list of numbers given in 2 arrays

The union array will be the result if all the elements from the 2 arrays are picked

const set1 = new Set(...arr1);
const set2 = new Set(...arr2);
const distinctArr = [...set1, ...set2];

Q33

Get the list of all distinct elements which are present in both list of numbers

The intersection array will be the result if the common elements from the 2 arrays are picked

const intersectionArr = arr1.filter(value => arr2.includes(value));
const distinctIntersectionArr = [...new Set(intersectionArr)];

const set1 = new Set(arr1);
const set2 = new Set(arr2);
const distinctIntersectionArr = [...set1].filter(value => set2.has(value));

Q34

Extract list of elements present only in the first list given.

The only present elements of 1st list will be the result when all the elements of 1st list not present in the 2nd are chosen

const set1 = new Set(arr1);
const set2 = new Set(arr2);
const intersectionArr = [...set1].filter(value => !set2.has(value));

Notes

Elements of 2nd list only can be obtained by checking for all the elements of lis 2 which are not present in list1

Q35

Create a function named "average" which can calculate the average of an array and should be available to be called from any Array object.

The function added to Array prototype are accessible to all the objects of Array

Array.prototype.average = function (){
    let total = 0;

    for(let index in this) {
      total += this[index];
    }
    return total / this.length;
}

Q36

Write a program to polyfill `filter` functionality of the Array

filter iterates over the all values of array and passes value, index and array (itself) as the arguments
Function returns a new array which filtering the values of the original array

if (!Array.prototype.filter) {
 Array.prototype.filter = function(callback) {
    
    if(typeof callback !== "function")
        throw new Error("Argument passed has to be a function");
 
    let newArray = [];

    for(let index in this) {
      if(callback(this[index], index, this)){
          newArray.push(this[index]);
      }
    }
    return newArray;
  }
}

Notes

The solution is a simple polyfill of filter and not intended to handle all the corner scenarios

Q37

Write a program to polyfill `map` functionality of the Array

map iterates over the all values of array and passes value, index and array (itself) as the arguments
Function returns a new array which is same as the length of the original array

if (!Array.prototype.map) {
 Array.prototype.map = function(callback) {
    if(typeof callbak !== "function")
        throw new Error("Argument passed has to be a function");
        
    let newArray = [];

    for(let index in this) {
      newArray.push(callback(this[index], index, this));
    }
    return newArray;
  }
}

Notes

The solution is a simple polyfill of map and not intended to handle all the corner scenarios

Q38

Write a program to polyfill `reduce` functionality of the Array

reduce iterates over the all values of array and passes value, index and array (itself) as the arguments
reduce accepts an optional initial value which when not provided can be skipped
Function returns a single value after all the iteration

if (!Array.prototype.reduce) {
 Array.prototype.reduce = function(callback, init) {
    let startPosition = 0;
    let accumulator = init ?? this[startPosition++];

    for(let index = startPosition; index < this.length; index++) {
      accumulator = callback(accumulator, this[index], index, this);
    }
    return accumulator;
  }
}

Notes

The solution is a simple polyfill of reduce and not intended to handle all the corner scenarios

Q39

Write a code to eliminate duplicate objects in an array where each object has an 'id' property which can be used to identify the object and the duplicate object with lower rank to be removed

// Example
const arr = [
    {
        id: 1,
        name: 'emp1',
        rank: 4,
    },
    {
        id: 2,
        name: 'emp2',
        rank: 1,
    },
    {
        id: 2,
        name: 'emp2',
        rank: 2,                // this is a duplicate object (id = 2) and has lower rank 
    },
    {
        id: 3,
        name: 'emp3',
        rank: 3,
    },
];

The duplicate objects cannot be removed using Set as the 2 objects with same structure and data have different references
Map can be used to have 'id' as the key and object as value
If 'id' is already present in the array, object with the higher rank can be retained

const map = new Map();

arr.forEach(obj => {
    if (map.has(obj.id)) {
        if (obj.rank > map.get(obj.id)) {
            map.set(obj.id, obj);
        }
    } else {
        map.set(obj.id, obj);
    }
});

distinctArr = [...map.values()];

Q40

Create an array which will only accept string values. (Homogeneous array of strings)

Array in JavaScript a collection of values of any type by default
Proxy can be used to validate and insert to the array only if the value satisfies the condition using set trap

var stringsArr = [];

var stringsArr = new Proxy(stringsArr, {
  set(target, prop, receiver){
    if(typeof receiver === "string"){
      target[target.length] = receiver;
    }
    return true;
  }
});

// driver code
stringsArr.push("Hello", 5, {}, "world", true, [1, 2, 3]);
stringsArr;                                                     // ["Hello", "world"]

Notes

The functionality of the code can be modified to make the array accept any one or more kind of values only

Q41

Create a Proxy object through which the array can be accessed as usual but also allow to access the values through negative indices

// Example
let arr = [10, 20, 30];

arr[-1];            // 30
arr[-2];            // 20

get trap of proxy can be used to map the negative index to the valid array position

arr = new Proxy(arr, {
    get(target, handler) {
        if (handler < 0) return target[target.length + Number(handler)];
        else return target[handler];
    },
});

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JavaScript interview code challenges on Collections

Q1

Show the different ways of creating an array

References

Q2

Write a program to iterate over an array and print all the values of it

Q3

Write a program to append a single or multiple values in to an array

Notes

References

Q4

Write a program to prepend a single or multiple values in to an array

Notes

References

Q5

Remove an element from the array from a given index

Notes

References

Q6

Insert an element to the array at a given index..

Notes

Q7

Show the different ways of emptying an array which has values

Notes

References

Q8

Check if given input is an array or not

Notes

References

Q9

Write a function which can concatenate 2 arrays. If only one array is passed it will duplicate it

Notes

References

Q10

Write a program to replace 3 center elements of the 1st array by center 3 elements of the 2nd array

Notes

Q11

Show how an array in JavaScript can act like a stack and queue

Q12

Sort the given array of integers in ascending or descending order

Notes

References

Q13

Square all the positive numbers of the array and return the output array

Notes

References

Q14

Check if the user with the name "John" exists in the array of objects

Q15

Generate an array of objects with properties id and full name from an array of objects where each object will have id, firstname and lastname

Q16

Create an array by removing all the holes of the array

Notes

Q17

Write a program to calculate the sum of all the values of an array

Notes

References

Q18

Get the maximum value from a numbers array along with its index

Notes

References

Q19

Find the number of occurences of minimum value in the numbers list

Q20

Create an array of length n with all the values of it set to 10

Notes

References

Q21

Optimize the given statements having lot of logical checks to use a compact and cleaner logic

Notes

Q22

Write a program to iterate over a 2 dimensional array and print all the values of it

Q23

Write a program to store values in to a set

Write a program to polyfill `filter` functionality of the Array

Write a program to polyfill `map` functionality of the Array

Write a program to polyfill `reduce` functionality of the Array