JavaScript Questions & Answers – Array and Related Methods

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This set of Javascript Multiple Choice Questions & Answers (MCQs) focuses on “Array and Related Methods”.

1. What is the observation made in the following JavaScript code?

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var count = [1,,3];

a) The omitted value takes “undefined”
b) This results in an error
c) This results in an exception
d) The omitted value takes an integer value
View Answer

Answer: a
Explanation: Array is defined with a null value when no value is mentioned. If you omit a value from an array literal, the omitted element is given an undefined value.

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2. What will be the output of the following JavaScript code?

var a1 = [,,,]; 
var a2 = new Array(3); 
0 in a1 
0 in a2

a) true false
b) false true
c) true true
d) false true
View Answer

Answer: a
Explanation: Array a1 is defined with null values. Therefore we can access the indexes 0, 1 and 2. But array a2 is only defined not declared. Therefore we cannot access index 0.

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3. The pop() method of the array does which of the following task?
a) decrements the total length by 1
b) increments the total length by 1
c) prints the first element but no effect on the length
d) updates the element
View Answer

Answer: a
Explanation: pop() function pops out that is delete the last element from the array. Hence pop() method (it works with push()) reduces the length of an array by 1.

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4. What is the observation made in the following JavaScript code?

if (!a[i]) continue;

a) Skips the defined elements
b) Skips the existent elements
c) Skips the null elements
d) Skips the defined & existent elements
View Answer

Answer: c
Explanation: The if loop in the above code checks whether the value of a[i] exists or not. For undefined, non existent and null values the if loop returns true.
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5. What will happen if reverse() and join() methods are used simultaneously?
a) Reverses and stores in the same array
b) Reverses and concatenates the elements of the array
c) Reverses
d) Stores the elements of an array in normal order
View Answer

Answer: a
Explanation: The array.join() method is an inbuilt function in JavaScript which is used to join the elements of an array into a string. The reverse() followed by a join() will reverse the respective array and will store the reversed array in the memory.

6. What will be the possible output of the following JavaScript code?

var a = [1,2,3,4,5];
a.slice(0,3);

a) Returns [1,2,3]
b) Returns [4,5]
c) Returns [1,2,3,4]
d) Returns [1,2,3,4,5]
View Answer

Answer: a
Explanation: .slice() function is a predefined function in JavaScript used to keep the elements from starting point and ending point mentioned in the function argument. The elements after the ending point and before the starting point are not shown.

7. What will be the shift() output of the following JavaScript code?

var a = []; 
a.unshift(1); 
a.unshift(22);
a.shift(); 
a.unshift(3,[4,5]); 
a.shift(); 
a.shift();
a.shift();

a) 1
b) [4,5]
c) [3,4,5]
d) Exception is thrown
View Answer

Answer: a
Explanation: The unshift() and shift() methods behave much like push() and pop(), except that they insert and remove elements from the beginning of an array rather than from the end. unshift() adds an element or elements to the beginning of the array, shifts the existing array elements up to higher indexes to make room, and returns the new length of the array. shift() removes and returns the first element of the array, shifting all subsequent elements down one place to occupy the newly vacant space at the start of the array.

8. The primary purpose of the array map() function is that it __________
a) maps the elements of another array into itself
b) passes each element of the array and returns the necessary mapped elements
c) passes each element of the array on which it is invoked to the function you specify, and returns an array containing the values returned by that function
d) pass the elements of the array into another array
View Answer

Answer: c
Explanation: map() is a predefined function in javascript used for mapping the array elements to be used for some other purpose. The map() method passes each element of the array on which it is invoked to the function you specify, and returns an array containing the values returned by that function.

9. The reduce and reduceRight methods follow a common operation called __________
a) filter and fold
b) inject and fold
c) finger and fold
d) fold
View Answer

Answer: b
Explanation: The reduceRight() method reduces the array to a single value. The reduceRight() method executes a provided function for each value of the array (from right-to-left). The return value of the function is stored in an accumulator (result/total). Hence it does the operation of injecting and folding.

10. The method or operator used to identify the array is __________
a) isarrayType()
b) ==
c) ===
d) typeof
View Answer

Answer: d
Explanation: The typeof operator is used to get the data type (returns a string) of its operand. The operand can be either a literal or a data structure such as a variable, a function, or an object. The operator returns the data type.

11. What will be the output of the following JavaScript code?

var val1=[1,2,3];  
var val2=[6,7,8];  
var result=val1.concat(val2);  
document.writeln(result);

a) 1, 2, 3
b) Error
c) 1, 2, 3, 6, 7, 8
d) 123
View Answer

Answer: c
Explanation: concat is a predefined function in the array library in Javascript. The concat function is used to combine the value of two arrays.
i.e.1, 2, 3, 6, 7, 8

12. What will be the output of the following JavaScript code?

var values=[1,2,3,4]  
var ans=values.slice(1);  
document.writeln(ans);

a) 1, 2, 3, 4
b) 2, 3, 4
c) 1, 3, 4
d) error
View Answer

Answer: d
Explanation: slice function is used to remove values from the array. The slice function accepts two arguments as the starting and ending values for slicing elements. As in the above function, only one argument is passed therefore it will result in an error.

13. What will be the output of the following JavaScript code?

int sum=0;
 
var arr = [10,15,20,30];  
 
arr.forEach(function myFunction(element) 
{  
    	sum= sum+element;  
});  
document.writeln(sum);

a) 70
b) 75
c) 10
d) error
View Answer

Answer: b
Explanation: forEach is a predefined function in Javascript which used to traverse through the array. It works in a similar way to for loop and iterates through each value in array.

14. What will be the output of the following JavaScript code?

var values=["one","two","Three"];  
var ans=values.shift();  
document.writeln(ans);

a) one
b) two
c) three
d) error
View Answer

Answer: a
Explanation: shift is a predefined function in array library and works like a pop function. It pops the first value of the array and returns its value. Therefore the answer will be one.

15. What will be the output of the following JavaScript code?

var arr=[1,2,3];  
var rev=arr.reverse();  
document.writeln(rev);

a) 1, 2, 3
b) 3, 2, 1
c) 3
d) 1
View Answer

Answer: b
Explanation: reverse function is a predefined function in array library in Javascript. The function is used to reverse the element of the array.
i.e. 3, 2, 1.

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Manish Bhojasia - Founder & CTO at Sanfoundry
Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He is Linux Kernel Developer & SAN Architect and is passionate about competency developments in these areas. He lives in Bangalore and delivers focused training sessions to IT professionals in Linux Kernel, Linux Debugging, Linux Device Drivers, Linux Networking, Linux Storage, Advanced C Programming, SAN Storage Technologies, SCSI Internals & Storage Protocols such as iSCSI & Fiber Channel. Stay connected with him @ LinkedIn | Youtube | Instagram | Facebook | Twitter