Top Multiple Choice Questions Loops in JavaScript:

This set of Javascript Multiple Choice Questions & Answers (MCQs) focuses on “Loops in JavaScript”

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

function printArray(a) 
{
     var len = a.length, i = 0;
     if (len == 0)
        console.log("Empty Array");
     else 
     {
         do 
         {
             console.log(a[i]);
         } while (++i < len);
     }
}
  • a) Prints the numbers in the array in order
    b) Prints the numbers in the array in the reverse order
    c) Prints 0 to the length of the array
    d) Prints “Empty Array”

Answer: a
Explanation: The do/while statement creates a loop that executes a block of code once, before checking if the condition is true, then it will repeat the loop as long as the condition is true. Hence the iterator traverses through the array and print them in normal order.

2. What are the three important manipulations done in a for loop on a loop variable?

a) Updation, Incrementation, Initialization
b) Initialization,Testing, Updation
c) Testing, Updation, Testing
d) Initialization,Testing, Incrementation

Answer: b
Explanation
: In a for loop, the initialization, the test, and the update are the three crucial manipulations of a loop variable. Firstly the loop initialiases the variable then test the condition and then after executing the statement increments its value.

3. What will the following JavaScript code snippet work? If not, what will be the error?

function tail(o) 
{ 
    for (; o.next; o = o.next) ;
    return o;
}

a) No, this will throw an exception as only numerics can be used in a for loop
b) No, this will not iterate
c) Yes, this will work
d) No, this will result in a runtime error with the message “Cannot use Linked List”

Answer: c
Explanation
: The above code uses a for loop to traverse a linked list data structure and return the last object in the list. This will perfectly work.

4. What will be the equivalent code of the following JavaScript code?

for(var p in o)
   console.log(o[p]);
a)

for (var i = 0;i < a.length;i++)
     console.log(a[i]);
b)

for (int i = 0;i < a.length;i++)
     console.log(a[i]);
c)

for (var i = 0;i <= a.length;i++)
     console.log(a[i]);
d)

for (var i = 1;i < a.length;i++)
     console.log(a[i]);

Answer: a
Explanation
: The in variable does the same task of traversing the array starting from the 0 index. The for/in loop makes it easy to do the same that we do using a for.

5. One of the special features of an interpreter in reference with the for loop is that ___________

a) Before each iteration, the interpreter evaluates the variable expression and assigns the name of the property
b) The iterations can be infinite when an interpreter is used
c) The body of the loop is executed only once
d) the iteration is finite when an interpreter is used

Answer: a
Explanation
: Interpreter translates the source code into machine code line by line, and stops when it encounters an error. Before each iteration, the interpreter evaluates the variable expression and assigns the name of the property (a string value) to it.

6. What will happen if the body of a for/in loop deletes a property that has not yet been enumerated?

a) The property will be stored in a cache
b) The loop will not run
c) That property will not be enumerated
d) The property will be enumerated

Answer: c
Explanation:
If the body of a for/in loop deletes a property that has not yet been enumerated, that property will not be enumerated. If the body of the loop defines new properties on the object, those properties will generally not be enumerated.

7. What will be the step of the interpreter in a jump statement when an exception is thrown?

a) The interpreter stops its work
b) The interpreter throws another exception
c) The interpreter jumps to the nearest enclosing exception handler
d) The interpreter throws an error

Answer: c
Explanation
: When an exception is thrown in a jump statement, the interpreter jumps to the nearest enclosing exception handler, which may be in the same function or up the call stack in an invoking function.

8. What will be the role of the continue keyword in the following JavaScript code snippet?

while (a != 0)
{
   if (a == 1) 
       continue;
   else 
       a++;
}

a) The continue keyword restarts the loop
b) The continue keyword skips the next iteration
c) The continue keyword skips the rest of the statements in that iteration
d) The continue keyword breaks out of the loop.

Answer: c
Explanation:
Instead of exiting a loop like the break keyword, the continue keyword moves to the next iteration from the place encountered. While the break statement breaks out of the loop.

9. What could be the task of the statement debugger in the following JavaScript code?

function f(o) 
{
     if (o === undefined) debugger;
}

a) It does nothing but a simple breakpoint
b) It debugs the error in that statement and restarts the statement’s execution
c) It is used as a keyword that debugs the entire program at once
d) It is used to find error in the statement

Answer: a
Explanation:
The debugger statement normally does nothing. If, however, a debugger program is available and is running, then an implementation may (but is not required to) perform some kind of debugging action. In practice, this statement acts like a breakpoint: execution of JavaScript code stops and you can use the debugger to print variable’s values.

10. Among the keywords below, which one is not a statement?

a) debugger
b) with
c) if
d) use strict

Answer: d
Explanation:
 use strict is a directive introduced in ECMAScript5. Directives are not statements because it does not include any language keywords. Also, it can appear only at the start of a script or at the start of a function body, before any real statement has appeared.

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