Test your JavaScript Skills: Logical Operators-27

Introduction

Namaste, In this blog I will discuss 10 code scenarios on JavaScript Logical Operators.

1. Identify the output of the below program

    if (10-10 || 20-20) 
      console.log("true");
    else
      console.log("false");
   

   a.true

   b.false

   c.0

   d.null

   e.error

   Answer: b

   Reason: The compound expression computes to 0 which is false.

2. Identify the output of the below program

    let a=20;
    if (console.log(a=a+5) || console.log(a=a+5)) 
      console.log("a:", a);
    else
      console.log("a",a);
   

   a.25,30

   b.30,30,30

   c.25,30,35

   d.25,30,30

   e.30

   Answer:d

   Reason:The console.log function prints 25,30. The compound expression yields into undefined which is false hence it prints 30.

3. Identify the output of the below program

    let res = NaN || undefined || 0 || null; 
    console.log(res);
   

   a.NaN

   b.undefined

   c.0

   d.null

   e.error

   Answer:d

   Reason:The compound expression is Falsey

4. Identify the output of the below program

    if (console.log("if")|| console.log("if2")) 
        console.log("true"); 
     else
      console.log("false");
   

   a.if,if2,false

   b.if,if2,true

   c.if,if2

   d.if,false

   e.if2,true

   Answer:a

   Reason:Both the console.log are printed. The compound expression returns undefined which is false.

5. Identify the output of the below program

    if (10-10 || 20-21)
      console.log("true"); 
    else
      console.log("false");
   

   a.0

   b.-1

   c.true

   d.false

   e.null

   Answer:c

   Reason:The compound expression yields into -1 which is true

6. Identify the output of the below program

    let res2 = NaN || undefined || 0  ; 
    console.log(res2);
   

   a.0

   b.undefined

   c.NaN

   d.error

   e.null

   Answer:a

   Reason:The compound expression returns 0.

7. Identify the output of the below program

    function fx() {
        return 10;
    }
   
    function fx2() {
        return 20;
    }
   
    console.log(fx() || fx2());
   

   a.20

   b.10

   c.null

   d.undefined

   e.error

   Answer:b

   Reason:The first Truthy value is 10.

8. Identify the output of the below program

    let res = NaN || undefined || -1 || null; 
    console.log(res);
   

   a.NaN

   b.undefined

   c.-1

   d.null

   e.error

   Answer:c

   Reason:The value -1 is Truthy.

9. Identify the output of the below program

    let res = null && undefined && NaN 
    console.log(res)
   

   a.undefined

   b.NaN

   c.null

   d.error

   e.0

   Answer:c

   Reason:The first Falsey value is taken.

10. Identify the output of the below program

    let x=1;
    (x>2) && console.log("first"); console.log("second"); console.log("third");
    

    a.first

    b.second

    c.third

    d.first, third

    e.second, third

    Answer:e

    Reason:The compound expression is falsey. Hence the output is second, third.