JavaScript : Logical Operators

What are Logical Operators ?

Logical operators are used for creating compound expressions i.e using logical operators the programmer can combine expressions to perform conditional executions.

In JavaScript the common logical operators are

• AND : && - The Compound expression is true when all the expressions are true

• OR : || - The Compound expression is true when any of the expressions is true

• NOT : ! - It negates the given expression

Using Logical Operators

Using the || (OR) operator

• Case 1

    if (10-10 || 20-20) // 0
      console.log("true");
    else
      console.log("false");  //false
  

  In the above code , the compound expression yields into 0 which is false.

• Case 2

    if (10-10 || 20-21) // -1
      console.log("true"); // true
    else
      console.log("false");
  

  In the above code, the compound expression yields into -1 which is true.

• Case 3

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

  In the above code, the compound expression yields into undefined which is false.

• Case 4

    let a=20;
    if (console.log(a=a+5) || console.log(a=a+5)) // prints - 25 30 
      console.log("true - a:", a);
    else
      console.log("false -a",a);   // false - 30
  

  In the above code, the compound expression yields into undefined which is false

• Case 5

    // first truthy value or last value
    let res = NaN || undefined || 0 || null  ; // null
    let res2 = NaN || undefined || 0  ; // 0
    let res3 = NaN || undefined ; // undefined 
    let res4 = NaN || undefined || -1 || null; // -1
    console.log(res,res2,res3,res4); // null, 0, undefined, -1
  

  In the above code , the first truthy value is taken or the last value.

• Case 6

    false || null || console.log("JS");  // console evaluation
  

  In above code, since the first two expressions are falsey hence 'JS' is printed

• Case 7

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

  In the above code the first expression (call to fx) is truthy hence 10 is printed

Using the &&(AND) Logical operator

• Case 1

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

  In the above code , the compound expression yields into undefined which is false.

• Case 2

    // First falsey value or last truthy value
    let res = null && undefined && NaN // null
    let res2 = 10 && 20 && 30; // 30
    let res3 = 10 && 20; // 20
    console.log(res,res2,res3); // null,  30, 20
  

• Case 3

    let x=1;
    (x>2) && console.log("first"); console.log("second"); console.log("third"); 
    // second , third not part of condition
  

  In the above code, the condition 1>2 is false hence it prints second, third.

• Case 4

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

  In the above, the compound expression yields into truthy hence prints 20.

Complete Code Listing

//Objective: Using Logical operators
//Author: Mahavir

// OR - operator
if (10-10 || 20-20) // 0
  console.log("true");
else
  console.log("false");  //false

if (10-10 || 20-21) // -1
  console.log("true"); // true
else
  console.log("false"); 

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

let a=20;
if (console.log(a=a+5) || console.log(a=a+5)) // prints - 25 30 
  console.log("true - a:", a);
else
  console.log("false -a",a);   // false - 30

// first truthy value or last value
let res = NaN || undefined || 0 || null  ; // null
let res2 = NaN || undefined || 0  ; // 0
let res3 = NaN || undefined ; // undefined 
let res4 = NaN || undefined || -1 || null; // -1
console.log(res,res2,res3,res4); // null, 0, undefined, -1

false || null || console.log("JS");  // console evaluation

function fx() {
    return 10;
}

function fx2() {
    return 20;
}

console.log(fx() || fx2()); // 10

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

// AND operator
// First falsey value or last truthy value
res = null && undefined && NaN // null
res2 = 10 && 20 && 30; // 30
res3 = 10 && 20; // 20
console.log(res,res2,res3); // null,  30, 20

let x=1;
(x>2) && console.log("first"); console.log("second"); console.log("third"); 
// second , third not part of condition

console.log(fx() && fx2()); // 20