Javascript Logical Operators

In this lesson, we will learn about Logical operators in JavaScript, to combine two or more conditions, which are necessary for working with conditional statements, return a boolean value, true if the condition is correct or false if the condition is incorrect.

And (&&)

You can check more than one condition using And (&&). Returns true if all conditions are correct. Returns false if one of the conditions is incorrect, because the entire expression will evaluate to false. Example:

javascript
          console.log(6 > 4 && 6 > 5);  // true

In this example it returns true, because all conditions are true. The first condition (6 > 4) is true because 6 is greater than 4. The second condition (6 > 5) is true because 6 is greater than 5.

javascript
          console.log(6 > 4 && 5 > 5);  // false

In this example, it returns false because there is a true condition and an incorrect condition. The first condition (6 > 4) is true because 6 is greater than 4. The second condition (5 < 5) is false because 5 is equal to 5.

Or (||)

You can check more than one condition. Returns true if all conditions are true or at least one condition is true. Returns false if all conditions are false. Example:

javascript
          console.log(2 > 5 || 3 > 5);  // fasle

The first condition (2 > 5) is false because 2 is less than 5. The second condition (3 > 5) is false because 3 is less than 5. In this example, it returns false, because all the conditions are false.

Not (!)

It reverses the boolean value. That is, it converts true to false, and false to true. Example:

javascript
          console.log(!true);  // false

In this example we convert true to false using the Not operator (!) before the true value.

javascript
          console.log(!false);  // true

In this example we convert false to true using the Not operator (!) before the false value. Example:

javascript
          console.log(!(5 > 3));  // false

In this example, we find that the condition (3 < 5) is correct and returns true, but because we used Not (!) before this expression, it converted true to false.

Javascript treats any value as true or false.

The following values ​​are considered false:

  • false
  • 0
  • "" or '' (Empty string)
  • null
  • Undefined
  • NaN

The following values ​​are considered true:

  • Everything else that is not falsy
  • true
  • '0' [0 in a string]
  • 'false' [false in a string]
  • [] (empty array)
  • {} (empty object)
  • function () {} (empty function)

Anything in a string is truthy, even a space.

Note: The AND operator (&&) returns the first falsy value it encounters, or the last value if all are truthy.

Example:

javascript
          console.log(0 && 10);  // 0

In this example the result is 0 because it is the first false value.

Example:

javascript
          console.log('John' && 'Jeff');  // Jeff

In this example Since both values are truthy, the last value ('Jeff') is returned.

Note: The OR operator (!) returns the first truthy value it encounters, or the last value if all are falsy.

Example:

javascript
          console.log(0 || 50 || 60);  // 50

In this example the result is 50 because it is the first truthy value.