Conditional Statements
Logical Operators

Working with conditionals means that we will be using booleans, true or false values. In JavaScript, there are operators that work with boolean values known as logical operators. We can use logical operators to add more sophisticated logic to our conditionals. There are three logical operators:

  • the and operator (&&)
  • the or operator (||)
  • the not operator, otherwise known as the bang operator (!)

When we use the && operator, we are checking that two things are true:

if (stopLight === 'green' && pedestrians === 0) { console.log('Go!'); } else { console.log('Stop'); }

When using the && operator, both conditions must evaluate to true for the entire condition to evaluate to true and execute. Otherwise, if either condition is false, the && condition will evaluate to false and the else block will execute.

If we only care about either condition being true, we can use the || operator:

if (day === 'Saturday' || day === 'Sunday') { console.log('Enjoy the weekend!'); } else { console.log('Do some work.'); }

When using the || operator, only one of the conditions must evaluate to true for the overall statement to evaluate to true. In the code example above, if either day === 'Saturday' or day === 'Sunday' evaluates to true the if‘s condition will evaluate to true and its code block will execute. If the first condition in an || statement evaluates to true, the second condition won’t even be checked. Only if day === 'Saturday' evaluates to false will day === 'Sunday' be evaluated. The code in the else statement above will execute only if both comparisons evaluate to false.

The ! not operator reverses, or negates, the value of a boolean:

let excited = true; console.log(!excited); // Prints false let sleepy = false; console.log(!sleepy); // Prints true

Essentially, the ! operator will either take a true value and pass back false, or it will take a false value and pass back true.

Logical operators are often used in conditional statements to add another layer of logic to our code.



In main.js there are two variables mood and tirednessLevel.

Let’s create an if...else statement that checks if mood is 'sleepy' and tirednessLevel is greater than 8.

If both conditions evaluate to true, then console.log() the string 'time to sleep'. Otherwise, we should console.log() 'not bed time yet'.

After you press “Run”, play around with the || operator and the ! operator! What happens if you negate the value of the entire statement with ! and switch to || instead of &&?

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