Course Review

Congratulations! You've finished the course and have mastered the beginning concepts. We've covered a lot of material, and now it's time for two final practice exercises where you will see how to use JavaScript to solve real-world problems. The first exercise reviews "assignment", "declaration", and "conditional statements" like if/else and switch, and "comparison" and "logical" operators using interactive examples.

Final Practice Exercise Review

logical operators

Logical operators: AND, OR, and NOT

If you want to test multiple conditions without writing nested if...else statements, logical operators can help you. When used in conditions, the first two do the following:

&& — AND; allows you to chain together two or more expressions so that all of them have to individually evaluate to true for the whole expression to return true.
|| — OR; allows you to chain together two or more expressions so that one or more of them have to individually evaluate to true for the whole expression to return true.

To give you an AND example, the previous example snippet can be rewritten to this:

if (choice === 'sunny' && temperature < 86) {
    para.textContent = 'It is ' + temperature + ' degrees outside — nice and sunny. Let\'s go out to the beach, or the park, and get an ice cream.'; 
 } else if (choice === 'sunny' && temperature >= 86) {
   para.textContent = 'It is ' + temperature + ' degrees outside — REALLY HOT! If you want to go outside, make sure to put some sunscreen on.';
  }

So for example, the first code block will only be run if choice === 'sunny' and temperature < 86 return true.

Let's look at a quick OR example:

if (iceCreamVanOutside || houseStatus === 'on fire') {
    console.log('You should leave the house quickly.'); 
 } else {
   console.log('Probably should just stay in then.');
  }

The last type of logical operator, NOT, expressed by the ! operator, can be used to negate an expression. Let's combine it with OR in the above example:

if (!(iceCreamVanOutside || houseStatus === 'on fire')) {
    console.log('Probably should just stay in then.');
   } else {
    console.log('You should leave the house quickly.');
 }

In this snippet, if the OR statement returns true, the NOT operator will negate it so that the overall expression returns false.

You can combine as many logical statements together as you want, in whatever structure. The following example executes the code inside only if both OR statements return true, meaning that the overall AND statement will return true:

if ((x === 5 || y > 3 || z <= 10) && (loggedIn || userName === 'Steve')) { // run the code 

A common mistake when using the logical OR operator in conditional statements is to try to state the variable whose value you are checking once, and then give a list of values it could be to return true, separated by || (OR) operators. For example:

if (x === 5 || 7 || 10 || 20) { // run my code } 

In this case the condition inside if(...) will always evaluate to true since 7 (or any other non-zero value) always evaluates to true. This condition is actually saying "if x equals 5, or 7 is true — which it always is". This is logically not what we want! To make this work you've got to specify a complete test either side of each OR operator:

if (x === 5 || x === 7 || x === 10 || x === 20) { // run my code }