If Statements

Kevin · January 31, 2022, 2:17am

Tutorials / JavaScript Tutorials / If Statements

If Statements

Now you know the fundamentals of JavaScript, and you’ve created your own functions and variables. You’ve also created interactive HTML by getting input from the user and modifying the page.

Here’s an example:

So far, all of the code we’ve seen runs line by line, executing each line one after the other. The code can show different things, e.g. the above program shows whatever name the user enters. But the code itself always does the same thing every time it runs.

This tutorial introduces if statements which let your code take different actions based on conditions that you specify.

Booleans

Most values and variables you’ve seen so far have been numbers or strings, like this:

let catLives = 9;
let accountBalance = -123.45;
let message = 'hello world!';

Number and String are called types because they specify what type of data a value contains. JavaScript also contains another type called Boolean.

A boolean can only be one of two values: either true or false.

let isCodingFun = true;
let isTheSkyGreen = false;

This might not seem very useful yet, but it will become more handy in a couple paragraphs!

Relational Operators

Boolean variables wouldn’t be very useful if you had to decide their values ahead of time. Instead, you can obtain boolean values through relational operators. You might have called these inequalities in algebra class, where you compared two sides with symbols like < less than, > greater than, or == equal to. Here’s an example:

let temperature = 85;
let isHot = temperature > 80;

This code creates a variable named temperature and sets it equal to 85. Then it creates a variable named isHot and sets it equal to the result of the inequality temperature > 80. In this case, the inequality is true because 85 is greater than 80. So at the end of this code, isHot is holding the boolean value of true.

If that line of code is confusing, try reading what’s on the right side of the = equals sign first. First you take the inequality temperature > 80 and get a boolean value from it (in this case, true), and then you point the isHot variable to that value.

Boolean Operators

Similar to how you can add two numbers using the + operator to get a third number, or subtract them using the - operator, you can also operate on two boolean values to get a third boolean value.

And

You can combine two boolean values using the and operator, which looks like two ampersands: &&. The and operator evaluates to true whenever the two boolean values on either side of it are also true.

let canSwim = true;
let canFly = true;
let isDuck = canSwim && canFly;

The isDuck variable will only be true when both canSwim and canFly are also true. If either one of them is false, then isDuck will also be false.

Again, it might make more sense to read the right side first. First the code evaluates the && operator, which creates a boolean value of true. Then it points the isDuck variable to that value.

Or

The or operator evaluates to true if either of the two boolean values on either side of it is true.

To use the or operator, type two pipes || (they’re above the enter key, or shift + \) between two boolean values:

let isTodaySaturday = true;
let isTodaySunday = false;
let isTodayWeekend = isTodaySaturday || isTodaySunday;

The isTodayWeekend variables will be true if either the isTodaySaturday or isTodaySunday variables are true.

First the code evaluates the || operator which creates a boolean value (in this case it’s true), and then it points the isTodayWeekend variable to that value.

Not

In addition to operating on two boolean values, you can also calculate the opposite of a single boolean value. The opposite of true is false, and the opposite of false is true.

This is called the not operator, and you use it by typing an exclamation point ! before the value you want to switch.

let sinks = !canSwim;
let falls = !canFly;
let isTodayWeekday = !isTodayWeekend;

Each line of this code evaluates the ! operator which creates a new boolean value based on the opposite of whatever follows it, and then it points a boolean variable to that value.

Combining Operators

You can also combine these operators to form more complicated logic. So you can do things like this:

let isMammal = !canSwim && !canFly;

First this code takes the opposite of the canSwim and canFly variables to create two new boolean values. It then takes those values and feeds them into the && operator, which creates yet another boolean value. Then it points the isMammal variable to that value.

(If this bothers you because it doesn’t account for animals like bats, beavers, and dolphins… come up with your own isMammal variable!)

There is a whole field of study devoted to boolean logic, so check that out if it sounds interesting. But for now, keep in mind that boolean values contain true or false, and you can use operators like &&, ||, and ! on them.

If Statements

An if statement checks a boolean value and only executes a block of code if that value is true.

To write an if statement, write the keyword if, then inside parentheses () insert a boolean value, and then in curly brackets {} write the code that should only execute when that value is true. That code is called the body of the if statement.

Here’s an example that says good morning, but only if the current time is before noon:

This code creates a currentHour variable and sets it equal to 10. Then it creates an isMorning variable, and sets it equal to the boolean value that’s generated from the expression currentHour < 12. In this case that evaluates to true. Then the if statement checks the value of isMorning, and because it’s true, it prints 'Good morning!' to the page.

Try changing currentHour to 14 (which would represent 2:00 PM) to see that the message is no longer displayed. That’s because the if statement only prints the message if currentHour is less than 12. If it’s greater than or equal to 12, then currentHour < 12 evaluates to false, and the if statement skips the code inside the { } curly braces, so nothing is printed.

Boolean Expressions

The above example separated the inequality and the if statement into two steps: the code first created a boolean variable isMorning from an inequality, and then it used that variable in an if statement. But you can combine them into a single step:

let currentHour = 10;
if (currentHour < 12) {
  document.write('Good morning!');
}

This does the exact same thing as the old code, except now the inequality (which evaluates to a boolean value) is inside the if statement instead of being split into its own step. This is the format that most folks use when writing if statements.

Else Statements

An if statement executes some code if its boolean value is true, and it skips that code if the boolean value is false. But what if you want to do one thing if the value is true and a different thing if it’s false? Sounds like a job for an else statement!

To use an else statement, type the else keyword after an if statement, and then inside curly brackets {} put the code you want to execute when the if statement’s boolean evaluates to false:

This code uses an if statement to check whether currentHour is less than 12. Since 22 is greater than 12, that inequality is false, so the code inside the if statement is skipped. Instead, the browser jumps to the code inside the body of the else statement, which prints 'Good night!' to the page.

You can translate the if statement pretty directly to English: “If the current hour is lass than 12, then display the ‘Good morning!’ message. Otherwise, display the ‘Good night!’ message instead.”

Else If Statements

An if statement executes some code if its boolean evaluates to true, and an else statement executes code if it evaluates to false. But what if you want to take different actions depending on multiple cases? For example, it probably doesn’t make sense to print 'Good night!' at 2:00 PM, does it? This is where else if statements come in handy.

An else if statement is like a mix between an else statement and an if statement. You put an else if statement after an if statement, and if the if statement evaluates to false, then the else if statement’s boolean is evaluated:

This code uses an if statement to check whether currentHour is less than 12. Since 14 is greater than 12, that inequality is false, so the code inside the if statement is skipped. The browser jumps down to the else if statement and checks the boolean expression inside the else if statement. Since 14 is less than 17, the inequality is true, and the code inside the body of the else if is executed. The code then displays the 'Good afternoon!' message.

You can think about the code like this: “If the current hour is less than 12, then display the ‘Good morning!’ message. Otherwise check whether the current hour is less than 17. If it is, then display the ‘Good afternoon!’ message instead.”

If currentHour was 10, then only the code inside the body of the if statement would run. The code inside the else if statement would be skipped. And if currentHour was 20, then the boolean expressions of both the if statement and the else if statement would evaluate to false, so neither one of their bodies would be executed.

If / Else If / Else Combinations

You can follow an if statement with multiple else-if statements, and you can follow an else-if statement with an else statement. So you could expand your program to detect every time of day:

This code uses a series of if, else if and else statements to run this logic:

Is currentHour < 5?
- If so, show the Go back to bed! message and stop checking any other conditions.
- If not, keep checking.
Is currentHour < 12?
- If so, show the Good morning! message and stop checking any other conditions.
- If not, keep checking.
Is currentHour < 17?
- If so, show the Good afternoon! message.
- If not, show the Good night! message.

Try changing the currentHour variable to see the different messages.

Avoiding Unnecessary Checks

Keep in mind that only one body of code is entered in a series of if else-if else statements. As soon as the code enters one of the if or else-if statements, it stops checking subsequent else-if statements.

Look at this line of code, from the above program:

else if (currentHour < 12) {

Notice that the code only checks whether currentHour < 12, and if so, it displays Good morning! in the page. But for very early times, like 2:00 AM, we don’t want to show Good morning! yet. So why doesn’t the code also check that currentHour > 5 on this line?

The code doesn’t check to make sure that currentHour > 5, because it already knows that it is. If currentHour was < 5, then the if (currentHour < 5) statement would have been entered, and the code wouldn’t even reach the else if (currentHour < 12) statement.

Think about it this way: first the code checks whether the time is before 5:00 AM. If it is, then it displays the Go back to bed! message and doesn’t check any other times since it already knows it’s before 5:00 AM. If it’s not before 5:00 AM, then it knows that currentHour > 5 and it keeps checking other times.

That’s why the code can check whether the current hour is in the morning by only checking whether currentHour < 12. It it is, then it displays the “Good morning!” message and doesn’t check against any other times since it already knows the time is in the morning. If currentHour is not less than 12, then the code knows that currentHour is greater than or equal to 12, and it continues that pattern for the rest of the program.

Truthiness

We talked a little bit about type coercion when you read about variables. For example:

let x = '21';
let y = '2';
let z = x * y;
console.log(z);

This code creates two string variables and then tries to multiply them together. Perhaps surprisingly, this code will work, because JavaScript coerces the strings into numbers before doing the multiplication.

Similarly, if you try to use a non-boolean value in an if statement, then the code uses the “truthiness” of that value to convert it to a boolean. Values like 0, '' (empty string), and undefined are “falsy” and convert to false, and values like '42' and 'hello world' are “truthy” and convert to true.

Here’s an example:

This code creates a volume variable, and then uses that variable in an if statement. Since it’s not a boolean, the code checks its truthiness to convert it to a boolean.

Try changing volume to 0 to see what happens!

Learn More

Homework

Modify the above time of day example to use times from your daily schedule.
Write code that shows a different message depending on the current temperature.
Replace the hard-coded currentHour variable with this: let currentHour = new Date().getHours(); and then modify the code to create your own daily greeting webpage.
Expand the boolean example to work for more animals. Write a showAnimalType function that takes a set of boolean parameters (as many as you want!) and shows a message that explains whether the animal is a mammal, bird, reptile, amphibian, or fish. How would you represent a bat (which can fly but isn’t a bird) or a penguin (which can’t fly but is a bird)? If you want an advanced challenge, do a google search for “animal identification key” for some handy guides.
Create a program that uses boolean logic to determine the outcome of a flowchart like this xkcd comic flowchart. If you want an advanced challenge, try this xkcd advance flowchart!

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