Conditional Statements

What are Conditional Statements?

In life, we make decisions based on circumstances. If we are tired, we go to sleep, else, we stay awake.

These if-else decisions can be modeled in code by creating conditional statements. A conditional statement checks a specific condition(s) and performs a task based on the condition(s).

In these notes, we will explore how programs make decisions by evaluating conditions and introduce logic into our code.

If Statement

We often perform a task based on a condition. For example, if the weather is nice today, then we will go outside. If the alarm clock rings, then we’ll shut it off. If we’re tired, then we’ll go to sleep.

In programming, we can also perform a task based on a condition using an if statement:

if (true) {
    console.log("This message will print!");
}
// prints: This message will print!

Notice in the example above, we have an if statement. The if statment is composed of:

The if keyword followed by a set of parentheses () which is followed by a code block, or block statement, indicated by a set of curly braces {}.
Inside the parentheses (), a condition is provided that evaluates to true or false.
If the condition evaluates to true, the code inside the curly braces {} runs, or executes.
If the condition evaluates to false, the block won't execute.

If-Else Statements

In the previous section, we used an if statement that checked a condition to decide whether or not to run a block of code. In many cases, we'll have code we want to run if our condition evaluates to false.

If we wanted to add some default behavior to the if statement, we can add an else statement to run a block of code when the condition evaluates to false. Take a look at the inclusion of an else statement:

if (false) {
    console.log("The code in this block will not run.");
} else { 
    console.log("But the code in this block will!");
}
// prints: but the code in this block will!

An else statement must be paired with an if statement, and together they are referred to as an if...else statement.

In the example above, the else statement:

Uses the else keyword following the code block of an if statement.
Has a code block that is wrapped by a set of curly braces {}.
The code inside the else statement code block will execute when the if statement's condition evaluates to false.

if...else statements allow us to automate solutions to yes-or-no questions, also known as binary decisions.

Comparison Operators

When writing conditional statements, sometimes we need to use different types of operators to compare values. These operators are called comparison operators.

Here is a list of some handy comparison operators and their syntax:

Less than: <
Greater than: >
Less than or equal to: <=
Greater than or equal to: >=
Is equal to: ===
Is not equal to: !==

Comparison operators compare the value on the left on the value on the right. For instance:

10 < 12 // evalutates to true

It can be helpful to think of comparison statements as questions. When the answer is "yes", the statement evaluates to tru, and when the answer is "no", the statement evaluates to false. The code above would be asking: "Is 10 less than 12?" Yes! So 10 < 12 evaluates to true.

We can also use comparison operators on different data types like strings:

"apples" === "oranges" // false

In the example above, we're using the identity operator (===) to check if the string "apples" is the same as the string "oranges". Since the two strings are not the same, the comparison statement evaluates to false.

All comparison statements evaluate to either true or false and are made up of:

Two values that will be compared.
An opeartor that separates the values and compares them accordingly (<, >, <=, >=, ===, !==).

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 the 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, onlly one of the conditions must evaluate to true for the overal statement to evaluate to true. In the code example above, if either day === "Saturday" or dat === "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 a false, or take a false value and pass back a true.

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

Truthy and Falsy

Let's consider how non-boolean data types, like strings or numbers, are evaluated when checked inside a condition.

Sometimes, you'll want to check if a variable exists and you won't necessarily want it to equal a specific value - you'll only want to see if the variable has been assigned a value.

Here is an example:

let myVariable = "I exist!";

if (myVariable) {
    console.log(myVariable);
} else {
    console.log("The variable does not exist");
}

The code block in the if statement will run because myVariable has a truthy value; even though the value of myVariable is not explicitly the value true, when used in a boolean or conditional context, it evaluates to true because it has been assigned a non-falsy value.

So which values are falsy - or evaluate to false when checked as a condition? The list of falsy values include:

0
Empty strings like "" or ''
null which represents when there is no value at all
undefined which represents when a declared variable lacks a value
NaN, or Not a Number

Here is an example with numbers:

let numberOfApples = 0;

if (numberOfApples){
   console.log('Let us eat apples!');
} else {
   console.log('No apples left!');
}

// prints 'No apples left!'

The condition evaluates to false because the value of the numberOfApples is 0. Since 0 is a falsy value, the code block in the else statement will run.

Truthy and Falsy Assignment

Truthy and falsy evaluations open a world of short-hand possibilities!

Say you have a website and want to take a user's username to make a personalized greeting. Sometimes, the user does not have an account, making the username variable falsy. The code below checks if username is defined and assigns a default string if it is not:

let defaultName;
if (username) {
    defaultName = username;
} else {
    defaultName = 'Stranger';
}

If you compine your knowledge of logical operators you can use a short-hand for the code above. In a boolean condition, JavaScript assigns the truthy value to a variable if you use the || operator in your assignment:

let defaultName = username || 'Stranger';

Because || or statements check the left-hand condition first, te variable defaultName will be assigned the value of username if it is truthy, and it will be assigned the value of "Stranger" if it is falsy. This concept is also referred to as short-circuit evaluation.

Ternary Operator

In the spirit of using short-hand syntax, we can use a ternary operator to simplify an if-else statement.

Take a look at the if-else example below:

let isNightTime = true;

if (isNightTime) {
  console.log('Turn on the lights!');
} else {
  console.log('Turn off the lights!');
}

We can use a ternary operator to perform the same functionality:

isNightTime ? console.log("Turn on the lights!") : console.log("Turn off the lights!");

In the example above:

The condition, isNightTime, is provided before the ?
Two expressions follow the ? and are separated by a colon :
If the condition evaluates to true, the first expression executes
If the condition evaluates to false, the second expression executes

Like if-else statements, ternary operators can be used for conditions which evaluate to true or false.

Else If Statements

We can add more conditionals to our if-else with an else if statement. The else if statement allows for more than two possible outcomes. You can add as many else if statements as you'd like, to make more complex conditionals.

The else if statement always comes after the if statement and before the else statement. The else if statement also takes a condition. Let's look at the syntax:

let stopLight = 'yellow';

if (stopLight === 'red') {
  console.log('Stop!');
} else if (stopLight === 'yellow') {
  console.log('Slow down.');
} else if (stopLight === 'green') {
  console.log('Go!');
} else {
  console.log('Caution, unknown!');
}

The else-if statements allow you to have multiple possible outcomes. if/else if/else statements are read from top to bottom, so the first condition that evaluates to true from the top to bottom is the block that gets executed.

In the example above, since stopLight === 'red' evaluates to false and stopLight === 'yellow' evaluates to true, the code inside the first else if statement is executed. The rest of the conditions are not evaluated. If none of the conditions evaluated to true, then the code in the else statement would have executed.

The `switch` Keyword

else-if statements are a great tool if we need to check multiple conditions. In programming, we often find ourselves needing to check multiple values and handling each of them differently. For example:

let groceryItem = 'papaya';

if (groceryItem === 'tomato') {
  console.log('Tomatoes are $0.49');
} else if (groceryItem === 'papaya'){
  console.log('Papayas are $1.29');
} else {
  console.log('Invalid item');
}

In the code above, we have a series of conditions checking or a value that matches a groceryItem variable. The code works fine, but imagine if we needed to check 100 different values! Having to write that many else if statements sounds like a pain!

A switch statement provides an alternative syntax that is easier to read and write. A switch statement looks like this:

let groceryItem = 'papaya';

switch (groceryItem) {
  case 'tomato':
    console.log('Tomatoes are $0.49');
    break;
  case 'lime':
    console.log('Limes are $1.49');
    break;
  case 'papaya':
    console.log('Papayas are $1.29');
    break;
  default:
    console.log('Invalid item');
    break;
}

// prints 'Papayas are $1.29'

- The switch keyword initiates the statement and is followed by (...), which contains the value that each case will compare. In the example, the value or expression of the switch statement is groceryItem.

Inside the block, {...}, there are multiple case keyword checks if the expression matches the specified value that comes after it. The value following the first case is 'tomato'. If the value of groceryItem equalled 'tomato', that case's console.log() would run.
The value of groceryItem is 'papaya', so the third case runs - Papayas are $1.29 is logged to the console.
The break keyword tells the computer to exit the block and not execute any more code or check any other cases inside the code block. Note: without break keywords, the first matching case will run, but so will every other subsequent case regardless of whether or not it matches - including the default. This behavior is different from if/else conditional statements that execute only one block of code.
At the end of each switch statement, there is a default statement. If none of the cases are true, then the code in the default statement will run.