TypeScript if else

Summary: in this tutorial, you will learn about the TypeScript if...else statement.

TypeScript if statement

An if statement executes a statement based on a condition. If the condition is truthy, the if statement will execute the statements inside its body:

if(condition) {
   // if-statement
}Code language: JavaScript (javascript)

For example, the following statement illustrates how to use the if statement to increase the counter variable if its value is less than the value of the max constant:

const max = 100;
let counter = 0;

if (counter < max) {
    counter++;
}

console.log(counter); // 1Code language: JavaScript (javascript)

Output:

1

In this example, because the counter variable starts at zero, it is less than the max constant. The expression counter < max evaluates to true therefore the if statement executes the statement counter++.

Let’s initialize the counter variable to 100:

const max = 100;
let counter = 100;

if (counter < max) {
    counter++;
}

console.log(counter); // 100Code language: JavaScript (javascript)

Output:

100

In this example, the expression counter < max evaluates to false. The if statement doesn’t execute the statement counter++. Therefore, the output is 100.

TypeScript if…else statement

If you want to execute other statements when the condition in the if statement evaluates to false, you can use the if...else statement:

if(condition) {
   // if-statements
} else {
  // else statements;
}Code language: JavaScript (javascript)

The following illustrates an example of using the if..else statement:

const max = 100;
let counter = 100;

if (counter < max) {
    counter++;
} else {
    counter = 1;
}

console.log(counter);Code language: JavaScript (javascript)

Output:

1

In this example, the expression counter < max evaluates to false therefore the statement in the else branch executes that resets the counter variable to 1.

Ternary operator ?:

In practice, if you have a simple condition, you can use the ternary operator ?: rather than the if...else statement to make code shorter like this:

const max = 100;
let counter = 100;

counter < max ? counter++ : counter = 1;

console.log(counter);Code language: JavaScript (javascript)

TypeScript if…else if…else statement

When you want to execute code based on multiple conditions, you can use the if...else if...else statement.

The if…else if…else statement can have one or more else if branches but only one else branch.

For example:

let discount: number;
let itemCount = 11;

if (itemCount > 0 && itemCount <= 5) {
    discount = 5;  // 5% discount
} else if (itemCount > 5 && itemCount <= 10) {
    discount = 10; // 10% discount 
} else {
    discount = 15; // 15%
}

console.log(`You got ${discount}% discount. `)Code language: JavaScript (javascript)

Output:

0

This example used the if...elseif...else statement to determine the discount based on the number of items.

If the number of items from less than or equal to 5, the discount is 5%. The statement in the if branch executes.

If the number of items is less than or equal to 10, the discount is 10%. The statement in the else if branch executes.

When the number of items is greater than 10, the discount is 15%. The statement in the else branch executes.

In this example, the assumption is that the number of items is always greater than zero. However, if the number of items is less than zero or greater than 10, the discount is 15%.

To make the code more robust, you can use another else if instead of the else branch like this:

let discount: number;
let itemCount = 11;

if (itemCount > 0 && itemCount <= 5) {
    discount = 5;  // 5% discount
} else if (itemCount > 5 && itemCount <= 10) {
    discount = 10; // 10% discount 
} else if (discount > 10) {
    discount = 15; // 15%
} else {
    throw new Error('The number of items cannot be negative!');
}

console.log(`You got ${discount}% discount. `)Code language: JavaScript (javascript)

In this example, only when the number of items is greater than 10, the discount is 15%. The statement in the second else if branch executes.

If the number of items is less than zero, the statement in the else branch executes.

Summary

• Use the if statement to execute code based on a condition.
• Use the else branch if you want to execute code when the condition is false. It’s good practice to use the ternary operator ?: instead of a simple if…else statement.
• Use if else if...else statement to execute code based on multiple conditions.