TypeScript if...else Statement

Note: If you're new to TypeScript, check our Getting Started with TypeScript tutorial first.

The TypeScript if...else statement is used to execute/skip a block of code based on a condition.

let quantity: number = 35;

// Check if quantity is a positive number
if (quantity > 0) {
    console.log("Positive number");
}
// Else, check if quantity is negative
else if (quantity < 0) {
    console.log("Negative number");
}
// If both conditions fail, quantity is 0
else {
    console.log("Zero");
}

// Output: Positive number.

TypeScript if Statement

We use the if keyword to execute code based on some specific condition.

The syntax of if statement is:

if (condition) {
    // Block of code
}

The if keyword checks the condition inside the parentheses (). If the condition is evaluated to:

• true - The code inside { } is executed.
• false - The code inside { } is skipped.

Note: The code inside { } is also called the body of the if statement.

Example 1: TypeScript if Statement

// Program to check if the student passed

let score: number = 96;

// Check if score is greater than 40
if (score >= 40) {
    // The body of the if statement
    console.log("You passed the examination.");
}

console.log("Program executed!");

Output

You passed the examination.
Program executed!

In the above program, the condition score >= 40 evaluates to true since the score is 96. Thus, the body of the if statement is executed.

Had the value of the score been lower than 40 (say, 35), then the body of the if statement wouldn't have been executed.

Finally, since console.log("Program executed!"); is outside the body of the if statement, it is executed regardless of the results of the if statement.

TypeScript else Statement

The else keyword executes a block of code when the condition in the preceding if statement evaluates to false.

Note: The else statement should always follow an if statement. In other words, the if and else statements are parts of a single conditional structure.

The syntax of the if...else statement is:

if (condition) {
    // Block of code to execute if the condition is true
}
else {
    // Block of code to execute if the condition is false
}

The if...else statement checks condition and executes code in two ways:

• If condition is true, the code inside if is executed. And, the code inside else is skipped.
• If condition is false, the code inside if is skipped. Instead, the code inside else is executed.

Example 2: TypeScript if…else Statement

// Program to check if the student passed or failed

let score: number = 35;

// Check if score is greater than 40
if (score >= 40) {
    // The body of the if statement
    console.log("You passed the examination.");
}
else {
    // The body of the else statement
    console.log("You failed the examination.");
}

console.log("Program executed!");

Output

You failed the examination.
Program executed!

Since score is 35, the if condition (score >= 40) evaluates to false.

Thus, the code inside if is skipped. And, the code inside else is executed.

let score: number = 35;

if (score >= 40)
    console.log("You passed the examination.");
else
    console.log("You failed the examination.");

// Output: You failed the examination.

TypeScript else if Statement

The else if keyword is used to check for additional conditions if the initial if statement is false.

The syntax of the else if statement is:

// Check for first condition
if (condition1) {
    // if body
}

// Check for second condition
else if (condition2){
    // else if body
}

// If no condition matches
else {
    // else body
}

Here:

1. First, the condition in the if statement is checked. If the condition evaluates to true, the body of if is executed, and the rest is skipped.
2. Otherwise, the condition in the else if statement is checked. If true, its body is executed and the rest is skipped.
3. Finally, if no condition matches, the block of code in else is executed.

Example 3: TypeScript if...else if Statement

let score: number = 59;

// Check if score is 80 or above
if (score >= 80) {
    console.log("Excellent!");
}
// Else, check if score 40 or above
else if (score >= 40) {
    console.log("Average");
}
// If both conditions fail, you fail the exam
else {
    console.log("Failure!");
}

Output

Average

Here, the if condition is false because score is 59. However, the else if condition is satisfied, so the program prints Average.

let score: number = 85;

// Condition for passing with second division
if (score >= 40 && score < 60) {
    console.log("Second division");
}
// Condition for passing with first division
else if (score >= 60 && score < 80) {
    console.log("First division");
}
// Condition for passing with distinction
else if (score >= 80 && score <= 100) {
    console.log("Distinction");
}
// Condition for failing the exam
else if (score > 0 && score < 40) {
    console.log("You failed the examination.");
}
// If all conditions fail, the score is invalid
else {
    console.log("Invalid score!");
}

// Output: Distinction

Nested if...else Statement

When we use an if...else statement inside another if...else statement, we create a nested if...else statement. For example,

let score: number = 60;

// Outer if...else statement
// Student passed if score 40 or above
// Otherwise, student failed

if (score >= 40) {

    // Inner if...else statement
    // Distinction if score is 80 or above

    if (score >= 80) {
        console.log("Distinction");
    }
    else {
        console.log("Passed");
    }
}

else {
    console.log("Failed");
}

// Output: Passed

Outer if...else statement

In the above example, the outer if condition checks if a student has passed or failed using the condition score >= 40. If it evaluates to false, the outer else statement will print "Failed".

On the other hand, if score >= 40 evaluates to true, the program moves to the inner if...else statement.

Inner if...else statement

The inner if condition checks whether the student has passed with distinction using the condition score >= 80.

If score >= 80 evaluates to true, the inner if statement will print "Distinction".

Otherwise, the inner else statement will print "Passed".

Note: Avoid nesting multiple if…else statements within each other to maintain code readability and simplify debugging.

let score: number = 40;
let result;

if (score >= 40) 
    result = "pass"
else
    result = "fail"

console.log(result);

// Output: pass

let score: number = 40;

let result = (score >= 40) ? "pass" : "fail";

console.log(result);

// Output: pass

let score: string = "C";

// Using if else for many conditions
// First condition
if (score === "A") {
    console.log("Excellent!");
}
// Second condition
else if (score === "B") {
    console.log("Good!");
}
// Third condition
else if (score === "C") {
    console.log("Average");
}
// Fourth condition
else if (score === "D") {
    console.log("Bad");
}
// Otherwise, execute else block
else {
    console.log("Fail");
}

// Output: Average

let score: string = "C";

// using switch...case
switch (score) {
    // First condition
    case "A":
        console.log("Excellent!");
        break;
    // Second condition
    case "B":
        console.log("Good!");
        break;
    // Third condition
    case "C":
        console.log("Average");
        break;
    // Fourth condition
    case "D":
        console.log("Bad");
        break;
    default:
        console.log("Fail");
}

// Output: Average

let age: number = 35;
let salary: number = 6000;

// Combine two conditions using the "and" operator &&
if (age >= 30 && salary >= 5000) {
    console.log("Eligible for premium membership.");
}
else {
    console.log("Not eligible for premium membership.");
}

// Output: Eligible for premium membership.

// Get user input from prompt(), which returns either string or null data
const userInput: string | null = prompt("Enter your exam score:");

// Check if user input is null
if (userInput === null) {
    console.log("Null input given!");
}
// Else, convert input to integer and check condition
else {
    let score: number = parseInt(userInput);

    // Check if the user passed or failed
    if (score >= 40)
        console.log("pass");
    else
        console.log("fail");
}

Also Read:

• TypeScript break Statement
• TypeScript continue Statement