TypeScript Conditional Statements

Conditional statements are used to execute different code blocks based on different conditions. TypeScript supports the same conditional statements as JavaScript.

1. if Statement

The if statement is the most basic conditional control structure. It executes a code block when the specified condition is true.

Syntax

if (condition) {
    // Code to execute when condition is true
}

Example

let num: number = 10;

if (num > 0) {
    console.log("The number is positive.");
}

2. if...else Statement

The if...else statement executes one code block when the condition is true and another code block when the condition is false.

Syntax

if (condition) {
    // Code to execute when condition is true
} else {
    // Code to execute when condition is false
}

Example

let temperature: number = 25;

if (temperature > 30) {
    console.log("It's a hot day!");
} else {
    console.log("The weather is pleasant.");
}

3. if...else if...else Statement

When you need to check multiple conditions, you can use the if...else if...else structure. It tests each condition in order and executes the code block corresponding to the first condition that is true.

Syntax

if (condition1) {
    // Code to execute when condition1 is true
} else if (condition2) {
    // Code to execute when condition2 is true
} else {
    // Code to execute when all above conditions are false
}

Example

let score: number = 85;

if (score >= 90) {
    console.log("Grade: A");
} else if (score >= 80) {
    console.log("Grade: B");
} else if (score >= 70) {
    console.log("Grade: C");
} else {
    console.log("Grade: F");
}

4. switch Statement

The switch statement is used to execute one of multiple code blocks based on the value of an expression. It's typically used as an alternative to if...else if...else statements, especially when all conditions depend on the value of the same variable.

Syntax

switch (expression) {
    case value1:
        // Code to execute when expression result equals value1
        break;
    case value2:
        // Code to execute when expression result equals value2
        break;
    // ...can have any number of cases
    default:
        // Code to execute if expression result doesn't match any case
}

• The break keyword is required. It terminates the execution of the switch statement. If break is omitted, code will continue executing the next case, which is called "fall-through."
• The default clause is optional and used to handle all other cases.

Example

let day: number = new Date().getDay(); // Get current day of week (0-6)
let dayName: string;

switch (day) {
    case 0:
        dayName = "Sunday";
        break;
    case 1:
        dayName = "Monday";
        break;
    case 2:
        dayName = "Tuesday";
        break;
    case 3:
        dayName = "Wednesday";
        break;
    case 4:
        dayName = "Thursday";
        break;
    case 5:
        dayName = "Friday";
        break;
    case 6:
        dayName = "Saturday";
        break;
    default:
        dayName = "Invalid day";
        break;
}

console.log(`Today is ${dayName}.`);