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TypeScript Optional Parameters - GeeksforGeeks

Last Updated : 22 Jan, 2025

Optional parameters in TypeScript allow functions to be called without specifying all arguments, enhancing flexibility and code readability.

• Denoted by appending a ? to the parameter name.
• Optional parameters must follow the required parameters in function definitions.

function functionName(param1: type, param2?: type): returnType {
    // function body
}

• param1: A required parameter of the specified type.
• param2?: An optional parameter; if omitted, its value is undefined.

function greet(name: string, greeting?: string): string {
    if (greeting) {
        return `${greeting}, ${name}!`;
    }
    return `Hello, ${name}!`;
}

console.log(greet("Alice")); 
console.log(greet("Bob", "Good morning"));

• The greet function has two parameters: name (required) and greeting (optional).
• If greeting is provided, it customizes the message; otherwise, it defaults to "Hello".

function greet(name: string, greeting?: string): string {
    if (greeting) {
        return `${greeting}, ${name}!`;
    } else {
        return `Hello, ${name}!`;
    }
}

console.log(greet("Alice")); 
console.log(greet("Bob", "Good morning")); 

• The greet function has two parameters: name (required) and greeting (optional).
• If greeting is provided, it customizes the message; otherwise, it defaults to "Hello".

Output:

Hello, Alice!
Good morning, Bob!

function createUser(username: string, age?: number, email?: string): string {
    let userInfo = `Username: ${username}`;
    if (age !== undefined) {
        userInfo += `, Age: ${age}`;
    }
    if (email !== undefined) {
        userInfo += `, Email: ${email}`;
    }
    return userInfo;
}

console.log(createUser("john_doe")); 
console.log(createUser("jane_doe", 28)); 
console.log(createUser("sam_smith", 30, "sam@example.com"));

• The createUser function accepts username (required), age, and email (both optional).
• It constructs a user information string based on the provided arguments.

Output:

Username: john_doe
Username: jane_doe, Age: 28
Username: sam_smith, Age: 30, Email: sam@example.com

• Place Optional Parameters After Required Ones: Always define optional parameters following all required parameters to maintain clarity and prevent errors.
• Provide Default Values When Appropriate: Assign default values to optional parameters to ensure predictable behavior when arguments are omitted.
• Use Union Types for Flexibility: Consider using union types for optional parameters to clearly specify the types that are allowed, enhancing type safety.

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