Optional Parameters and Default Parameters

This lesson covers parameters that can be omitted (optional parameters) and parameters with a built-in initial value (default parameters). It's for anyone searching "TypeScript optional parameter default value."

Adding ? after a parameter name makes it "okay to omit." Writing paramName: type = initialValue instead sets a default value used whenever the caller doesn't pass one.

The example sets a default greeting with greet(name: string, greeting: string = "Hello"). The caller can either omit the greeting or specify one.

A common early mistake is confusing when to use an optional parameter (?) versus a default parameter. If you want a missing value to be treated as undefined, use ?; if you want a fallback value to be filled in automatically, use a default value — that distinction is worth remembering.

In real projects, the more configuration options a function has, the more setting common values as defaults keeps the caller's code simple.

📖 Reference code

function greet(name: string, greeting: string = "Hello"): string {
  return `${greeting}, ${name}!`;
}
console.log(greet("Alice"));
console.log(greet("Professor", "Good morning"));

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🧪 This site can't compile or run TypeScript directly, so it checks on the spot whether what you typed matches the reference code (scoring happens entirely in your browser — nothing is sent anywhere).